Inverter size advice

[Turboponzy]

Could someone mind confirming what I’m about to buy will work.
I’m ok fitting everything but when I comes to amps, watts etc it just boggles my mind.
I have an AGM lp 100amp leisure battery already fitted running the 12v in the van.
I want to fit an inverter to run 2 x 4amp MTB chargers, they will need to be used sometimes while parked up, most often the van will be driving whilst charging.
I am going to buy a Victron 12/12 30 non isolated charger kit.
Renogy 1000w invertor.
Will these 2 do the job and how long could the battery last without driving the van?
Thanks.

 

[Argent]

Hi @Turboponzy,

There is some info missing from your post, but I will try and fill the blanks as best possible to give you an answer:

Assuming the MTB chargers are 4A DC @ 42V:

• Power per charger = 42V × 4A = 168W
• 2 chargers = 336W total
• Add 10–15% for inefficiency = ~380–400W AC draw from inverter

On the invertor side @ Renogy claimed 90% efficiency: 360W / 0.9 = ~400W DC
So at 12V, 400W = ~33 amps.

Your AGM is best preserved by staying within 50% of its discharge capacity but lets allow for 65% discharge. 65 / 33 = ~2 hours.

Again with the limited information and variables such as efficiency this could be really wrong but hopefully gives you a ballpark figure.

I would recommend investing in a Lithium batter which for the same nominal Ah rating would nearly double your capacity whilst halving the weight.

 

[Turboponzy]

Hi Argent, thank you for this info. Much appreciated

 

[T6 Syd]

Hi Argent, thank you for this info. Much appreciated

Part of the missing info is the capacity of your MTB batteries. Without recharging the leisure battery you have a fixed pool of Wh available to transfer to your MTB batteries.

The 100Ah battery contains about 1200Wh, of which 65% is about 750 - 800Wh. It is not just about how long the battery could run the chargers, but also how much juice the MTB batteries need. In turn that is not just their capacity in Wh, but how far they have been drained. If the chargers are what you already use at home you may already know how long it takes to charge the batteries under your normal usage pattern.

However, the Li-ion chargers are likely to have a charge profile, which won't be constant Amps throughout the charge period. Your situation isn't limited by the max Amp output of the LB or the inverter, so the comparison of the capacity required (Wh) by the MTB batteries and that available in the source battery is perhaps most important - but allow for some losses in the transfer process.

 

[Turboponzy]

That’s food for thought. Looking at it like that the numbers do start to pile up, easy days we wouldn’t use more than 50% but a good day in the mountains my wife can rinse her 750w battery and I can be around 70-80% down on a 700w. That would mean I’d need a second LB really. I would also have to think about how long the LB would take to charge when driving. We travel around Wales with the bikes so driving to another area would put some energy back into the LB. Cheers

 

[Argent]

Like I mentioned previously, a 100AH Lithium battery almost equates to a 200Ah AGM battery. Also, check what spec alternator you have and/or what size SCR or B2B you have.