Two Banks. Different ah and brand - possible?

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gb232 asked Feb 4, '22

Two Banks. Different ah and brand - possible?

I have 2 banks of 16 x CALB 180ah making up 24v 720ah. They have seperate BMS and isolation breakers. One bank is in poor health with between 0.3 and 0.77ohm impedance using a LiPo4 test meter, and certain cells don't accept charge or discharge without the BMS turning the whole bank off.

I'm considering changing out the bad cells by adding one bank of 16 x EVE 280ah cells - but keeping one bank with the better health CALB 180ah (less then 0.3ohm impedance).

The battery spec is the same the only difference being ah and impedance. CALB is greater impedance but a lot depends on how the impedance is measured and not sure if it's the same.

If I keep the same configuration - seperate banks with seperate BMS and breakers, can I operate them both online at the same time with charge and discharge from the same sources (Multiplus, Skylla and Alternator) allowing for not allowing the banks to get out of sync with different voltages causing one bank to charge the other, (the intention is to have both online together).

Anyone have any advice on this - it would be much appreciated.

battery

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snoobler answered · Feb 04, 2022 at 02:29 PM

Given that you've had problems with the CALB cells, it's probably worth your time to actually capacity test each cell. IR is a good measure of a cell's health, but it won't tell you if it's 180Ah or 120Ah.

I also trust that you plan to do an actual top balance of each battery prior to build. If you're not planning it, do it.

Paralleling two different capacity batteries is never recommended; however, with certain considerations, it can be done safely. Obviously, they should be wired in accordance with best practices to minimize voltage drop - making sure every cell terminal and all other connections are properly torqued. Each string should be fused.

Avoid paralleling LFP batteries if they vary by more than about 0.2V as there can be a BMS-tripping current surge.

Once in parallel and operating, subject battery bank to a maximum load. Use a clamp ammeter to confirm that the current flowing through each string is within specification.

Expect the two strings to wear at different rates depending on the C rates observed.

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gb232 commented · Feb 05, 2022 at 01:47 AM

Thanks Snoobler. The CALB were just great until they were allowed to drop below damage level due to multiple reasons!

I will check the capacity on the 180ah bank using the Cerbo graph - good idea. That will give me an idea if they are worth keeping.

I'm hoping to fit a new bank of 280ah, use both 280+180 banks together but they are switched separately so could be isolated if any unexpected issues come up. (avoiding voltage differences and cross charging). Hope to get more years out of the 180ah bank but eventually change them and have all the same 280ah batteries in both banks but with different ages.

From what I read it is possible to operate 2 seperate banks with 2 different ah capacities - but if anyone has any advice on what problems to watch out for that would be appreciated.

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snoobler gb232 commented · Feb 05, 2022 at 02:56 AM

Not the "pack". Since you are operating a battery with parallel cells, you really need to capacity test individual cells and match them up accordingly. The last thing you want is for the series elements to have different capacity. It's tedious, but it's worth it.

You could get the Eve bank online as your exclusive battery while you take the time to test the 180Ah cell and do a proper build.

As I mentioned above, difference capacity parallel bank elements are possible, but you have to confirm that the cells are operating within spec. The problems are primarily from the IR disparity between the two parallel batteries, and making sure that all of your connections are perfect.

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gb232 snoobler commented · Feb 05, 2022 at 03:29 PM

Hi again Snoobler. Thanks for the advice.

As you say - it's a 24v bank so 16 x 3.2v. cells are paired up to make 8 x 3.2v = 28v total.

From what you say I understand that I should take the mOhm readings of each 180ah cell (I have access to a LiPo4 test meter), and match the paired up/parallel cells.

I have the readings already. Of the 16 cells x 180ah the best is 0.16mohm and the worst is 0.19mohm so I'm happy that I can pair them very closely as you suggest.

Not sure how I can test the capacity of individual cells through - the Cerbo can give me the consumed amphours for the whole bank from fully charged at 28v, down to BMS shut-off at 20v. I imagine that would be a good indicator. If the ah capacity is reduced that will just bring forward the purchase of the next 16 x 280ah cells.

I will be able to get the voltage of each pair within 0.2v using the Smart 123 BMS for the initial set-up.

I think I'm on the right track - please jump in if I'm missing something.

Look forward to doing as you suggest then putting the 280ah and 180ah banks online and checking for the unexpected.

As the two existing banks of 180's are already compromised one being healthy and one in a poor state, but operating together - I can't imagine two healthy banks - even though different ah, would not get on together.

Am I missing something?

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Answer 1 · 2022-02-04T04:47:06Z

snoobler answered · Feb 04, 2022 at 04:47 AM

0.3Ω is horrific. I would hesitate to use any battery with this kind of resistance. That's about 100 times higher than quality lead acid. Big LFP cells should be down in the <0.5mΩ range.

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Answer 2 · 2022-02-04T13:40:09Z

gb232 answered · Feb 04, 2022 at 01:40 PM

Maybe I'm wrong and the reading is m-ohm

That's why I want to take out 16 cells above 0.3(milli) ohm. The very best 'new' cells I have come in at 0.16

Brand new Eve 280ah cell spec is

CAL-B 180ah cell spec is

But any advice on having a bank of 16 x 180ah and a bank of 16 x 280ah all at 24v - all matched as close as possible in pairs with similar resistance.

Will the ah rating cause an issue?

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