Monitoring WF-139 with DMM to terminate before 4.2V

[bob_ninja]

This is a follow-up question based on these WF-139 tests:

http://www.candlepowerforums.com/vb/showthread.php?t=209785

http://www.candlepowerforums.com/vb/showthread.php?t=205036

The key assumptions are:
- using quality protected 18650/14500 cells such as from AW
- willing and able to monitor WF-139 charger
(meaning I won't forget to check it and such)

My question is:
Can I simply use a DMM to monitor charge voltage on WF-139 and terminate at 4.1V (happy with 80%-90% SOC) in order to ensure:
- safe charge process, eliminate all risk
- long cell lifespan

In other words, is using a simple DMM to stop charging at 4.1V practical and sufficient assuming I will remember to check? Is a simple DMM good enough? Are there other steps necessary?

thanks

 

[SilverFox]

Hello Bob,

I think you would be better off monitoring the charge and terminating it when the cell voltage reaches 4.2 volts.

At this point the cell should be about 70% charged.

Tom

 

[CaseyS]

4.2V = 70% charge :huh:? I thought 4.2V = 100%.

And if that's true, why would you want to stop charging at a 70% charge level?

 

[bob_ninja]

I am also confused. I thought 4.2V is 100% SOC.
If it is only 70% SOC then why is it unsafe to charge above 4.2V? Why in other discussions I posted links for there is such a concern for charging voltage of 4.3V and especially for 4.4V?
At what charge voltage is cell supposed to be 100% SOC?

thanks

 

[smiley1437]

I am also confused. I thought 4.2V is 100% SOC.
If it is only 70% SOC then why is it unsafe to charge above 4.2V? Why in other discussions I posted links for there is such a concern for charging voltage of 4.3V and especially for 4.4V?
At what charge voltage is cell supposed to be 100% SOC?

thanks

Remember that lithium cells should be charged in 3 stages -
http://www.batteryuniversity.com/partone-12.htm

If you just do stage one (bring cell up to 4.2 v) you're not quite done - I think this is where the 70% comment came from. You can certainly use the cell at that point, you just won't be getting as much run-time as you could with a full charge.

For stage 2, the charger should hold the cell at 4.2v while allowing current to slowly drop off. This is where the battery is going from 70% to 100%. Once the cell stops taking current (going below 0.03C) then your battery is considered fully charged.

Stage 3 is just a topping charge once every 500 hours. Not usually a concern unless you intend to leave the battery in the charger indefinitely

So I guess you could say that, while charging a cell, 4.2v could indicate a SoC of anywhere from 70% to 100%. Just depends on your charger.

 

[Th232]

Tacking on to what Smiley said, remember that what you're doing when you're charging a battery is literally charging, you could say that you're putting charge (electrons) into the battery. I believe that the chemistry of a Li-Ion is such that voltage is not proportional to the charge put in, hence why Li-Ions need a constant-current then constant-voltage charging algorithm.

Smiley's link covers overcharging quite well and why it's not good for the battery (and possibly user).

 

[bob_ninja]

Got it, thanks 🙂
So I could simply stop at 4.2V getting between 70% to 90% SOC
Again for my own needs I don't mind as low as 70% SOC if it is going to ensure much longer cell lifespan.

cool

 

[ampdude]

The protected lithium cobalt cells (RCRs and P17500/P18500, ect.) SHOULD cut off at around 4.3V if the WF charger fails to stop charging at 4.25V, however I wouldn't say this is always the case, as the WF charging rate is probably too low to cause the protection circuit to engage on my protected lithium cobalt cells. These are the "black" cells that AW sells, the PILAs and the Ultrafires and such. Charging over 4.3V with lithium cobalt cells is dangerous, I've heard people say they've had packs start to swell at that voltage. Years ago I've had RCR's come off the charger at 4.28V that I accidently left in there overnight, as when it does cut off the WF charger will still have a slow 10-15ma charge on the cell. But I think the normal 250-300ma charge of this charger still is too slow for some protection circuits to engage.

This issue may have been fixed in newer WF chargers, one of mine is several years old and the other is about a year and a half old, my issues came with the older charger, I haven't tested the newer charger in this fashion.

I only use IMR cells these days anyways and don't worry about it too much. They are completely safe and a hair lighter as well. And they don't have the fit issues that lithium cobalts do in some lights as they don't need a protection circuit. And they can be drained at a much higher rate safely and with very little loss in amp hours. I like the cells to come off with a 4.23-4.24V charge on them for the best runtime, but still have a useful service life. I have done the same thing with RCR type cells in the past with no problems.

After the charging cycle I usually snap them back into place for another charging cycle and they usually charge for about another minute to around that voltage when measured on a DMM. I have never had any problems charging IMR's or RCR's like this.

 

[Wattnot]

I believe it would be difficult and unnecessary to try and beat the charger at it's own game. The WF139 has proved reliable and most of the experienced folks here are in agreement that the only thing you should do is remove the batteries asap when the charger says they're done.

How would you go about checking anyway? Are you going to pull the battery every couple of minutes and test it? Because that's the only way you're going to know the voltage of the cell. Doing that would probably do more harm than good, let alone the hassle. You can't measure the battery while it's being charged because your meter will read the voltage the charger is putting out. To charge a battery to 4.2 volts it is necessary to put MORE than 4.2 volts in or else the current will not flow into the battery. Current will flow from high to low voltages. So if you measure 4.4 volts and rip the cord out of the wall, you'll never end up with fully charged cells.

Just don't let the cells linger after the charge is complete and you'll be fine. Otherwise, buy the Pila.

 

[buickid]

One problem I can see is that you may not pull the two cells at equal voltages. The WF-139 is pretty good at stopping the cells at a consistent voltage. If you pull the cells at different times, the voltages may be different and result in one cell reverse charging the other when used in a pair.

 

[bob_ninja]

... You can't measure the battery while it's being charged because your meter will read the voltage the charger is putting out. To charge a battery to 4.2 volts it is necessary to put MORE than 4.2 volts in or else the current will not flow into the battery ....

That is exactly what I want to monitor, charge voltage. The actual cell voltage is always going to be lower, of course. From the linked discussions, the concern is that charge voltage should never exceed 4.2V
Seems WF-139 doesn't have CV stage. Instead it keeps pushing charge voltage past 4.2V during CC stage to get full charge. Their concern is that charge voltage greater than 4.2V even for few minutes does a lot of hard to cells. I am new to Li chemistry, just repeating what I read here.

So monitoring charge voltage to stop at 4.2V should avoid the damage while ending up with less than 100% SOC charge. I imagine I could simply connect a DMM to cell terminals to monitor charge voltage.

 

[KiwiMark]

That is exactly what I want to monitor, charge voltage. The actual cell voltage is always going to be lower, of course. From the linked discussions, the concern is that charge voltage should never exceed 4.2V
Seems WF-139 doesn't have CV stage. Instead it keeps pushing charge voltage past 4.2V during CC stage to get full charge. Their concern is that charge voltage greater than 4.2V even for few minutes does a lot of hard to cells. I am new to Li chemistry, just repeating what I read here.

So monitoring charge voltage to stop at 4.2V should avoid the damage while ending up with less than 100% SOC charge. I imagine I could simply connect a DMM to cell terminals to monitor charge voltage.

I would suggest you buy this Accucel-6 charger instead because it does the proper CC/CV charge to 4.2V using the Li-Po setting or to 4.1V using the Li-ion setting. A CC/CV charge to 4.1V will give you 85-90% of a full charge while allowing the battery to be used for more cycles (easier on the battery). A CC/CV charge to 4.2V will give you the maximum capacity without unduly damaging the battery. This charger can also provide a storage charge, charge LiFePO4 cells, charge small cells like 10440, charge large cells like 32600 and charge NiCd, NiMH & Pb.

Note:
You would need to supply 12V DC power to run this charger - if you have a spare computer PSU then you could utilise that for free.