Charging a 100Ah LiFePO4 with a CC/CV Power Supply - Revelations

[OnTheRoadAgain]

Quite a few times I read about people overcharging their LiFePO4 battery packs and I just didn't understand.

Now that I have one, it makes perfect sense. But ONLY now that I'm trying to charge my own. Yep, I'm not the brightest of bulbs.

So, I want to charge my battery to 13.4v. So I set my CC/CV Power supply to 13.4v. The battery is at 13.2v

The charge rate is 2.8amps (goverened by laws of electronics) and will take 21.03 hours according to the BMS at this moment. The temptation is to reach over and crank up the voltage so that it charges at the full 10amps the power supply can offer......but that's the trap. People walk away, fall asleep or forget they have turned the voltage way up and when they have that OS moment later and remember, it's too late. The battery is all swollen and at 15volts

I'll bet you a shiny nickel more people do this than confess to it. They just don't forget to monitor it.

Now, you might ask.......why is the battery charging so slow? If it's at 13.2v and I'm trying to charge it to 13.4v?
The answer is because that difference in voltage is not great enough to "push" the voltage along faster.
You need more voltage to do that.
Current is like water.....voltage is like water pressure. If the water on both sides of the faucet is at the same pressure, no water will move when you open the valve.
Likewise, if voltage is the exact same on both sides of a circuit, no current will flow. The greater the difference, the faster the flow.

This is of course assuming (uh-oh..here comes Q-Dog) that there is no BMS or there is a BMS but it fails to protect the battery.
Personally, I never like to trust the BMS to be the only sentry on duty protecting my xpensive battery

So for those new to this....that's the reason people increase the voltage to dangerous (for the battery) levels on a power supply, to cut down on the charge time.
If I crank the voltage way up, the charging amps jump way up and the charge time drops to 5.7 hours.
As obvious as that seems now, it didn't really make sense until now.

Oh, and as the battery charges, it will draw less current so the voltage will change and it will charge even slower.

So a dilemma....how to charge your LiFePO4 faster....yet still safely?

I guess you can buy a dedicated charger, but be careful some of them rely on your BMS to stop the charging.
I tried to find one that can be programmed and set at 13.4v, but came up short. The only solution was over $300 (too much)

 

[snoobler]

The charge rate is 2.8amps (goverened by laws of electronics) and will take 21.03 hours according to the BMS at this moment. The temptation is to reach over and crank up the voltage so that it charges at the full 10amps the power supply can offer......but that's the trap. People walk away, fall asleep or forget they have turned the voltage way up and when they have that OS moment later and remember, it's too late. The battery is all swollen and at 15volts

Or they make the adjustment with the leads attached, and they go well over a safe level.

The risk is absurdly low with a functioning BMS, and Will routinely tests battery BMS via this exact method to confirm HVD works.

I'll bet you a shiny nickel more people do this than confess to it. They just don't forget to monitor it.

Now, you might ask.......why is the battery charging so slow? If it's at 13.2v and I'm trying to charge it to 13.4v?
The answer is because that difference in voltage is not great enough to "push" the voltage along faster.
You need more voltage to do that.
Current is like water.....voltage is like water pressure. If the water on both sides of the faucet is at the same pressure, no water will move when you open the valve.
Likewise, if voltage is the exact same on both sides of a circuit, no current will flow. The greater the difference, the faster the flow.

It's less the voltage delta and more the wiring resistance. 0.2V can flow gobs of current though low resistance wires.

0.2V / .0005Ω (4S resistance of big cell LFP) = 400A (what the battery can accept based on its internal resistance).

Heck even FLA can take, 0.2V/.004Ω = 50A

You're well under "safe". You could crank it to 14.6V and have no concerns about something going wrong, assuming the BMS works.

This is of course assuming (uh-oh..here comes Q-Dog) that there is no BMS or there is a BMS but it fails to protect the battery.

So for those new to this....that's the reason people increase the voltage to dangerous (for the battery) levels on a power supply, to cut down on the charge time.
If I crank the voltage way up, the charging amps jump way up and the charge time drops to 5.7 hours.
As obvious as that seems now, it didn't really make sense until now.

We learn best by doing!

Oh, and as the battery charges, it will draw less current so the voltage will change and it will charge even slower.

So a dilemma....how to charge your LiFePO4 faster....yet still safely?

I guess you can buy a dedicated charger, but be careful some of them rely on your BMS to stop the charging.
I tried to find one that can be programmed and set at 13.4v, but came up short. The only solution was over $300 (too much)

Fabricate quality leads with ring terminals (no alligator clips), Disconnect PS, set voltage to 14.6V while disconnected, and take what you get.

13.4 is relatively low voltage and is unlikely to fully charge the battery.

If this is a canned battery with a built in BMS, you should test that the HVD works. Most have HVD of 3.8V or lower (3.75±.05), so 15.2V should trip the high cell.

 

[OnTheRoadAgain]

If this is a canned battery with a built in BMS, you should test that the HVD works. Most have HVD of 3.8V or lower (3.75±.05), so 15.2V should trip the high cell.

Is that charger voltage or battery voltage ?

May I assume it's charger voltage since that would kill a LiFePO4 battery ? My PS won't go over 13.85 connected to the battery.

There go those darn ASSumptions again.

 

[snoobler]

Charger.

PS won't go over 13.85V because battery is pulling the full 10A.

"kill the battery" - not typically true.

LFP is tolerant to charging up to 4.2V; however, there is so little gain between 3.65 and 4.20, it is not warranted, and reducing the peak charge voltage to 3.65 is key to LFP's longevity. Does it place greater stress on the battery? Yes. If done routinely, will it shorten life? Yes. If done occasionally, will it shorten life? Not measurably.

You always set your PS voltage with the leads disconnected from the battery, otherwise, the PS reports the battery voltage.

 

[Kcchris816]

What I would do is set alarms with Siri when I started charging cause sure as shit I would get distracted. I ended up buy an Junsi icharger then using the power supply into junsi when I need for solar bank. And the rest of the time I use solar to junsi for all other cells, lights, tools etc