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Discussion Starter · #1 ·
Hay all, I have a less than ideal setup and would like peoples thoughts on the best way forward.

I managed to purchase 12 Lifepo4 Valence batteries second hand from the local scrap yard that had no idea what to do with them. Tested them all and they are all about 70-80% SOC.

Each battery is 12S, 38.4v nominal with 43.8v charging. Each battery has its own balancer.

I am running 3 batteries in series to give ~120v with 4 sets in parallel to increase the AH.

My question is the best way to charge this setup? My thoughts,

1) 12 separate 43.8v Lifepo4 chargers so each battery balances itself
2) A Single 131.4v Lifepo4 charger, risks strings falling out of balance
3) 4 131.4v Lifepo4 chargers, lowers the balancing issues but still there
4) Install balancers to balance each set of 3 batteries
5) remove all balancers from the batteries and install a new balancing system, with a combination of battery chargers above?

Or something else, I am not massively worried about these balancing perfectly, from the testing I have done these Lifepo4 batteries can take quite an overcharge without becoming damaged. They appear way more sensitive to under voltage.

So throwing it out there!

Dave
 

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IMO, the least complicated the better. I would try one charger that charges them all in series. (I mean all the parallel strings in series). Parallel strings will all stay at the same voltage. I will caution you though, when using parallel strings that are old and can't deliver the same current you will have current imbalance between the parallel modules and you can melt wire. Make sure you connecting wires can handle the current. For example, one module may sag in voltage at 50 amps and the other doesn't sag that much till 75 amps.... so one will be supplying most of the current.
 

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Discussion Starter · #3 ·
Solid advice cheers, currently I just have 3 batteries in series and am noticing what I feel is a lot of sag under load from those three. 120v down to 90v.

I was hoping that adding in the further 3 sets in parallel would help avoid such a large sag.

These are old batteries so I am pushing my luck.

I am in the process of purchasing an Elcon charger, slap it on and see what happens! As these batteries are not going to balance correctly between them I am going to at least make sure the batteries with the most similar characteristics live together in series.

Ill keep my eye on the wiring!
 

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Yes past usual EoL, need more attentive care.

It is completely false that undercharging is a problem. In fact 3.45Vpc will get you to as Full SoC as they should go, and help with keeping them in balance as well as longevity.

If that had been followed rather than 3.6V charge termination they may not have hit the scrap yard for another 5-10 years or more.

Going anywhere near that maximum rating stress point reduces longevity, and allowing any higher than that - on ANY one cell - risks a major unquenchable FIRE, so put in redundant failsafe layers to guard against that being possible.

That means 41V charging per battery for normal cycling. If you want maximum range, say another 5-7% then hold that Absorb / CV until amps accepted falls to 0.05C

But otherwise, just use a CC-only profile, HVC when the first cell hits say 3.50V or maybe 3.55

For rebalancing at the battery level, if you use the internal BMS for that, see if you can lower the voltage where balancing starts, so you don't need to increase Absorb / CV voltage, nor hold that harmfully for a longer time than you would normally.

If the BMS forces you to hold over 3.5V for hours just to finish balancing (stupid design but very common low cost protocol to implement) then consider balancing using different hardware. maybe even middle or bottom balancing as a rarely needed maintenance procedure.

Accept that the bank usable capacity is determined by the weakest cell, no way around that but removing the weak links replacing with new.

The question as to how "best" to accomplish these goals is complex, depends on the detailed behaviour of the packs, bith cells and current BMS, and also on your preferences.

Personally I would not bulk charge at high voltages unless any of the BMS could terminate charging as soon as any cell/1S group approaches 3.5V
 
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