I use lithium-ion batteries, which I abbreviate to Li-Ion, the same way other people use them. But I have noticed that the fully-charged voltage of each one is not the same.
There is a WiKiPedia article, which explains well enough for my needs, how Lithium-Ion batteries work. One question which I had not previously had an answer to, was, ‘If I was to design a Lithium-Metal battery, aside from using Lithium as the anode, what material would I use as a cathode – as the oxidizer?’ And the above answer provides possible solutions.
One fact which I have noted before, is that I have ordered a battery charger from Ebay, for ‘Type 18650′ batteries, and that these batteries usually have a fully-charged voltage of 3.7 Volts, and that some compatible batteries only hold 3400mAh of charge, while others hold 9000mAh of charge.
Well, when the battery in my phone has a voltage of about 3.7V, it is only indicated as 60% charged. At 90% charged, my phone battery has 4.1V, and a typical charging-cycle will bring it up to 4.2V – enough to fry certain other batteries.
All of these observations could well be explained, by the phone battery being a Lithium-Cobalt-Oxide battery, while certain other, exchangeable batteries may simply be Lithium-Iron-Phosphate batteries, or yet other batteries. ‘Other exchangeable batteries’ could include the ones in my camera, etc.. The fact that the cathode can have different compositions, will lead to different voltages.
But, when I do receive the batteries and charger I ordered, which are supposed to have a 9000mAh capacity, I will need to verify something. The Type 18650 batteries need to have a fully-charged voltage of 3.7V . Yet, there seem to be high-capacity batteries which hold more charge than merely 3400mAh.
There is a possible, little trick which the makers of my battery and charger could be using. They could have programmed their charger, only to charge the batteries to 3.7V – as usual. But the high-capacity batteries there, too, could be of the Lithium-Cobalt-Oxide type, which can theoretically be charged to 4.2V. At 3.7V, that charger could simply stop charging them.
When I have received my charger and the batteries, what I will have to do after charging those, will be to measure their voltage. The reason for this will be the fact that other battery-types are only allowed to be charged to 3.7V . I will need to know whether it would be safe to insert a Lithium-Iron-Phosphate battery into the charger, instead of the higher-capacity batteries it ships with.
(Edit 12/11/2016 : Additionally, I will want to measure the voltage of the same battery, 30 minutes after taking it out of the charger, without having connected any load to it. I could expect, to see a voltage of 4.2V , and then one of 4.1V . This would tell me that the same charger cannot be used with the lower-capacity batteries. But all of this thinking is pure guesswork, until I have measured the battery. I am insinuating that the batteries are mislabeled as 3.7V batteries, and as soon as something is mislabeled, we need to measure values. )
If the yet-to-arrive batteries test out as having 3.7 V or close to it, I will also know that I can trust the charger I ordered, with the lower-capacity battery-type, which it does not ship with.
The possibility should not be dismissed outright, that the lithium ions might not be distributed evenly in the cathode, immediately after a full charge. This would just not be taking place, for the reason I first gave it.
It could be that when power-charged, the battery is at 4.2V when full, and that 5 minutes after stopping the charging current, the voltage would be at 4.1V . The battery might still be fully charged, only with a lower voltage than it had immediately after reaching full charge.
The voltage will not drop all the way down to 3.7V for this reason.
Charging the battery more slowly can fix this problem. But otherwise, this problem should also be why, we cannot leave the same battery connected to a 4.2V supply, once it is charged. It will overcharge if we do.