Testing voltage on li-ion cells

When is it the best time to test the voltage on a li-ion cell to make sure it has not been overcharged?

With my charger, if I left it to its own devices and charged the cells and wait for the light to turn green, the voltage gets to around 4.30v after removing it straight after from the charger and checking it with a DMM. That is why I have instead, resorted to time the charges.

When I time the charges, I make sure that the voltage is 4.10v straight off the charger. However, if I were to leave the cells alone for a day and check them again with a DMM, the voltage goes up to around 4.24v and some other cells get up to as high as 4.30v.

In future, should I instead aim for 4.00v straight off the charger as I could possibly expect it to go up another 0.20v?

no they shouldnt be floating up after a charge ???
A proper charge alogrythm is supposed to (which doesnt mean they all have to, or do) max at 4.20V (+- .05) on the final part of the charge, that means a proper charge alogrythm could NEVER have a battery reach 4.30 at all no way shape or form.

now there is stuff that instead of having a max voltage of the 4.2V they have a peak voltage of 4.2 where it terminates, they still shouldnt Ramp UP in voltage after removed from the charger. but that can have in it more swing up to the termination point, wheras the charger that is parked at a max 4.2v for its final point will slowly ramp up to that chargers max voltage.

something isnt making enough sence. what Rate is this charger running at anyway? and what current flow is happening at the end of charge?

Max = the maximum voltage the charger applies at end of charge
Peak = having a higher voltage for faster charge, and stopping when the microcontroller sees that peak voltage point.

if I were to leave the cells alone for a day and check them again with a DMM, the voltage goes up to around 4.24v and some other cells get up to as high as 4.30v.
if you mean when you leave it ON the charger , the charger Overcharges the battery, then you should throw the charger out, as simple as that, a charger that goes out of spec (assuming your meter is reading correctally), should just not be used at all. It is a recipe for disaster, it gets left on the charger or your timer doen't click off, or somone else uses it, your going to ruin more batteries in the chargers lifetime than the charge is even worth, and in the process, putting yourself and anyone else in the home in a very slight risk area.

$8 bucks enough people AT that site said it was WRONG, why did you buy it <---- that is me being mean ,just because of junky poorly made stuff, not being mean to you nessisarily.

if you must use the thing, then stop it around 4.10 - 4.15 , that is a good place for medium longevity, how your going to get that to work out everytime i donno, for $20 you could probably have a Life and a charger that works that you dont have to be AS paranoid about.

VidPro said:

something isnt making enough sence. what Rate is this charger running at anyway? and what current flow is happening at the end of charge?

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IIRC, on an open circuit (no battery in the charger, but is left on), the DMM reads out 4.20v. As for current flow, I have no idea how to measure that. If it is of any help, according to the back of the charger the output is 4.2v at 500ma.

More than likely it's probably to do with my $10 DMM I got from Bunnings (it's a massive DIY warehouse store here).

I'll test the cells out on my uncle's DMM (who happen to be an electrician).

Rod911 said:

IIRC, on an open circuit (no battery in the charger, but is left on), the DMM reads out 4.20v.

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then i wonder again how a battery could self charge over that rate i am sure there is some way for that to happen:shrug: like temperature changes or something that wouldnt exist in a "normal" charging.

i swear i have read before about someone saying the same thing, but it was probably about a DX charger that was also overcharing slightly.

you still need to define if the battery rises to this high point, While ON the charger , or when taken off, just because i am still confused about that, and intrigued. you know it would be really cool and different information to see a battery self rise like that, you may have even discovered a perpetual energy source yet unknown to mankind

The temperature here is a cool 17C (or thereabouts). As for the battery temps themselves, they never even get warm to the touch whilst charging or remotely warm when I pop them off the charger. All the voltage measurements on the cells are done once they are out of the charger and not when they are charging.

As for why I chose this charger over the usual WF-139? I did read the reviews for both chargers on DX and here. They each have a lot of positive and negative reviews, with the odd post stating that it had the same guts as the WF-139. I pretty much came to the conclusion of the typical 'DX pot-luck draw', hoping to get a not so flaky charger.

Once I do have access to a decent DMM, I'll compare the results of that with my own DMM to determine which piece of equipment is at fault here.

I really do think it is my cheap DMM at fault here. Even after a day's rest, an 18650 that I charged last night, when put under my DMM, the voltage gets to 4.20v straight off the bat, but it slowly creeps up and then stops to a higher-than-the-start voltage.

Rod911 said:

I really do think it is my cheap DMM at fault here. Even after a day's rest, an 18650 that I charged last night, when put under my DMM, the voltage gets to 4.20v straight off the bat, but it slowly creeps up and then stops to a higher-than-the-start voltage.

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must determine this still:
creeps up WHEN?
1) when off the charger
2) when On the charger

really really cheap meters will not show that the battery in them is low on the display, and could (i have seen it) read a higher voltage (of all things) when the battery in the meter is low.
when i learned that one of my meters was doing that, i made sure that the battery in it was good, then eventually discontinued using it, because it was going to get me in trouble. so that IS possible.

Rod911 said:

When I time the charges, I make sure that the voltage is 4.10v straight off the charger. However, if I were to leave the cells alone for a day and check them again with a DMM, the voltage goes up to around 4.24v and some other cells get up to as high as 4.30v.

Click to expand...

Rod911 said:

All the voltage measurements on the cells are done once they are out of the charger and not when they are charging.

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VidPro said:

must determine this still:
creeps up WHEN?
1) when off the charger
2) when On the charger

Click to expand...

Answer to your above question was over in post 1 and 5. If I have time after work, I might youtube this odd phenomena.

I have the same charger, sometimes it reads 4.22 and sometimes it reads 4.25. It dumps full current until the battery reaches the 'max' voltage the charger thinks it should have, then it varies the current to keep the voltage at that level.

Measuring the voltage of the charger without a cell in it is a good indication of what voltage it wants to charge your battery to. You should be able to add a variable resistor to these to tweak the 'max' voltage that it wants to send your battery to. I opened mine up to see if there were any internal adjustments, but did not find any. Someone did modify the ultrafire one to add a knob to it however.

After a charge the cell should be 4.200 or 4.196 or thereabouts. Using my hobby charger, I'm dropping about 100mA into the battery when it is around 4.17 volts resting, so if you wanted to prematurely pull the battery out, I would aim for 4.17 and above.

On that note, when the cell hits 4.2 volts while charging at 1C, you've only filled 40-60% of the battery, it spends about 3-4x longer to fill the remaining half than the first half.

At home now, back from work, I haven't even had dinner yet (it's 8pm here), fired up the old camera phone, DMM and cells in question and got...no increasing voltage.

After two days worth of rest (well, more like one and a half days), the TrustFire, UltraFire and SoShine cells are no longer behaving oddly.

I stopped the SoShine cells charge at 10.30pm 22/09/09 and the voltage measured right off the charger was 4.10v. Later the next day (around 6.30am), voltage creep kicked in and stopped at 4.20v. As of now, it's settled itself to 4.09v.

Likewise, with both the TrustFire and UltraFire cells, they measured 4.20v and 4.30v respectively off the charger. Eight hours later, they're creeping up to 4.24v and 4.34v respectively. Now, 4.09v and 4.13v.

Too tired (and hungry) to head off my uncle's place right now. I might do the DMM comparison in the weekend.

edit: I'm happy for the cells to find their happy place at around 4.10v to (hopefully) increase their useful life. I'm planning to keep the spares in the fridge with 4.05v or lower.

I've never observed lithium cells to creep up in voltage while sitting. Methinks the protection circuit might have something to do with it?

Tested my multimeter and found that the readings weren't accurate due to the battery in it being nearly dead. After I changed it to a fresh battery and compared its results with my uncle's DMM, it was perfectly fine (only around a 0.1-0.2v difference in comparative readings).

I haven't discovered perpetual energy after all, doh!