Take the 2-minute tour ×
Super User is a question and answer site for computer enthusiasts and power users. It's 100% free, no registration required.

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?

share|improve this question
add comment

3 Answers

up vote 3 down vote accepted

None of the other answers to this question are hitting at the actual reason behind limiting battery charge.

First of all, battery charging electronics aren't dumb. They aren't going to pump energy into a battery that is fully charged or close to fully charged. Doing so would require that the extra energy be dumped as heat, because the battery not be able to store it, and is why overcharging a battery can lead to overheating and fires. In fact, Lithium-ion battery chemistry requires a lower amount of energy as the battery reaches its full capacity, precisely because it can't accept all of this extra energy at a very fast rate (without overheating.) You've probably seen that the last 90-99% of your laptop charge takes almost as long as the first 0-50%, and this is why.

The Lithium-ion/Lithium-polymer battery chemistry is also the reason why limiting battery charge is a good idea. Storing Lithium batteries close to being fully charged means they degrade more quickly, losing their capacity to store as much energy as when they started. If you've had an old laptop which was kept always plugged in, you've probably wondered why the battery became completely useless after a few years - this was the reason.

Why does this matter?

The 80% and 50% charge limits on Sony laptops are good defaults for trading off performance and long-term battery capacity. 80% allows you to use the laptop on battery with most of its capacity, while avoiding the really damaging 90-100% charged state. 50% is closer to the optimal amount of energy to store in the battery to extend its life, and is what you should use if you are just on AC power almost all of the time.

Also, see this graph for how using these functions will affect battery capacity over time.

share|improve this answer
    
This is really good information, especially on the 80% and 50% settings. Thanks! –  Ben Richards Oct 24 '13 at 19:48
add comment

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

share|improve this answer
    
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
add comment

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.

share|improve this answer
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.