Charging 24S LiFePo4 battery with 20S lithium charger

[GijsW]

Hi everyone!

I'm building a 24S LiFePo4 pack. The chargers for these packs have a cutoff at 87.6V.
Could I also use a 20S regular lithium charger for my pack? It should work, right? As far as I know, they're both CCCV chargers.
The only thing is that the 20S charger has an end voltage of 84V, meaning I would charge to only 3.5V per cell, or 96% of max capacity.
This isn't an issue for me, and it would even reduce degradation. Am I missing something here? I'm pretty sure it would work, but burning my house down because I missed something would be less than ideal, so a confirmation would be much appreciated.

Oh, another thought just popped in: There's quite a lot of periods where I don't use my battery. Could I limit the SoC by putting a couple of diodes between the charger and battery? With a forward voltage drop of 0.6V, could I put 6 in series to reduce the cell charge voltage to 3.3V (which equates to about 70% SoC)? Limiting pack SoC when putting it away for large periods of time greatly improves pack life, but is this an accepted way of doing so? I can't really find chargers which have this option, except for one in a US store (with 110V rating and expensive shipping).

Thanks!

 

[999zip999]

85volt is fully charged for lfepo4 3.5 v is all lifepo4 holds the BMS takes 3.6v per cell to balance most common ones. Lifepo4 can handle a little over charge to bleed down by BMS for balancing. I have no BMS and charge to 85.1v or 3.5v to 3.48v as I always monitor so I know. 7yrs 1,480 cycles 24,900 miles. Must monitor the secret.
Plus what quality of cells ?

 

[john61ct]

Yes 84V for 24S LFP is excellent, better than going higher, much better for longevity.

At rest isolated 3.33-3.35Vpc is as Full as, any higher is just stressful for no reason.

Even CC-only just stop at 84V you'll be well over 90% from vendor rating max version of 100%. Depends on the C-rate of course.

If the charger does hold CV / Absorb a while, it should cut out before current trails below say 0.1C.

Just watch out for BMS "start balancing" voltage. Ideally should be adjustable, set to say 3.3Vpc.

 

[john61ct]

For storage voltage, yes you should **not** let them sit full for long. Just run a load, bring down to 3.2V or so resting.

A bit higher is OK.

If you leave the BMS hooked up BEWARE it pulling SoC down, start checking every couple weeks, then monthly etc.

Recharge if you hit 3.1Vpc back up to over 3.2V this time, but under 3.3Vpc.

Battery Murdering Systems if you aren't careful. Ideally can be easily disconnected or at least fully powered off.

 

[GijsW]

Thanks! I learned a lot!
I didn't think about the bms balancing voltage.

My bms isn't programmable, and the specifications list:
"Balance detect voltage=3.5V"
"Balance release voltage=3.5V"

So maybe using a 84 V charger isn't a great idea.
Or I could buy a cheap tiny 87.6V charger (1A or so), and use that every 10-20 cycles to balance.

And I'll keep an eye on bms consumption, thanks for the tip.

 

[john61ct]

I would rather see you replace the BMS, adjustability is important IMO.

Or just use another device for your balancing needs

staying below 3.5Vpc in effect disables that function on such a BMS.