A short question on LiIon charging

[Fallingwater]

If a charger charges a LiIon cell with a voltage that goes up to 4.3V or so, but two or three minutes after the cell is removed from the charger its voltage goes back down to 4.2V... is it being significantly harmed (overcharged) or not? (I assume some damage is being done in any case)

 

[coppertrail]

You're pushing it once you hit 4.3, anything over that can be dangerous. I believe you're significantly reducing the number of cycles out of the cells if you charge to 4.3, not sure about the issue of their returning to 4.2 after charge.

Do you mean right off the charger they're 4.3, and after sitting for a couple hours or overnight they drop to 4.2?

Also, what charger are you using and I'm assuming these are unprotected cells?

 

[Fallingwater]

I'm trying to figure out how exactly my Photon Rex is misbehaving. Charging voltage goes up to even 4.34V! The cell does recover once it stops charging, but if it's left to terminate when it sees fit the final resting voltage can be as high as 4.25.

I connected a protection board from a 18650 cell to the circuit, but it doesn't seem to be stopping the charge process once 4.2V is reached, and I don't quite understand why. Thought maybe it was figuring the charge current was low enough that it wasn't damaging the cell much; on the other hand, I doubt the board has the smarts for this sort of action.

Edit: just tested the board, and it accepted a 4.5V input at the battery terminals without issue. Crappy thing doesn't have overcharge protection at all.

 

[SilverFox]

Hello Fallingwater,

The RC people have discovered that some of the chargers they use bump the voltage above 4.2 volts. They have also discovered that their Li-Po battery packs don't last as long when they use those chargers.

The general agreement is that any charger that exceeds 4.2 volts at anytime during the charge process is detrimental to the batteries. If the charge is limited to 4.2 volts it takes a little longer to complete the charge, but that is a small price to pay for longevity.

Tom

 

[rantanplan]

[...]
The general agreement is that any charger that exceeds 4.2 volts at anytime during the charge process is detrimental to the batteries. [...]

And that´s why the Ultrafire WF139 is such a piece of cr*p ... it bumps up to 4.4 (and more) volts because it doesn´t use a proper CC/CV-charging, but only a CC-technique ...

But it fits ... the manufacturer of this charger sells tons of batteries too

 

[Kremer]

I was just going to ask the same question, I have a insten brand generic charger for the kodak camera li-ion battery. last night I hooked clamps to the terminals and got 4.5x V with the charger plugged in and nothing connected, then I hooked a fully charged 18650 (resting voltage 4.16V, sony, unprotected, from a laptop) to it. The terminal voltage dropped to 4.20 with the battery then started rising, after 5-10 minutes it was to 4.23, where I pulled it off. instantly after removing the charger the battery went to 4.18V I guess the original and my question is which is the critical voltage? or are we just way overthinking this?

 

[Fallingwater]

I guess the Rex doesn't use proper CC/CV charging either. It still overcharges though; if left to its own devices, when it ends the process and the cell is left to rest, its voltage ends up being 4.25V or thereabouts.

Still have to determine if it's just my Rex being defective, of if it's a behaviour shared by all of them. It might be that the cell gets damaged, but not enough for most people to notice.