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Charging voltage for LifePO4 4S pack
#1
Wondering if someone can answer this question or point me to an article.

I have a 4s/5p pack built from K2 26650 LifePO4 batteries.  It has a simple Chinese BMS.

I have a Li-Ion charger (16.8V/3A) that I've used to charge a 4s/8p pack built with 18650's.  Can I use this charger to charge the 4s/5p LifePO4 pack?

Thanks!
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#2
No.

LiFePo4 max is around 3.65v per cell (depends a bit on manufacturer)

Unless you can change the end voltage that Charger goes to high.
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#3
(10-05-2020, 03:29 PM)daromer Wrote: No.

LiFePo4 max is around 3.65v per cell (depends a bit on manufacturer)

Unless you can change the end voltage that Charger goes to high.

Thanks.  I'll get a charger with a 14.4V to 14.8V max voltage.
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#4
If you want the cells to last I'd only charge to about 3.45V a cell.
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#5
get a buck converter and that will lower your charger voltage down to 14.6 volts which is the max for a 4s lifepo4. You can find a buck converter for less then 10 dollars. It's what I use to charge my 4s lifepo4 and 3s li-ion, you can program the max volts/amps to charge at. With a buck converter you can even use a laptop charger and charge at even higher amps.

buck converter 280 watt


300 watt buck converter with 4 dollar volt/amp meter I built. Having a volt/amp meter lets you know when the battery is almost fully charged.
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#6
Problem with using a buck converter to charge with is that you can't properly do CC/CV charging on lithium. Which means you aren't getting as much power stored as is capable of the batteries. If the buck converter can be controlled with a modification and an arduino, then this would be more accurate to what is needed.

Bucks are fine, but not recommended for long term use. Fine for initial testing though.
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#7
Why wouldn't it be fully Charge? A converter like that is exactly like most lithium chargers are out There except No time limit. Thats how an mppt work...
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#8
If it's able to do constant current output (vs overload current limiting) up to the set voltage it should be OK.

Like I said above, charging LiFePo4 to 3.65V doesn't help & they reach "full charge" if the CV voltage is set to 3.45V (13.8V for a 4 cell pack).
Have a look at a typical LiFePo4 voltage curve, stay on the linear parts, the "J" curves at either end aren't helping cell life.
Holding cells at 3.65V is definitely bad for them & very likely to cause swelling & early failure.
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#9
3.45 is not full on all LiFe though. Some say 3.5 or even 3.55 before they can go full and 3.65 is their max. (But this is just extra stuff and not needed in this thread)

We all know longeivity is to be in the middle no matter what type of lithium cells we talk about.
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YouTube / Forum system setup / My webpage  Diy Tech & Repairs

Current: 10kW Mpp Hybrid | 4kW PIP4048 | 2x PCM60x | 100kWh LiFePo4 | 20kWh 14s 18650 |  66*260W Poly | ABB S3 and S5 Trip breakers
Upcoming: 14S 18650~30kWh
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#10
(10-07-2020, 12:09 PM)daromer Wrote: 3.45 is not full on all LiFe though. Some say 3.5 or even 3.55 before they can go full and 3.65 is their max. (But this is just extra stuff and not needed in this thread)

True. The trick is to find the point where your cells start to kick up & stop there. The charge current takes a bit longer to drop away here but worth it IMHO.
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