DC-DC Charger (for leisure battery) -- How I Done It --


VIP Member
T6 Legend
DC-DC Charger (for Leisure battery) -- How I Done It --

Dellmassive`s -- "how I Done It" -- Thread
Kit List And Stuff -- How I Done It & What I Use --

I thought it was about time i addressed the whole DC-DC charger thing. . . .

The subject and viability is discussed at length over here > Stop/start...regen...smart Alternator... Dc-dc Charger For Leisure Battery Or Not?


Long story short the conclusion was that if you want a decent Leisure battery setup that will last a weekend, week, month etc etc ...... Then you need a DC-DC charger.


Well as we found out VW went for the option of a SCR (Split Charge Relay), which worked fine for all of history before "EU6 engines with stop/start"(and EU5 T6 with stop/start as highlighted below) . . . . this new engine tech changed everything! Not only on the VW T6 but but all vehicles from all manufacturers..... Even the trusted converters got caught on the hop.


The reason for this change is "emissions standards" and MPG/fuel economy . . . the clever guys at say VW worked out that if you used the kinetic energy from deceleration/braking to "charge" the battery then you didn't need to waist precious fuel and MPG`s while driving. . . . . the net effect is that the alternator is mostly free-wheeling and only dumps power into the battery when braking.

Now this is an extremely simplified version of events .. . . in truth there is a very complex system of electronics, sensors and software controlling the whole process to ensure that we dont end up with a flat battery on Monday morning.



What does it do?

The old "pre-EU6" system ensured a constant supply of power from the alternator when the engine was running . . . this could be seen with a multi-meter and would show a healthy 13.8v constantly that would charge the starter battery as well as any leisure battery/s that were connected to the system via the SCR. . . . This has worked great for years.


In the EU6 stop/start setup its all different, the alternator mostly free-wheels and will develop chunks of power under braking or as needed, all controlled by a computer ECU. . . . you can see this with a multi-meter, you will see 12.6v battery voltage rising to 15.2v under braking then dropping back to 12.6v when accelerating. . . . .this is no good for charging your leisure battery via a SCR.

The Ideas is the ECU will keep the starter battery at around 80% full to leave room for the REGEN braking . . .this all helps MPG and emissions. when you then connect your leisure battery and a SCR to the system you will find that your leisure battery will only ever achieve 80% SOC (state of charge) and that can fluctuate wildly (and thats assuming you are driving long enough to let the battery absorb the charge), also there is and issue of back drain . . . where by your 100% solar charged leisure battery will effectively drain its-self back down into the starter battery via the SCR once the van starts. (that has just wasted all your solar harvest)

The issue being is that in a leisure situation on a SLA (sealed lead acid) battery, you can only use the the top 50% of the battery power, its referred to a 50% DOD (depth of discharge) . . . it means on a 100Ah battery you can only use 50ah . . . beyond this the voltage drops too low to be usable and battery damage can occur... that means that if your leisure battery is only 80% charged in the first place . . . . and you can only use 50% . . . then you end up with 30% or less of your battery for your leisure applications . . . . . (this is not good and why your battery wont run your fridge and LED lights for a 3day camping trip =[ )



A DC-DC charger will totally separate your starter battery and leisure battery systems to prevent these problems.

They work very similar to the SOLAR MPPT controllers . . . they have internal boost and buck converters that can reduce or increase the voltage/power coming from the engine bay...

Now "power is power" (that means it cant be created, but can be changed) and thats physics but what these cleaver boxes will do is take the lower voltage 80%(SOC @13.2v Float) starter battery and boost the voltage to a decent charge for your leisure setup (>100% 14.2v bst/abs/flt) . . .

They will also reduce the massive surge from the regen phase to a voltage (15.2v regen) suitable for charging.... effectively they will take the massively changing voltages a EU6 engine produces and turns it into a stable steady charging voltage to keep you leisure system at 100% charged. (>100% 14.2v bst/abs then drop to 13.2v flt as per the selected charge smart algorithm)

The great thing is these work like standard EHU battery chargers . . . they have smart 3-7 stage charging algorithms including Lithium, that will constantly monitor the SOC of your leisure setup and keep it charged to 100%. They have many built in safety features and are basically fit and forget units. . . . . Some units even have built in Solar MPPT charge controllers that offer a one-stop-shop charge solution. . . . .

Then once the battery is fully charged, the DC-DC will drop to float/maintenance mode to keep the battery at 100%.


What happens when i turn on my Camper loads? (Inverter, Kettle, Lights, heater) -
well this is the cleaver part, most DC-DC chargers will disconnect the charging power every so often (every 100sec in the case of Redarc) to "sample" the AUX battery voltage, They use this data combined with the Current thats being drawn to determine the SOC of the battery or the Loads that are connected to the battery. . . . When you draw Loads or current from the AUX battery it will cause a slight volt drop that the DC-DC charger will detect. Using this reading plus the current drawn from the charger will make the charger move into full charge mode in an attempt to keep the battery fully charged.

The net effect is that even if your AUX battery is 100% charged @ 12.6V and the engine is running . . . . . when you switch on your loads the DC-DC charger will raise the output voltage to 14.6v or so and deliver up-to its maximum current... So effectively all your loads are being powered from the DC-DC charger and no battery power is lost (until you switch off the engine that is)


What size DC-DC charger do i need?

This is a good question, and the answer is very similar to the "what size solar do i need?" . . . Its not just about charging your battery . . . . its also about charging you battery WHILE running your loads. So you need to account for your worst case scenario - which will be a low AUX battery while remote camping while running all your loads - at nighttime - in the rain - and its snowing etc etc ..

Joking aside i believe that the sizing of a DC-DC should account for the above, whats the point in getting in that situation, then having to switch off all your lights and fridges etc . . . just to give the battery as much charge as possible while running the engine for 20mins at nights . . . . . nope thats not right.

So take your AUX battery size say 100Ah and go with 10-20% which is a recomended charge rate for SLA/AGM etc - thats 10-20A charger .

Then measure all your loads, either with a current clamp meter, or add up all your Watts or any other way get an idea of what your worst case scenario is . . . an example may be 6amp, 10amp, 15amp, 20amp . . . whatever. Lets take 10Amp for thius example.

Add the Battery charge rate of 10-20A plus the Loads draw rate of 10A give you a figure of 20-30A.

Now you know that a 30A DC-DC is needed to fully charge your AUX battery while drawing a Load.

If you are a POWER USER like me then the sky is the limit . . . . i have 1000-1500W inverters that will draw 100A easy frona SLA/AGM battery which causes large volt SAG and depleates the battery rapidly . . . im my case i use DC-DC to offset the current draw from the battery . . . . So ill draw say 40A from the DC-DC charger, then the rest from the Aux battery. - doubling up DC-DC chargers in parrallel is also posdsible so 2x 40A DC-DC chargers will give 80A then only 20A will be drawn from the battery. (remember the cables on the system will need to handle the 80A current)


What are the smart charger stages?

Adaptive three step charging, DC-DC chargers are configured for a three-step (or more) charging process: Bulk – Absorption – Float.

Bulk During this stage the controller delivers as much charge current as possible to rapidly recharge the batteries.

Absorption When the battery voltage reaches the absorption voltage setting, the controller switches to constant voltage mode. For lead acid batteries it is important that during shallow discharges the absorption time is kept short in order to prevent overcharging of the battery. After a deep discharge the absorption time is automatically increased to make sure that the battery is 3 EN NL FR DE ES SE IT completely recharged. For lithium batteries absorption time may be limited. The fixed or adaptive mode can be chosen on the battery settings (Victron unit).

Float During this stage, float voltage is applied to the battery to maintain it in a fully charged state. When the battery voltage drops substantially below this level, due to a high load for example, during at least 1 minute, a new charge cycle will be triggered.



Next is to look at the available DC-DC chargers and they can/cant do:

The main contenders are below:


there are other makes around.


The things to look for in a DC-DC or B2b charger are:

- Max though-put or charging Amps ie 20/30/40/50/60 A.
- input voltage range ie 10-16V input for start/stop engines.
- output voltage ie - 12v as some are 24v or even 36V.
- Battery types they work with ie - AGM, SLA,LIFEPO4, Gell.
- Multi-stage smart Charging? - most are.
- Does it have a temp sensor? - some do, some dont.
- type/size of input/output terminals..
- Does it have a MPPT solar reg built in? - some do, some dont..
- can they be parralled up for power users? (some can have 4x in parrallel).


The T6 "Engine Run" feed from the BCM is shown here:

T6 - Second battery wiring diagram
Second battery for transporter ( not California)

+12v switched on when RPM engine run is detected.

used by the OEM setup to switch a 80A relay that connects the second battery (old skool SCR)


conection : J519 BCM - T73a/3


And BCM coding to activate output:

credit: @Deaky - Byte 17 Bit 4 Battery Seperating Relay active


credit: @rod_vw



I personally have been running the Redarc BCDC1240LV,

and will be upgrading to the BCDC1250D shortly (added Lifepo4 battery + 50A throughput)

I will also be getting a Victron 12/12/30 to test (to see if it will work with VE-Direct)


Diagrams, Schematics & Wires -- How I Done It --



I've started a excel sheet detailing the most important details of the various DC-DC charger units as an easy to read chart.

I've attached the sheet below to download, if you have any of the missing DC-DC chargers or want to add your own . . .

download the chart edit/add in your own unit then email it back to me for uploading : dellmassive@gmail.com



Last edited:


VIP Member
T6 Legend

First up we Have the Renogy DC-DC charger:


12V DC to DC On-Board Battery Charger


This 20A/40A DC to DC Charger is designed to charge secondary batteries while driving using the primary battery connected to the alternator. It is able to charge: Flooded, Gel, AGM, and Lithium batteries using multi-stage charging.

If you have any questions regarding this product, please call us at 1 (909)287-7111 or email sales@renogy.com.

  • Multi-Chemistry Battery Types ( Flooded, Gel, AGM, and Lithium )
  • Smart Protection Features ( Battery Isolation, Over-voltage safety, Overheat protection, and Reverse polarity protection)
  • S3-phase Charging (Bulk, Boost, and Float)
  • Built tough for all conditions
  • Able to charge an auxiliary battery to 100% state of charge
  • Increases run time of loads, such as fridges and lights
  • Overcomes voltage drop caused by long cable runs
  • Prolongs Battery Life
- Nominal Voltage: 12V Nominal Voltage: 12V.
- Rated Charge Current: 20A Rated Charge Current: 40A.
- Output: 250W Output: 500W.
- Idle Power Consumption: <0.4A Idle Power Consumption: <0.4A.
- Input Voltage Range: 8V-16V Input Voltage Range: 8V-16V.
- Equalization Voltage: N/A Equalization Voltage: N/A.
- Float Voltage: 13.2-13.8v Float Voltage: 13.2-13.8v.
- Boost Voltage: 14.1-14.7v Boost Voltage: 14.1-14.7v.
- Operating Temperature: -20°C to +50°C Operating Temperature: -20°C to +50°C.
- Dimensions: 8.31 x 6.89 x 2.68 inches Dimensions: 10.28 x 6.88 x 2.68 inches.
- Weight: 2.93lbs Weight: 4.14lbs.



Edit: Renogy have released a 60A DC-DC version - 12V 60A DC to DC Battery Charger


Renogy now have a combined DC-DC + MPPT,

(25+25=50A or 50+0=50A 0r 0+50=50A)



DCC50S 12V 50A DC-DC On-Board Battery Charger with MPPT


Renogy's DCC50S DC to DC MPPT on-board Battery Charger is designed to give you options! This multi-stage, multi-input battery charger can charge a service battery from an alternator connected to a starting battery or by utilizing solar panels connected directly to the unit. While your vehicle's alternator prioritizes your starting battery, this unit will allow your solar system to charge your service battery first! It's the smart way to keep yourself fully charged and off-grid longer! And with multi-stage charging and multi-chemistry functionality, Renogy's DC to DC MPPT Charger has you covered whether you're using Flooded, Gel, AGM, or Lithium batteries.

If you have any questions regarding this product, please call us at 1 (909) 287-7111 or email sales@renogy.com.

  • Designed to charge service batteries from two DC inputs—solar panels and alternator.
  • Built-in Maximum Power Point Tracking (MPPT) to maximize the solar power.
  • 3-phase charging profile (Bulk, Boost, and Float) ensures your service battery will be accurately charged at the correct voltage levels to 100%.
  • Built-in Voltage Sensitive Relay (VSR) for easy setup with traditional alternators.
  • Compatible with smart alternators (with variable output voltage).
  • Trickle charges the starting battery via solar panels if the service battery is fully charged.
  • Isolation of the starting battery and the service battery.
  • Temperature and voltage compensation features prolong battery life and improve system performance.
  • Smart Protection Features: battery isolation, over-voltage protection, battery temperature protection, over-current protection, overheat protection, reverse current protection, solar panel and alternator reverse polarity protection.
  • Compatible with multiple battery types: AGM, GEL, Flooded, and Lithium.
  • Compact with a sturdy design, it was built tough for all conditions.
System Voltage : 12VDC Battery Voltage Range : 9-16VDC
Maximum Charging Current : 50A (25A Alternator / 25A PV) Battery Type : SLA, GEL, FLD, LI, USER
Backup Battery Charging Mode : MPPT Charging Efficiency : Up to 97%
Max. Solar Input Voltage : 25V Maximum Solar Input Power : 660W
Alternator Input Voltage : Traditional Alternator: 13.2-16VDC Smart Alternator (Euro 6): 12-16VDC"

Maximum Alternator Input Power : 660W
Self-consumption : 60mA Operating Temperature Range : -22°F~149°F / ﹣35°C~65°C
Dimensions : 9.6 x 5.7 x 3.0 in / 244 x 146 x 77 mm Weight : 3.13 lbs / 1.42 kg

Accessories 1-year material warranty





Next we have the Redarc


DC to DC Battery Charger - Best Charging Systems | REDARC Electronics


The REDARC BCDC1250D In-vehicle Battery Charger is a 12V 50A In-vehicle DC to DC battery charger. It is the latest variant of the award-winning BCDC Dual range and features REDARC’s Smart Start technology.

Styled with a red line running down its back tells you there’s something different about this dual battery charger. A 50-amp charging output makes it the most powerful in REDARC’s BCDC range.

The BCDC1250D has been developed to provide a higher current output, it features an additional charging stage known as SoftStart which has been engineered to handle more demanding applications.

Featuring a Maximum Power Point Tracking (MPPT) solar regulator, the BCDC1250D will charge from both solar and the alternator simultaneously. And, with built-in 'Green Power Priority' it will select solar charging first, meaning less load on the alternator.

Suitable for 12 or 24-volt vehicle systems, the unit will also work with either standard or variable/smart alternators.

It is compatible with all common types of batteries used in automotive and recreational vehicles such as AGM, Gel, standard lead acid, Calcium and LiFeP04.

As a guide, the BCDC1250D is used for battery bank sizes over 200AH. For smaller batteries of 75-200AH REDARC recommends the BCDC1225D.

Proven and Tested

The award-winning technology in the BCDC has been independently verified and tested to ensure battery life is maximised, charging your battery to a proven 100%.

Built in Australia and tested for our unique conditions, the DC to DC battery charger will work in the extreme heat of the Simpson Desert, up to 80°C and can handle deep water crossings at Cape York – water, dust and vibration are no match for this charger.

If you’re heading out into the bush and want to get there and back again safely – you can trust REDARC.

  • 50amp output makes it the most powerful in REDARC's BCDC range
  • Separate DC and solar charging inputs
  • Maximises the performance and life of your auxiliary battery
  • Simultaneous DC and solar charging with Green Power Priority
  • Suits 12V/24V standard and variable voltage/smart alternators
  • Suitable for AGM, GEL, standard lead acid, calcium and LiFeP04 batteries

- Input (Operating) Voltage 9 - 32VDC.
- Output System Voltage 12VDC.
- Maximum Charging Voltage 15.3V.
- Output Current Rating (Cont.) 50A.
- Input Current (Max.) 55A.
- Input Fuse Rating 60A.
- Output Fuse Rating 60A.
- Standby Current Draw <100mA.
- Output Power 750W.
- Width (mm) 120.
- Height (mm) 37.
- Length (mm) 165.
- Weight (kg) 1.
- Warranty 2 Years.




Next we have the New Kit-on-the-block Victron:

Orion-Tr Smart DC-DC Charger Isolated

https://www.victronenergy.com/dc-dc-converters/orion-tr smart

The Orion-Tr Smart DC-DC charger is a professional DC to DC adaptive 3-stage charger with built-in Bluetooth.

For use in dual battery systems in vehicles or on boats where the (smart) alternator and the start battery are used to charge the service battery. The unit can be monitored and programmed via Bluetooth and can be remotely controlled via a remote on/off switch and has an engine running detection mechanism.

The Orion Tr-Smart charger can be used in 12V or 24V systems and is suitable for both lead acid and lithium batteries. Models are available up to 400W and unlimited multiple units can be connected in parallel to increase output power.

Suitable for high temperatures up to 55°C with a full rated output up to 40°C.




NO YT Vids yet !


Next Up CTEK




Product information

Dual input 20A charger with selectable charge voltages

Enjoy more and relax

With award winning Swedish technology, the dual input D250SE allows you to enjoy your vehicle more with maximized battery capacity as the ideal charger for vehicles without access to a mains power supply.

The D250SE is a fully automatic, 5 step charger that supplies up to 20A of power to any 12V lead-acid or lithium* service battery from 40–300Ah. It has selectable charge algorithms for AGM and lithium* batteries and can use power through its dual input from alternator, solar panel and wind power. When the service battery is fully charged, the D250SE will automatically redirect maintenance charge power to the starter battery. The D250SE can maintain a stable output up to 20A to vehicles fitted with smart ECU controlled alternators and also has a temperature sensor for optimised charging, regardless of weather conditions. *) 12V lithium batteries (LiFePO4, Li-Fe, Li-iron, LFP)

    • 20A fully automatic temperature compensated charging for batteries from 40-300Ah while you’re on the move
    • Dual input (solar and alternator in parallel)
    • Smart alternator compatible
    • Selectable lithium* and lead-acid mode
    • Solar panel regulator with Maximum Power Point Tracking (MPPT)
    • *) 12V lithium batteries (LiFePO4, Li-Fe, Li-iron, LFP)

    • Battery separation replacing diodes and VSR relays
    • Selectable AGM option – 14.4V or 14.7V
    • Maximised charging for better battery life and performance
    • Temperature sensor to compensate for hot or cold conditions
    • Splash and dust proof (IP65)
    • Compatible with SMARTPASS 120 for the ultimate 140A power management solution
    • 2-year warranty






AMC12-12-30 | 0161 745 7697

This AMC12 charger works on vehicles with a 12V electrical system to charge nominal 12V battery packs. The AMC charger works with Ignition control, Constant Voltage Alternators, Euro 5 & 6 engines, STOP/START, Smart Alternator charging systems & Vehicles with Regenerative Braking, ( Mercedes Benz Blue Efficiency Vehicles, VW Blue Motion Vehicles, Ford ECOnetic vehicles, Vauxhall ecoFlex vehicles to name a few). The unit delivers 30A charging currents to charge upto to 300Ahr battery packs .

The unit operates with from 9-30V and so works with long cable runs and large swings in input voltages .The unit is protected against current overload and over temperature protection with a fully automatic reset . The software optimises the battery charging cycle and is configured to suit most types of lead acid including Gel, AGM but can easily be reprogrammed to fit a particular type of battery or customer specification using the Ablemail Software Interface (ASI). The optional Interface Card adds remote output voltage monitoring for when the secondary battery is a long way from the charger & Battery Temperature monitoring for operating in extremely hot or cold conditions. The unit can be programmed to charge Lithium batteries .

The unit is assembled in an aluminium extruded case which can be filled with resin for operation in particularly harsh environments. It can be supplied with a variety of connection options see the Ablemail Mechanical Options. This product is part of a large range & if this product specifcation does not meet your requirement please contact our technical department to see if we can meet your exact needs.

- Output Load Current:30A.
- Input Voltage:12V nominal (10-15V).
- No Load Input Current:2mA @ 12v input.
- EMC:E11 Approved CE approved.
- Protection:Automatic overload and over temperature protection with fully automatic reset.
- Output Voltage:12V.
- Isolation:Non-isolated.

- Spec Sheet:

AMC12-12-30E DATA SHEET.pdf

No YT vid????


Next we have Votronic

Charging Converter VCC 1212-50

VOTRONIC - VCC 1212-50

Battery-to-Battery (B2B)
Charging Technology


Starter Battery Supply Battery


    • The efficient charging converter ensures high charging capacity, already within short distances.
    • Full charging when driving longer distancesIt increases/reduces the voltage to the level, which is required for precise charging of the board battery with the optimum charging characteristic line.
    • Cable losses and considerable voltage fluctuations of the generator, known from Euro 6 vehicles (intelligent alternators), are completely compensated.
    • The charging converters excel by their compact design, low weight (high-frequency switch mode technology) and powerfully dimensioned power components for safe operation.
    • The energy balance of the board battery is considerably improved, without intervention into the starter circuit.
    • The simultaneously supplied 12 V consumer loads are protected against overvoltage and voltage fluctuations.
    • The charging voltage is free from peaks and is controlled in such a way, that overcharging of the batteries is excluded.
    • Fully Automatic Operation: The unit is permanently connected to the batteries, and it is automatically activated by the running generator of the vehicle. Battery discharge in case of an engine stop is avoided.
    • Parallel and Floating Operation: In case of simultaneous consumption, the battery will either continue to be charged or maintained via trickle charging. Calculation and control of the adaptation of the charging times is effected automatically by the unit.
    • Unattended Charging: Multiple protection against overload, overheating, overvoltage, short circuit, incorrect behaviour and back discharge of the battery by electronically controlled gradual reduction down to complete separation of unit and battery.
    • Integrated On-board Mains Suppression Filter: Unproblematic parallel operation of charging sources (EBL, chargers, motor-driven and petrol-driven generators, solar systems) on one battery.
    • Charging Cable Compensation: Automatic compensation of voltage losses on the charging cables.
    • Connection for Battery Temperature Sensor (included in the standard delivery scope): Lead batteries (acid, gel, AGM): In case of low outside temperatures, full charging of the weak battery is improved by automatic adaptation of the charging voltage to the battery temperature, and in case of summery temperatures unnecessary battery gassing will be avoided. LiFePO4 Batteries: Battery protection in case of high temperatures and particularly in case of low temperatures < 0° C. Highly recommended, if the battery temperature might drop below 0 °C during operation.
    • Charging Aid for Deeply Discharged Lead Batteries: Gentle preliminary charging of the (lead-acid, gel, AGM) battery to 8 V, followed by powerful support of the battery, in case of possibly switched-on consumers.



next we have Sterling.

Pro Batt Ultra Battery to Battery Chargers, Non Waterproof, (Drip Resistant IP21)

Pro Batt Ultra Battery to Battery Chargers, Non Waterproof, (Drip Resistant IP21)



Sterling's range of Battery to Battery Chargers (B2Bs) has grown significantly over the past few years. This is in an effort to supplement the ever growing demand from the commercial vehicle, recreational vehicle and marine industries. The B2Bs have become extremely popular as they fast charge batteries as you cruise along without the need for complex wiring, touching your alternator, voiding the alternator's warranty and tampering with the electronic control units (ECUs). You can provide the onboard batteries with a fast 4 stage charging profle with a very simple and speedy installation. All of the benefits of advanced charging without any of the drawbacks. Simply connect the B2B between the battery being charged and the battery you wish to charge.

DC V (In) DC V (Out) Amps In Weight Kg L x W x D mm Part Number
- 12 12 30 1.2 190 x 160 x 70 BB1230.
- 12 12 60 1.4 190 x 160 x 95 BB1260.
- 12 24 70 1.4 190 x 160 x 95 BB122470.
- 12 36 70 1.4 190 x 160 x 95 BB123670.
- 12 48 70 1.4 190 x 160 x 95 BB124870.
- 24 24 35 1.4 190 x 160 x 90 BB242435.
- 24 12 35 1.4 190 x 160 x 95 BB241235.
- Remote Control with 10m cable BBURC.


1) Multiple Activation Modes: Automatic for most non vehicle applications,

Manual for remote activation.

Regenerative braking (requiring ignition feed),

Automatic regenerative braking, requiring no ignition feed.

2) Output charging at 12V, 24V, 36V and 48V. Input voltages at 12V and 24V, Up to 800W rating.

3) Much larger model up to 3000W coming soon.

4) Current is NOT taken from the input battery and given to the output battery. The B2B uses alternator power to charge the output battery. Manual mode and regenerative braking mde allows you to override this.

5) Safety features:

100% fire proof plastic box.

No screws to corrode.

Thermal power reduction.

Multi stage fan cooling

6) Night Time Setting: Allows the unit to ran at half the power so the fan noise is kept down.

7) Boost / Reduce Charging: The B2Bs ensure batteries get the correct charging profile irrespective of high or low input voltages.

8) B2B turns on at 13.3V and turns off at 13.0V (x 2 for 24V). Thus, does not drain input battery. Regenerative braking mode allows the input voltage to drop to 12.2V (x 2 for 24V).

9) Customizable Profile: Choose your own charging profile on the front panel.

10) 9 preset battery chemistry options: Including AGM, LiFePO4, Gel, Flooded and sealed lead acid.

11) 4 stage battery charging: Thge B2B charges batteries between 5-20 times faster than a stand alone alternator.

12) Very Simple to install: No electronic Control Unit (ECU) issues. No complex wiring. No warranty issues. Fully prepared for smart alternators (regenerative braking).

Remote control optional.




Nemo DX1230 DC-DC charger



The NEMO series DC-DC Charger Converter has maximum two inputs, can draw power from smart alternator or traditional alternator, as well as solar panel, to perform a proper charging for service battery while you are driving. • NEMO is compatible with smart ECU controlled alternator (Euro 6) which delivers a variable output when it is not needed. NEMO could maintain the charging with a stable output in this situation, to assure your battery getting a fully charging in the shortest time. • It will protect or maintain your starter battery even if engine was stopped without issue of starting problems. • Dual input from alternator and solar panel • Compatible with smart alternator – Euro 6 • TBB premium II multiple stages charging algorithm for lead acid battery • Built in automatic temperature and voltage compensated battery charging • Multiple battery chemicals including AGM, GEL, LFP(LiFePO4) etc • Max Power Point Tracking (MPPT) technology • Compact and Water Proof, IP65


no YT vid . . .


any more you want me to add?

Last edited:


VIP Member
T6 Legend
Part2 added, part3 to come . . . . . . .

Part3 is more about the install,

once you have decided on what unit you want to go for, its time to get it fitted.

Now even though i have the BCDC1240 which has built in solar MTTP i chose to go with a seperate Victron MPPT and just run them side by side.

Now you have a few options: If you have the factory fitted leisure battery or not?

If you have the Factory fitted kit then VW will have done all the hard work for you and run all the cables, fitted fuses etc . . . the even give you a nice ignition/engine run wire. (which is nice)

Vw fit a SCR like this : (this is under the front seat (double bench in my case)

This needs to be removed and replaced with your shiny new DC-DC charger.

They will also have fitted a second battery for you: again here under the double bench:

Now at this point depending on the Amp rating of your new DC-DC you may be able to just replace the relay with this mod from @travelvolts https://www.travelvolts.net/product-page/plug-and-play-upgrade


Which make the install plug-n-play and leaves the original factory setup in place.

If not then you can do as i done and just cut out the factory supply fuse and relay and replace it with your own . . . . i went for MIDI style fuse holders and crimps.

I also chose to add some battery monitors at the same time . . . at this point it was a couple of BM2 battery monitors . . . . like these :


Because i had the factory wiring, the link to the starter battery was already in place and fused,
the link to the AUX battery was there and fused,
The ignition feed wire was already there,
all i needed to add was a new ground cable for the DC-DC.

i used velcro to stick the DC-DC to the seat basr, and made all the connections with MIDI fuses and crimps.

I also decided to fit a new fusebox to the AUX battery at the same time . . .

once complete i used the BM2 battery monitor to keep an eye on the performsnce of the new DC-DC charger . . . as we can see here it works perfectly (on the EFB+ AUX battery)

it also worked great as a jumpstart for a dead AGM battery when camping charging the battery at the full 40Amps with the engine running. see here > Mobile Solar Panels ? . . . - How I Did It -

and some final battery testing . . .



more in part 4
Last edited:


VIP Member
T6 Legend
part4 DC-DC install

What if i dont have the factory install kit/wires ?

This is where thing can get tricky . . . . . you need to install all the new cabling and fuses etc yourself . . . .

now luckily there some kits available that have all you need.


here is an example from @travelvolts https://www.travelvolts.net

Extra long wiring kit for all DC-DC chargers. Has enough cable to reach to the rear of the van. Includes all fuses, connectors etc. All lugs pre crimped.



and here from Splitcharge: Split Charge Kits, Systems, Components & Tools | SplitCharge.co.uk

REDARC BV1240D - B2B Charger 40 Amp - Full Installation Kit | SplitCharge.co.uk


There are various guides on what goes where and how, but basically you need to.

fit a new Aux battery,tray,hold down kit.
run new HD fused battery cable from starter battery to Aux battery to feed the DC-DC
run a new HD fused cable from the DC-DC to the Aux battery
run a new HD ground cable from the DC-DC to ground
run a new HD ground cable from the Aux battery to ground
run a new ignition feed cable from the center fuse box to the DC-DC


TravelVolts has a great guide here> "Redarc 1240D DC-DC charger."



CTEK DUAL 250/ 250SA /250SE DC-DC charger. > https://docs.wixstatic.com/ugd/d959f6_3ed319f968954720a73c38807b437278.pdf


Victron DC-DC >


ABLEMAIL ABMC12-12-30 and AMS12-12-30 DC-DC chargers.




This was the setup i went with:


i removed the factory relay, fuse and fuse holders:


and cut the ends off and replaced with MIDI style fuses and crimp ends:




some extra wires went in for some other bits:


A new HD ground cable was fitted to the seat base bolt, (for the DC-DC charger)
The existing feed for the OEM relay was used as the ignition trigger for the new DC-DC.

A new MIDI style fuse box was fitted:


which fits down the side of the AUX battery:


and with the rubber battery cover refitted the final outcome looks like this:


more battery testing . . . . .

Screenshot_20180827-102136_Battery Sense.jpg

and we end up with a very happy Aux battery that is 100% fully charged via the DC-DC, that is totally isolated from the front starter battery.

Screenshot_20180827-102221_Battery Monitor.jpg


That was all stage 1,2,3.

were now at stage 4 with is the Lifepo4 Lithium upgrade . . . . that will entail replacing this DC-DC with one more suited to Lifepo4 charging and new Lifepo4 battery's with associated LBD & LTD etc.


You can also find out more info over here>

Ignition Live Under Passenger Seat - How Its Done -
Installing 12v Socket /s - How Its Done -
12v Igntn Feed + 5xusb Charging From Ign Feed - Today's Install
12v Aux Feed + 5xusb Charging From Aux Battery - Today's Install
Stop/start...regen...smart Alternator... Dc-dc Charger For Leisure Battery Or Not?

and other related info here>

Mobile Solar Panels ? . . . - How I Did It -
What Solar Panel & Controller ?
Solar Panel Cutting Hole In Roof !!!
Battery Chargers - How I Did It -
Which Jump Starter?
Lithium Lifepo4 12v Batteries - - - Time For An Upgrade ? - - -


Last edited:


Senior Member
VIP Member
T6 Guru
Nice write up Dell.

if anyone is looking at lithium batteries, I’d go the largest Amp charger you can use for the lithium battery you buy.

I’m running a votronic triple 30 amp charger and the 60amp version would have been better for short distances.

but I am running induction cooking and hot water off my system, which I hadn’t planned on when I decided to go 30 amp version, but as we have been doing a lot of km, it hasn’t been a major issue.

I may even change over to it, but I’ll most likely just build a new camper on a crafter/sprinter MWB next year.

I’m only pointing this out as a lot of people like to freecamp in the one spot for a while, and a short drive for you may be an hour or so if solar hasn’t covered your needs, you can easily top up the battery for a longer stay


VIP Member
T6 Legend
Part5 - MORE POWER !!

Paralleling up chargers:

(Note: Part1 has new edits to include DC-DC sizing)

This is a feature that can increase the power output from DC-DC chargers.

Same brand and models are advised . . . Here is an example from Redarc:


Connecting REDARC BCDC Dual Inputs in parallel

For batteries requiring a higher charge rate than REDARC's 25A or 40A chargers, the good news is that up to 4 BCDC-Dual chargers can be used in parallel.

Both wiring diagrams below are suitable depending on the installation requirements. Appropriate fuses should be used (fuse sizing depends on the charger size).

BCDC-Dual chargers feature a fault detection mode which ceases the charging output if the battery is removed; it is important to note that this does not function when BCDC-Dual chargers are used in parallel. The result is that BCDC-Dual chargers in parallel may continue to run a load indefinitely or until the BCDC turns off due to the inputs falling below the turn off thresholds. Before commencing work on loads connected to the auxiliary battery, ensure that the BCDCs have ceased charging so that the load is not powered by the BCDC even with the battery is disconnected.




  • Power wires and fuses must be appropriately sized to suit the current rating of the charger(s) and cable length.
  • The chargers should be installed in areas which receive airfl ow. Refer to the user manual for further information.
  • The Solar input Positives (A) should not be connected in parallel.
  • All chargers should be set to the same charging profile.
1. Refer to the battery manufacturer’s data sheet to ensure the battery can accept the combined maximum charge currents from all chargers whilst at operating temperature (e.g. Engine Bay).

2. - BCDC1225D: 40 Amp fuses (as found in REDARC FK40 Fuse Kit)
- BCDC1240D: 60 Amp fuses (as found in REDARC FK60 Fuse Kit)

This tech tip detailing how to connect dual input BCDC's in parallel is also available as a PDF.



VIP Member
T6 Legend
New units are in........

Thank you @travelvolts

Here you see the first BCDC1250D in the UK, apparently.

20191018_142803.jpg 20191018_142751.jpg


A quick look at the Victron showed a instant firmware update....

But seemed to have no VE-direct for lithium temp and battery monitoring.


First unit on test is the Victron Trio 12/12/30.

I've swapped out the Redarc BCDC1240LV and fitted the 12/12/30 so we have a like for like test . . .


its a bit busy down there , , , , but basically the 12/12/30 is massive compared to the 1240 so i had to temporarily mount it to the side of the battery...


i made up a loom using 8AWG silicone cables for the power in/out and the Two grounds . . . . Yes Two grounds !!!

because this unit is "isolated" the in/out ground feeds need there own cable, so i just run both to a ground block . . .

I also connected the "engine run" 12v trigger to the Remote-H input, this enables/disables the unit.

here we can see the Bluetooth monitoring . . . voltage in and out . . . i set the battery at a AGM/Gell preset. . . .


and this pic shows the Graph view . . . .


one thing to note is that it didn't seem to have any VE-direct functionality . . . ?

i was hoping that it could connect up to the VE-Direct and get temperature information (from the BMV712) to disable charging of lithium battery's at low temps . . . ?

also i couldn't find any info on the metrics for the three charge states . . . . i know its rated for 30A and can surge for 10Sec at 40A . . . but there is no info given on howmuch current is being supplied at a set point . . .

for exacmple i know that when the Redarc sees less than 4amps being drawn for a minute for so it drops down to float stage . .

The victron on the other has was delivering 400mA (monitored via the BMV712) @ 14.3V for ages . . . . so far I've not seen it drop to float stage.

more to follow.



the 12/12/30 has been running for a few weeks now, as you would expect from Victron its been issues and fault free and just works straight out the box.....

one thing i noticed was that it didnt drop the charging output to monitor the Aux battery voltage like the Redarc does . . . . . it must be doing some internal fancy monitoring to suss out whats what...

Its kept the Aux battery fully topped up as the Redarc did, the 30A charge current is solid and stable, its had no problems keeping up with any loads connected...

The bluetooth monitoring app is simple and basic and intergrates well with the other victron devices . . . . its just shame that there is NO VE-Direct connection ( i wanted the 12/12/30 to be able to get the temp info from the BMV712 to stop the dc-dc charge in lifepo4 mode - but i doesnt...... lets hope its just a firmware mod?)

so lets see some screen shots:

new firmware released . . .


BM2 . . .

Screenshot_20191029-071340_Battery Monitor.jpg

below shows the alternator charging the starter at 14.7v . . . . but the Aux is fully charged, so just 13.8v float.


below shows the steady power with no monitoring drop-outs

Screenshot_20191029-134258_Battery Monitor.jpg

The BMV showing 100% SOC


below shows the DC-DC reacting to the stop/start.

Screenshot_20191030-132349_Battery Monitor.jpg

and below is a 24hr BM2 voltage log for the Aux battery
Screenshot_20191120-054357_Battery Monitor.jpg
and temp temp data


Last edited:


Senior Member
VIP Member
T6 Guru
Dell, I noticed you also had the factory aux battery, I’m assuming VW supplies as bigger alternator as standard with the factory aux battery setup.

Which would be helpful, as I’m guessing you will be drawing 80 amps with those two Dc-dc chargers

looking forwards to seeing it set up:thumbsup:


VIP Member
T6 Legend
Dell, I noticed you also had the factory aux battery, I’m assuming VW supplies as bigger alternator as standard with the factory aux battery setup.

Which would be helpful, as I’m guessing you will be drawing 80 amps with those two Dc-dc chargers

looking forwards to seeing it set up:thumbsup:
Yes... as part of the leisure package VW give you a 180A alternator, cheapy EFB+ second battery, plus a splitcharge relay and fuse.... also a most handy "engine run" switched 12v from the BCM, which is more preferable than a straight ignition switch feed. ( also the standard cable set is fused at 80A with a 100A relay . . . . So it should be good for short 80A blips and sustained 50A ish or so . . . . But will have to test and monitor the feed cable temp at high loads.


Senior Member
VIP Member
T6 Guru
Yes... as part of the leisure package VW give you a 180A alternator, cheapy EFB+ second battery, plus a splitcharge relay and fuse.... also a most handy "engine run" switched 12v from the BCM, which is more preferable than a straight ignition switch feed. ( also the standard cable set is fused at 80A with a 100A relay . . . . So it should be good for short 80A blips and sustained 50A ish or so . . . . But will have to test and monitor the feed cable temp at high loads.
I really should have looked hooking up my ignition feed to there instead of piggy backing in to the main fuse board, It’s not a hard job to redo it, if I have to lift the rubber and seats for any other reason.

Although at the time, I end up running 16mm2 cable to my DC-DC from the main battery as I wasn’t convinced about the 10mm2 aux battery feed from the factory aux battery.

It’ll be interesting to see those cable temps, and thanks for the confirmation about the larger alternator, when I ordered my van, the dealer didn’t know anything about the options and what they entailed, but for the price at the time the factory aux option was an easy fix for an aux battery.


New Member
Thanks Dell, I was holding off on buying a DC-DC charger until I saw what Victron came out with with the Orion TR Smart, I guess I'll sit tight a little longer. I was expecting VE-Direct functionality.
Hi All, Great thread here!

As some of you know I am technical sales manager for the exclusive UK and Ireland distributor for Redarc.

I can confirm that your unit is the first in the UK the other pcs are sitting in our warehouse :), majority of people are sticking with the 40A model as its is adequate and more cost effective!

In terms of performance and reliability Redarc is the superior option and is now standard fit for a number of large vehicle convertors where they are used in utility, service and blue light applications.
In over 5 years we are still yet to see a BCDC failure.

The biggest issue with some of the other products on the market is that they de-rate massively in ambient temperatures over 25 degrees (which can be common in some places these are installed) this can reduce the charging output significantly and means you are not getting the full advertised output current.
Redarc has full output up to 60 degrees and will continue operating up to 80 degrees!

Feel free to give me a shout if you have any technical enquires on the BCDC etc.


Senior Member
VIP Member
T6 Guru
Good to hear about the operating temperature as we got back to the van in Slovenia having been parked in the sun for a few hours and the erberspacher thermostat was reading 52 degrees C. Admittedly the van is bamboo green and the thermostat is close to the metal.


VIP Member
T6 Legend
John never fitted a DC-DC charger. . .

He relied on the Sargent for charging a Lifepo4 !!!!

Nor did he have a Smart charger on his EHU.....

Let's see what happened......

. . .

. .

Now hes fitted a VSR!!!!!

Lets hope his camper isn't Eu6 with stop/start.


If it was me I would have fitted a Dc-dc for lithium, and a smart charger for EHU........ let's see how he gets on with the VSR.



Senior Member
VIP Member
T6 Legend
John never fitted a DC-DC charger. . .

He relied on the Sargent for charging a Lifepo4 !!!!

Nor did he have a Smart charger on his EHU.....

Let's see what happened......

. . .

. .

Now hes fitted a VSR!!!!!

Lets hope his camper isn't Eu6 with stop/start.


If it was me I would have fitted a Dc-dc for lithium, and a smart charger for EHU........ let's see how he gets on with the VSR.

I also watched this last week and did wonder how someone who prides himself on his tech could be so behind the curve on this stuff. I don't know if Ducatos of that age had stop/start and regen braking or not, the new ones certainly do


New Member
If it was me I would have fitted a Dc-dc for lithium, and a smart charger for EHU........ let's see how he gets on with the VSR.

What charger for EHU do you recommend? I was looking at a Noco Genius but I gather that they don't have any provision to prevent charging at freezing temperatures.
I have two Valence U27-12XP LiFePO4s connected in parallel.


VIP Member
T6 Legend