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My laptop lets me limit how much of the battery will charge before reaching full charge:

enter image description here

At first, I understand what the intent is. I know that overcharging a battery will result in degradation of its charge capacity. However, with smart chargers these days detecting a full charge and turning charging off when reaching a full charge, wouldn't this mean that overcharging is impossible?

Furthermore, my laptop appears to be able to do this (as indicated in the screenshot below). Therefore, am I getting any benefit to having it turn charging off at 80% or 50% capacity, rather than letting it charge to 100%?

enter image description here

My thought is that perhaps those settings are more useful when the battery is already degraded. Let's say it can only hold 90% of its original charge. Would the laptop try charging it to 100% even when after 90% it cannot hold any more? Perhaps these limits are more useful, then, to drop the full charge limit so that overcharging doesn't happen and further degrade the battery even more quickly?

Or am I missing something, here?

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2 Answers

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When you charge a battery you obvisouly have to supply a higher voltage than the battery is currently delivering - so that power will flow into the battery!

When it is full the battery is at its rated voltage and so the charger circuitry has to supply a higher voltage than this. So if the battery sits in a machine that is connected to line power when it is full - the charger will constantly be supplying a higher than maximum voltage to the battery. This isn't good for the internal chemistry.

Smart chargers only charge to 70-80% so that it never supplies more than the final battery output voltage.

For details see battery university

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But once the battery reaches 100% capacity, wouldn't it stop trying to charge the battery? Before I turned this on, I did see something similar to my screenshot above, where it said "100% available, not charging". Wouldn't that mean it isn't supplying more power than its maximum voltage? – Ben Richards Nov 7 '12 at 19:21
Ah, I can see in the link that it mentioned that holding it at max charge is itself stressful, and that some chargers, while terminating voltage at 100%, will still top it off if it drops any, so while it avoids overcharging, it does still keep it at a level that will degrade the battery's life. – Ben Richards Nov 7 '12 at 20:09
@sidran32 - yes that's the problem. It charges to 100%, harger turns off, the battery drops to 99% it turns on at max voltage, etc etc. – Martin Beckett Nov 7 '12 at 20:57
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I think this mechanism is more about the battery being at 100% and over time losing charge (not through use) so it goes down to 99%. Then the charger kicks in and brings it back up to 100%. The point is that filling it up this one percent wears it a lot compared to the juice you got into it (compared to a charging filling it from 30% for example). That is what this setting is supposed to avoid i believe. I saw the feature on some notebooks as well.

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