Strange issue with DC-DC charger [Resolved]

[Matt_camps]

Any help would be greatly appreciated with the below

Am just wondering if anyone has come across this issue before with a DC-DC charger. I first thought it was a faulty unit (CTEK 250SE) so changed to the RENOGY DCC50S but am still having the same issue.

When the wire from the alternator is not attached to the DC-DC charger it reads as it should (14V~ while running and 12~ while off) but as soon as I then connect this wire to the alternator input on either the CTEK or RENOGY unit the voltage drops to <10 even though the voltage at the starter battery itself stays 12V or 14V. This means that it never swaps over to charge the leisure battery.

There is a good ground connection from the starter battery to the DC-DC charger and everything else works as it should when on EHU.

Has anyone come across this before by any chance?

 

[roadtripper]

Is the wire really from the alternator or from an ignition live source?

Pictures of your install will help get context.

 

[t0mb0]

Any help would be greatly appreciated with the below

Am just wondering if anyone has come across this issue before with a DC-DC charger. I first thought it was a faulty unit (CTEK 250SE) so changed to the RENOGY DCC50S but am still having the same issue.

When the wire from the alternator is not attached to the DC-DC charger it reads as it should (14V~ while running and 12~ while off) but as soon as I then connect this wire to the alternator input on either the CTEK or RENOGY unit the voltage drops to <10 even though the voltage at the starter battery itself stays 12V or 14V. This means that it never swaps over to charge the leisure battery.

There is a good ground connection from the starter battery to the DC-DC charger and everything else works as it should when on EHU.

Has anyone come across this before by any chance?

Sounds like you have a bad connection on either the positive from the alternator or the ground - or some other cause of high resistance. The resistance, and subsequent voltage drop during load is why it measures ok when it’s not connected and drops off when connected.

 

[Matt_camps]

Wire comes from positive terminal of the battery as far as I’m aware, fused at 30A in the engine bay.
Pictures show voltage measurement while running at both points.

Is it under load as soon as is connected to the DC-DC charger unit? It isn’t actually being called upon to charge leisure battery at this point. Also isn’t a new install though this could point at a connection coming loose over time?

Sounds like you have a bad connection on either the positive from the alternator or the ground - or some other cause of high resistance. The resistance, and subsequent voltage drop during load is why it measures ok when it’s not connected and drops off when connected.

Is the wire really from the alternator or from an ignition live source?

Pictures of your install will help get context.

 

[t0mb0]

I took from the original post that the voltage dropped when the DC-DC was running but I may have misunderstood. Is the voltage low at the DC-DC end whether or not it’s running? And are you sure you’re measuring the DC-DC input there, rather than the output? If so, still looks like a dodgy connection on either positive or ground to me.

 

[Matt_camps]

I took from the original post that the voltage dropped when the DC-DC was running but I may have misunderstood. Is the voltage low at the DC-DC end whether or not it’s running? And are you sure you’re measuring the DC-DC input there, rather than the output? If so, still looks like a dodgy connection on either positive or ground to me.

Apologies, voltage drops as soon as it is connected to the alternator terminal even when not under load. Yes definitely measuring from the DC-DC input, output is connected to the leisure battery so is at 12V~ (same issue even with leisure battery disconnected, just as soon as cable connects to DC-DC alternator terminal the voltage drops)

The ground I have measured back to the battery negative terminal and is showing as 0 ohms, haven’t done the same with the positive yet but will try this as the next step.

 

[CJW]

Think I’m right in saying that the reading across neg to alt on the charger should be exactly the same within a couple of % (to allow for drop along the pos cable) as the voltage across the starter battery terminals *at all times* regardless of whether the IGN signal is high.

 

[Dellmassive]

Volt drop over the cable run.

The cable is to thin for the length of run.

Replace the main feed cable with a big fat beefy cable.

 

[t0mb0]

Apologies, voltage drops as soon as it is connected to the alternator terminal even when not under load. Yes definitely measuring from the DC-DC input, output is connected to the leisure battery so is at 12V~ (same issue even with leisure battery disconnected, just as soon as cable connects to DC-DC alternator terminal the voltage drops)

The ground I have measured back to the battery negative terminal and is showing as 0 ohms, haven’t done the same with the positive yet but will try this as the next step.

So it measures decent high voltage before touching the DC-DC terminals but then immediately drops as soon as it makes contact? If so, and the DC-DC isn’t even on, that is quite weird.

 

[t0mb0]

Volt drop over the cable run.

The cable is to thin for the length of run.

Replace the main feed cable with a big fat beefy cable.

Except, if we’ve understood correctly, it seems to be the case whether or not the DC-DC is running.

@Matt_camps - are you absolutely sure this is the case? Can you disconnect the DC-DC output to be absolutely sure?

 

[t0mb0]

Think I’m right in saying that the reading across neg to alt on the charger should be exactly the same within a couple of % (to allow for drop along the pos cable) as the voltage across the starter battery terminals *at all times* regardless of whether the IGN signal is high.

Volt drop over a cable is a function of current flow (V = IR), as well as resistance, and hence, if resistance is too high then voltage would naturally drop when the DC-DC is on and high current is flowing.

 

[Matt_camps]

Think I’m right in saying that the reading across neg to alt on the charger should be exactly the same within a couple of % (to allow for drop along the pos cable) as the voltage across the starter battery terminals *at all times* regardless of whether the IGN signal is high.

Volt drop over the cable run.

The cable is to thin for the length of run.

Replace the main feed cable with a big fat beefy cable.

This is an existing set up that was installed by Snugcampers and has been working fine over the last 2 years as far as I know so can’t see this being the issue or it would have been there since install?

 

[Matt_camps]

Volt drop over a cable is a function of current flow (V = IR), as well as resistance, and hence, if resistance is too high then voltage would naturally drop when the DC-DC is on and high current is flowing.

The DC-DC is not on. Just connected to the alternator terminal on the DC-DC charger

 

[Matt_camps]

Except, if we’ve understood correctly, it seems to be the case whether or not the DC-DC is running.

@Matt_camps - are you absolutely sure this is the case? Can you disconnect the DC-DC output to be absolutely sure?

yes, have tried this with the output disconnected and still the same issue

 

[t0mb0]

yes, have tried this with the output disconnected and still the same issue

And are you taking the reading from exactly the same point on the cable, before and after touching the terminal?

I’m just wondering whether you’re taking a reading from a coated bolt head on at the terminal or something that’s giving some extra resistance.

 

[CJW]

Volt drop over a cable is a function of current flow (V = IR), as well as resistance, and hence, if resistance is too high then voltage would naturally drop when the DC-DC is on and high current is flowing.

If the cable is sized correctly, then drop shouldn’t be more than 3-4% and the measurement at the charger should always track with starter battery voltage. What I’m saying is if OP is seeing different, it points to a fundamental problem eg cable cross section, bad earth.

I think we’ve just found a slightly odd way of agreeing with each other.

 

[CJW]

Is the 30A fuse slow-blow? Wonder if it could have partly blown.

 

[Matt_camps]

And are you taking the reading from exactly the same point on the cable, before and after touching the terminal?

I’m just wondering whether you’re taking a reading from a coated bolt head on at the terminal or something that’s giving some extra resistance.

Yes taking the reading from the same point (can also see the voltage on the Renogy app, same voltage as indicated on the app as on the meter when connected)

 

[Matt_camps]

Is the 30A fuse slow-blow? Wonder if it could have partly blown.

It is a 30A midi fuse, not sure if it is a slow blow

 

[Matt_camps]

If the cable is sized correctly, then drop shouldn’t be more than 3-4% and the measurement at the charger should always track with starter battery voltage. What I’m saying is if OP is seeing different, it points to a fundamental problem eg cable cross section, bad earth.

I think we’ve just found a slightly odd way of agreeing with each other.

I’m thinking the same but wondering how this could have happened as it has been working fine for the previous 5 months of me owning the van and 2 years or so for the previous owner. Scratching my head haha