Enlarging Battery Bank - Max Current Questions

Hi Everyone - thanks for the time you spend here helping people. It's been such a resource for me, like so many.

To the matter at hand - I have a 5th wheel RV, with (2) MPP LV2424s (hybrid green) and 2 Li Time 200ah LiFePO4 batteries in series.

The battery pack is undersized for our needs, but worse still, these two batteries only have 100a max current draw - newbie mistakes I made last year, when I should have put my project plan to the experts here, but I was time crunched.

I need help understanding the best choice for expanding the bank, likely by adding another series-wired pack in parallel. My first thought is to add on two larger (200-300ah ea) batteries in the new pack, but also this time getting the 200a BMS. In this case, how will current draw end up divided between the two (dissimilar in regards to max current) packs? Is it worth it to sell my existing batteries and start over to avoid all this? I guess finally I could split the bank between the two inverters, but that seems like another whole can of worms.

Anyway, thanks again for any advice - it will be appreciated.

I would look at 24V batteries rather than series connect two 12V ones. With a series connection, you could have one at 11V and one at 13V.

Adding more in parallel will reduce the load on the batteries because the current will split. I'm 12V in my trailer and have 3 in parallel. When my inverter is pulling 100A to run the microwave, each of my batteries provide about 33A. With 100A BMSs the bank could provide 300A load but my needs (2000W inverter) never tax that capacity.

Thanks for that, Joe. What you say makes sense - I should add to my original post that we're not spending weeks boondocking, our use case is more about being able to pop a bag of popcorn or run the a/c for an hour to cool off the trailer when we're on the road for a night and doing Harvest Hosts or something like that. Certainly capacity is a concern, but max current is at least as important.

To prod you further - in my shoes would you ditch the two 12v batteries, then? My big worry here is having different max current ratings and how that interaction will affect the operation of the bank as a whole.

I would keep them. Different max current ratings is only an issue when you approach the max of the lowest rated one, 100A in this case. Keeping them holds another 100Ah of capacity.

I have a shunt in addition to the bluetooth BMSs on my 3 batteries so I'm able to mine all sort of data from them. Bottom line is that they pretty much take care of themselves and I try not to outthink them.

Yeah, but that's the thing - I'm up against that max current rating all the time as is.

mattb291 said:

Yeah, but that's the thing - I'm up against that max current rating all the time as is.

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You have 2x 200Ah batteries, 100A max current each right now. In series for 24v output, i assume since you didnt specify your battery voltages but you did say series.

So You have 100A max output from batteries, thats 2400W max. Feeding your 2x 2424 Inverters, 24v and 2400W, which can take 4800W but you can only supply power for one of them really.

If you double your batteries, in parallel, you'd be able to power both of them. Current would be 100A to each set of batteries still at full inverter pull, not great to be maxing stuff out.

How much do you REALLY pull? Problem is putting dissimilar batteries in parallel, but at least it's better than series... So long as you're not pulling max Amps, all the parallel sets will just split the load fine.

1. I would keep batteries, and buy a 24V 200Ah battery with 200A discharge ability to put in parallel. Or just 2 more of the same as youve got, and don't max the inverters out.

mattb291 said:

Yeah, but that's the thing - I'm up against that max current rating all the time as is.

Click to expand...

If you parallel another battery you will halve the current. Your 100A load will split and each battery will only push 50A. You can run the original up to 100A, so with another battery the total would be 200A, or 300A if you add the third. The simple math would be take the lowest battery max current, time the number of batteries in parallel.

To add detail - The LV2424s are 24v, yes - and the batteries are 12v in series. I definitely will not put dissimilar batteries in series - if I add on, it would be batteries in parallel to the existing 2x12v, 200A set. But I was considering new batteries that had a 200A max current - that's the root question. Let's imagine a hypothetical - I pull 1800w on one inverter, and 1300 on the other, so 3100w. Let's also imagine I have the existing 2 batteries (2x12v,200A,100A max in series) and then I added 24v, 300A, 200A max in parallel. Is it correct that the load would be split equally between the two 'packs' in parallel, and run that way until the 200A pair gave out, and then sort of gracefully 'fail' to using only the new battery, at (approx, I get that) 100ah left? If we assume the BMS won't let me do something truly stupid like overdischarge, then is this an ok setup for not trashing my batteries? I'm thinking that at 4800 total possible load, this would work. . .

I mean the above would be an extreme scenario, capacity is actually usually not our problem nearly as much as max current - we use the @#$% out of the system for a hour or two one evening, then get up and drive the next day.

Thanks again for helping me think through this - I hope I'm not a pest. Sizing is so simple on paper with unlimited funds. . .

Just add another 2 battery series in parallel to your existing if you're comfortable with it and want the flexibility of 12v down the road. Being able to convert or reuse 12v batteries in a mobile application is a strong plus.

The balancing issue is frequently overstated and can be easily accommodated either by occasionally topping up manually or using an ha01 balancer or similar.


All these lifepo4 batteries are series internally anyway.they need balancing maintenance too.

If you were talking about a house system that was getting hammered with loads and charging constantly then maybe there would be concern.

I would just swap them out now for a pair of battle born game changes.

Start by wiring them in parallel and fully charge them to be pairs.

Discharge some, charge again.

Now wire them in series. They don't have that problem with hardly any power.

Thanks everyone - I definitely feel more confident now.

4800w at 24v is 200 amps. I would get enough 24v batteries so that the sum of the discharge capacity is 400 amps minimum.
Put the LiTimes in parallel with the main batteries if you like or sell them or use them as an auxiliary 12v source for the other accessories.