Mixing cell capacity

RRRRRRRRRRudy

New Member
I did a bad thing !

I started to top balance my cells for a second battery BUT I adjust my power supply to 4.60 instead of 3.60.

Now the first cell is not reusable (I think) so I ordered a new cell from my Alibaba seller BUT she is out of 150ah.

My question, can I put a different cell capacity to complete my battery ? I would by a 173ah and put it in series with the other 3 cells of 150ah.
 

rickst29

Solar Addict
This is largely SWAG, because I am not an expert of BMS microcode or circuitry. But I feel that your result will depend on the quality of "management" within your BMS. In general, I feel that to be a bad idea, kind of like building a battery with "3 bad cells" on purpose. In particular, "top balancing" might not work correctly - balancing might begin with the lower-capacity cells reaching the top-balance threshold (e.g. 3.4 Volts on the lowest cell), reducing current to all cells significantly long before the big cell has actually approached that threshold. Then, with only low charging current being applied, it could take a very long time to bring that cell up to full voltage. It might never reach a high state of charge.

But, AFAIK, LFP batteries are not affected by any kind of "memory effect", and I don't think that their lifespan is significantly reduced by failure to reach a high State-Of-Charge in long term use. It could be completely harmless, or it could cause a loss of "capacity" which you will never use anyway.

With my post providing only guesses and a bit of background, let's see if a more expert person can advise both of us. :)
 

Lt.Dan

Photon Sorcerer
How high did the voltage actually get? Have you tested the battery to see if it is still good?
 

RRRRRRRRRRudy

New Member
How high did the voltage actually get? Have you tested the battery to see if it is still good?
The cell is at 3.29 (or something like that).
Is it still good ? this is really THE question.
My seller just wrote me 'not to use it'. So is it because there are risks ? or just because it is no good.
I will wait to see what experts here say about it.
 

Lt.Dan

Photon Sorcerer
The cell might be at 3.29 now, but how high DID it get? Did it reach the full 4.6v? Is the cell "Bulging"?

Can you do a capacity test on this cell? You can get one of the cheap 180w testers on Amazon for $50~.
 

RRRRRRRRRRudy

New Member
The cell might be at 3.29 now, but how high DID it get? Did it reach the full 4.6v? Is the cell "Bulging"?

Can you do a capacity test on this cell? You can get one of the cheap 180w testers on Amazon for $50~.
I will do a capacity test today.

And it did get to at least 4.60

And yes the cell is quite big.

gonflée.jpg
 

Lt.Dan

Photon Sorcerer
With it bulging that much, its questionable if it will suffer from any thermal runaway or not. Chances are it won't, but you won't know until if/when it does!
 
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