Effect of charge current on termination voltage

[pae77]

I found it quite interesting that the final termination voltage that my hobby charger will charge a particular Li-ion battery to appears to be affected to some extent by the initial charge current selected. For example, when I charged up a Trustfire 18650 "true" 2400 mah red and black battery setting the charger to use 2.4 amp (2400 ma) or approximately 1C for the starting charge current, the voltage of the battery that was achieved at termination was 4.09 volts as measured by my DMM and this was repeatable.

However, when I subsequently tried charging the same battery with a considerably lower starting charge current of .6 amp (600 ma) or ~.25 C, the final voltage achieved was 4.14 volts.

I guess I'll be using the lower (slower) charge rate in the future with those cells. But I was wondering if there was any explanation for why the different starting charge currents produced such different results in the final termination voltage?

 

[VidPro]

yes, i find that to be true also.
the balance modes slow down more to finish the balance, but even those you can get more charge going in with a slower rate.

probably has to do with how it works. from what i have observed (which was not for long this time) they pound the power in at the current selected, stop the charge for a Very short period of time, then read the battery voltage.
SO
when the charge current is lower, the short stopping to read the actual battery voltage, doesnt have as false of a reading. the false reading being that it just had charge going into it less than a second ago.
(even though the current goes lower as the charge reaches the end , current goes lowest with the lower current settings)

A) just charge normal and be happy with the final which is a bit low and will have the battery last longer.

B) charge at a lower current one time, and the final charge level will be a bit higher

C) charge normal, and dont worry that its not 99% charged, and AS desired, put it BACK on the charge at a lower current , and top it off.
(ONLY for this type of charger, only for li-?? charging)
I prefer method C, depending on the use of the cell you might find it works "better" with a faster charge, not to be confused with a too fast charge.

i have not seen a situation yet , where the cheap hobby charger will Overcharge Li-??, because it is put back on or restarted. but that is a reason to never tell anyone to put it back on, some chargers WILL. usually these wont. Doing that with Ni-?? V-drop termination can overcharge for sure.

 

[45/70]

That is correct, pae. With a CC/CV charger the lower the initial charge current (CC stage) the higher the end voltage (and capacity) of the cell will be. It is also interesting to note that the CV stage takes longer when using a high charging rate than when using a slow rate. Therefore, when a lower rate is used, such as 0.5C vs. 1C, the amount of time the charge takes is longer, but nowhere near twice as long.

This all has to do with why a true CC/CV charger is easier on the cells than other types. Current won't be forced through the cell as with many other types that use CC with voltage checking, for instance. CC/CV is much better for the cells.

Dave

 

[Mr Happy]

Actually, I think this effect occurs because you are "telling" the charger to behave this way. You give instructions to the charger and the charger follows those instructions.

For example, a typical charging procedure for lithium ion batteries is that you charge using constant current using some specified charge current (e.g. 2.4 A) until the voltage reaches 4.2 V, then you switch to constant voltage mode at 4.2 V until the current reduces to (say) 3% of the starting current (3% of 2.4 A is 72 mA), and then you stop charging. Evidently this will charge the battery to within 72 mA of a full charge.

Now if you start the charge at 0.6 A, the CV part of the charge will stop at 3% of 0.6 A, which is 18 mA. Therefore this will charge the battery to within 18 mA of a full charge.

You can understand, of course, that 18 mA is closer to a full charge than 72 mA, and this therefore can explain your observations.

If you are consequently wondering what current to use for the CC stage at the start, you should use the manufacturer's recommendation. For large cylindrical cells like the 18650, the recommended charge current is often 0.8 C, but for most cells you can't go wrong at 0.5 C.

 

[Aquanaut]

Aren't you forgetting about surface charge? The faster the charge rate, the greater the surface charge. You must let the battery sit for a long time to measure its true SOC.

The charger thinks that the battery is fully charged sooner at higher rates. Once the battery rests, the surface charge dissipates and you measure a lower voltage.

This phenomenon is well known for lead acid batteries, and I would be willing to bet it exists for Li-Ion too.

 

[Mr Happy]

I do not believe the surface charge phenomenon is very significant for lithium cobalt cells.

 

[45/70]

I do not believe the surface charge phenomenon is very significant for lithium cobalt cells.

Yeah, if a new or little used LiCo cell charges to, say 4.18 Volts, it'll likely still read that days, or even a week later. With older, or somewhat used cells though, the voltage usually drops a bit within a half hour or so. Also, a lot of the cheap cells available drop in voltage. This is most probably because they are old when purchased.

Great explanation in your first post, as to the effects of fast charging vs. slow charging, Mr H. Honestly, I hadn't thought about it from that perspective.

Dave