200ah beginner build. Better to wire up two ~100ah batteries or just build one bigger 200ah battery?

Hi I have a simple question.

For a ~200ah LiFePo4 battery build, is it better to build two 100ah batteries and put in series, or build one larger 200ah?

I am leaning to having two ~100ah batteries as I like the flexibility of having the potential ability for 100ah batteries for use in potential future solar projects (i.e. outbuilding lighting, camper trailer builds, miscellaneous projects, easy swapability with future battery tech, etc...). Other than the additional cost of the additional cost of 2 BMS and some extra wiring, tell me why having 2 seperate 200ah batteries would be a problematic or ill advised.

Background. This is my first solar system. (cargo van campervan conversion). Because van storage is tight, custom battery build gives some flexibility in battery /storage layout behind driver's seat of van. I know just buying 2 battleborn 100ah batteries would be the easiest way to go, but I want to save some $ and most of all learn more about electronics.

I've lightly scoured Will Prowse's excellent content on his website, YouTube channel and this forum, and wasn't able to see any opinions on this. Perhaps it's a stupid questions but being a beginner maybe there is something I am just totally unaware. I want to know before I buy one pack of 8 balanced cells or buy two separate packs of 4 balanced cells.

Two batteries put in parallel can become unbalanced. One pack of parallel cells avoids that problem. It is also cheaper, with just one BMS.

If you can, avoiding parallel batteries is a good idea.

Nathantrogers said:

For a ~200ah LiFePo4 battery build, is it better to build two 100ah batteries and put in series, or build one larger 200ah?

Click to expand...

Series or parallel? Will this be a 12v or 24v system?

Either way, I like one battery with one BMS for simplicity...but if you have a good reason to do two separate batteries, that's fine too.

12 or 24 V ? 100AH cells or 200AH cells ?
In Series configuration: 4 cells make a 12 volt pack, 8 cells make a 24V pack.
Series / Parallel config: 8 cells (4s2p) makes 12V 200AH or 400AH, 16 Cells (8s2p) makes 24V 200AH / 400AH respectively

Cells are available up to 1000AH but the price is a shocker too.... Best bang per buck ATM are 200AH @ $70 ea or 280AH $84 each cell PLUS delivery.
Cheap BMS add $75 +/- Better BMS $125 +/- Good BMS $400+/- Best BMS 500+++.... Daly & LLT = cheap, Chargery = better, TinyBMS = Good, Orion Jr etc = Best.

sajjen said:

Two batteries put in parallel can become unbalanced. One pack of parallel cells avoids that problem. It is also cheaper, with just one BMS.

If you can, avoiding parallel batteries is a good idea.

Click to expand...

How do two batteries in parallel become unbalanced? I thought that could not happen because they both experience the same voltage anyway and that balance is all about voltage differences between components. I agree with one BMS needed instead of two.

a battery can degrade and then its behavior is not following the standard curve than the other one it is attached too.
that is why usually parallel batteries must be fused separately.
if a battery goes bad, it can go either open (best case) so it simply cease to exist.
In that case the remaining cell will fail soon after because it takes all the drain that is splitted on 2 cells on the other parts of the battery.
so when a pair of cell will give each half of content, the cell alone will be empty.
the worst case is when the bad cell goes short. If alone, it will heat and drain fast and die.
if you are lucky, you will just get melted plastic.
but with a good cell in paralllel, the good cell will start to discharge into the bad one, increasing the heat.
a slow dying cell could not go totally short, but just a bit more than the other cell, so the power of the good cell with discharg into the bad cell , aslo killing it.
fusing parallel battery is just a clever way to turn a short circuit into an open circuit

I would suggest that you build a big single 200AH
pack. To make it simple with a bigger pac SOC will be pulled down less, meaning longer life.

Thanks @nosys70 for these explanations!

A link to a nice thread about this question that I just went through quickly: https://diysolarforum.com/threads/incrementally-adding-dc-batteries.186/

I understand better now why putting two batteries in parallel can have issues. Indeed paralleling two batteries (or two cells for that matter) entails the risk of them having different internal resistance. This seems to be where all problems come from. During the charge, all cells/batteries experience a voltage drop at their poles, which are different if internal resistance are different. This means that while during the charge the voltages experienced by the batteries are the same, when the charge is stopped the batteries go back to a resting voltage which can be different. Then current flows between the batteries to equalize their voltage. I guess this happens at rather at high C (just a wire between batteries).

Now, I don't know what the order of magnitude of this effect is (maybe anyway batteries usually have the similar enough internal resistance if they are the same chemistry, especially if all the cells are identical), and how it affects aging (LiFePO4 batteries can tolerate high C), there would be some back of the envelope computation to be done here. So maybe paralleling batteries/cells is fine as long as they have matched internal resistance? I'm just saying it might be a bit too cautious to just ban it entirely, especially since we don't ban cells in parallel and that the same principles apply.

Anyway to answer the OP I also side with the unique battery design, simpler, and you can always break it into smaller banks when you'll need.

I have 2, 8 cell batteries that I can't easily dissemble. I want my cake and I want to eat it too. What I would like is to have the batteries in parallel at the cell level and also be able to split them and cycle one down and up, while leaving the other one fully charged. To do that I need some balance cables that can carry a few amps, with plugs on the ends so that I can split the batteries apart or make them as one. Anyway, I am working toward that goal while working on other things as well. So it will be a while. My batteries are for emergency standby. I want to test them, but I also want them to recharge after test from solar. If it is cloudy for a few days, I don't want my standby power to be low because of bad timing. So if I can cycle one of the batteries per month, I will always have one ready in case I need it. And I can have one large well balanced battery when I am not cycle testing.

SolarRat said:

Series or parallel? Will this be a 12v or 24v system?

Either way, I like one battery with one BMS for simplicity...but if you have a good reason to do two separate batteries, that's fine too.

Click to expand...

It's will be a 12v system. And from the multiple comments it sounds like just building a single ~200ah battery is the best course of action.

Thank you all for the quick (and even easy to understand) reply!!

And thanks Steve_S for the tips.

This is going to be fun ?

if you have parallel cell, you can keep the battery working while removing a cell.
you just need to wire them in a way that it is easy to disconnect one pole of the battery.
even better, if you you build such connection , you can as well use it for a fuse.

Nathantrogers said:

Hi I have a simple question.

For a ~200ah LiFePo4 battery build, is it better to build two 100ah batteries and put in series, or build one larger 200ah?

I am leaning to having two ~100ah batteries as I like the flexibility of having the potential ability for 100ah batteries for use in potential future solar projects (i.e. outbuilding lighting, camper trailer builds, miscellaneous projects, easy swapability with future battery tech, etc...). Other than the additional cost of the additional cost of 2 BMS and some extra wiring, tell me why having 2 seperate 200ah batteries would be a problematic or ill advised.

Background. This is my first solar system. (cargo van campervan conversion). Because van storage is tight, custom battery build gives some flexibility in battery /storage layout behind driver's seat of van. I know just buying 2 battleborn 100ah batteries would be the easiest way to go, but I want to save some $ and most of all learn more about electronics.

I've lightly scoured Will Prowse's excellent content on his website, YouTube channel and this forum, and wasn't able to see any opinions on this. Perhaps it's a stupid questions but being a beginner maybe there is something I am just totally unaware. I want to know before I buy one pack of 8 balanced cells or buy two separate packs of 4 balanced cells.

Click to expand...

I'd ask Will Prowse. IMHO.