Why should one not use 18 x 3.2v 280ah cells on a 48v bank?

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Jaco Reinecke asked Dec 11, '21

Why should one not use 18 x 3.2v 280ah cells on a 48v bank?

It is common knowledge that Lifepo4 cells follow a rather flat voltage curve. If one keeps within this curve staying well clear of the top (3.65v) / bottom (2.8v), it could mean the cells last much longer than their 6000 "advertised" cycles.

So my idea is to run 18 x 3.2v 280ah cells, charged at max 3.45v (18 x 3.45v = 62.1v), as Victron can handle max 66v with the BMS also well with its max volts limits, being able to handle 20 cells.

Settings are:
BMS sees 3.45v as 100% SOC.
Min SOC of 20% / 3.2v.

My question is: Why would one not do this, why is this a bad idea?

Ps. I have tried this already with 17cells at 3.45v. It worked very well.
Pss. Whilst waiting for the new 280ah cells to arrive, I also tested using 12 x 100ah 2nd Life cells with 5 x 280ah A Grade cells as a 100ah bank. It works.

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Answer 1 · 2021-12-11T12:17:06Z

Paul B answered · Dec 11, 2021 at 12:17 PM

You can do whatever you like as long as you stay withing the victron specs - but its just non standard and thus not recommended.

In regards to mixing cell A/H sizes that would not be a good idea. I will work short term but long term the bigger cells will not be charged up to there correct cell voltages

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Jaco Reinecke commented · Dec 11, 2021 at 12:50 PM

Thank you @Paul B

I concur the long-term use of mixed cells is not conducive to longevity. As a temporary measure/emergency, it is possible PROVIDED the cells are of similar quality/resistance, after having top balanced the bank first.

Because of importing cells and the issues with warranty replacements seeing that one can run a min of 15 cells for a 48v system, more commonly 16, buy 2 more as "warranty" cells and use them. If a cell goes, remove it, till you get to min 15 cells ... knowing by then that you have bought dud cells. :-)

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Answer 2 · 2021-12-11T12:49:42Z

kevgermany answered · Dec 11, 2021 at 12:49 PM

Balancing might be tricky. But a high end built in BMS would handle it. Don't get caught by all the fake cells out there

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Jaco Reinecke commented · Dec 11, 2021 at 01:36 PM

O, I've "crashed and burned" with cells and incorrect BMS. EVE cells from Amy in China, all A Grade Batch Matched which solves the balancing to a very large extent, if not all the way.

280ah cells because one needs fewer busbars than 3 x 100ah banks.

JBD BMS with an RS485 port that interfaces it with the Venus/Cerbo, leaving the Bluetooth functionality open to keep an eye on each cell via the app.

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