Rockbox Development > Feature Ideas

Battery management while charging

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torne:
It comes down to this: there's two (main) things that reduce the capacity of a battery: the passage of time, and recharging it. A battery that's just sitting around doing nothing will lose capacity no matter what; the rate depends on temperature and on the level of charge, and indeed it will lose capacity faster if it's left fully charged. However, you're going to kill the battery in your consumer electronics through repeated charge cycles long before you notice this effect.

RC cars typically aren't actually used very often; they spend days, weeks or months at a time sitting on a shelf. Mine has certainly had less than a hundred charge cycles *ever* :) This means that the capacity loss is dominated by how much it's lost over time, and thus it's important not to leave it fully charged when you put it on that shelf.

Actual hybrid cars switch between charging and discharging on an extremely frequent basis, often second-by-second, with a load that varies even faster, and then on top of that *also* get left sitting around for long periods without any consideration to their state of charge when doing so. This is basically the worst treatment you can give a battery and any attempt to make it last longer is subject to a zillion variables that hopefully your car manufacturer understands, but you probably don't :)

My smartphone, for a different example, can chew its entire battery from 100% to 0% in about 20 hours if I use it as much as I normally do. I end up charging it multiple times per day as a result. Even then, it's not fully charged for a very large proportion of the time, and the number of charge cycles massively outweighs the loss-over-time effect.

An MP3 player is not quite so bad, hopefully, since they are not usually turned on 24/7 and their power consumption even when in use is lower compared to their battery size, but they are much closer to the smartphone example than the RC car example.

If your MP3 player lasts long enough on a single charge that you can use it for several days without charging it, then it's very likely that the best *simple* thing to do in terms of preserving its battery lifetime is just to.. use it several days without charging it. When you think it's too low to go another day without charging it, charge it. The difference between letting it charge to 100% and letting it charge to a lower voltage is pretty minor compared to just not charging it as often. :)

Anyway, if you feel like writing a patch to control charging behaviour in this way, then go for it; I'm just noting that the effect is likely to be much smaller than you imagine, and may in fact be unmeasurable.

FaN:
I understand, and you're probably right.
so... I don't know if such a patch could correct my problem. Probably not...

But... I'm convinced it can be great for other users, with other kind of usage of their players...

Not sure...

Don't know what to think about!  ???

blr_p:

--- Quote from: torne on February 24, 2012, 11:45:58 AM ---However, you're going to kill the battery in your consumer electronics through repeated charge cycles long before you notice this effect.
--- End quote ---
I'd like to know the basis for why you say repeated charge cycles will kill the battery.

From BU, the detrimental effect is marginal wrt to charging li-ion/poly ceterius paribus.



Normal charging should only get a Li-ion lukewarm. Whats more dangerous  is fast charging and harsh discharges. Think super fast wall chargers and deep discharges.



--- Quote from: torne on February 24, 2012, 11:45:58 AM ---If your MP3 player lasts long enough on a single charge that you can use it for several days without charging it, then it's very likely that the best *simple* thing to do in terms of preserving its battery lifetime is just to.. use it several days without charging it. When you think it's too low to go another day without charging it, charge it. The difference between letting it charge to 100% and letting it charge to a lower voltage is pretty minor compared to just not charging it as often. :)
--- End quote ---
If you go several days without charging and then charge when it is too low, then you have to deal with a higher DoD (Depth of Discharge)  See Table 2 here

The smaller the DoD the better. That means frequent charges with small DoD is better than less frequent charges with a larger DoD. Temps are lower with partial charges than longer ones. And we all know elevated temp is the biggest battery killer.

The reason i posted here is I am in agreement that RB needs a feature that allows the user to set the max charging level or voltage. Going with Table 4 of the previous posted link that is at 3.9 - 4.0V.

3.9V corresponds to 70% using this battery benchmark log .

Therefore the optimal charging level is not 100% but rather closer to 70% (or whatever the corresponding voltage). If there was way to stop charging at that level via a user configurable setting then battery life could be improved considerably.

The magic formula for li-poly battery longevity is...

charge to 70%, use till down to 45% and then recharge again.

This provides a low DoD of just 25% and keeps battery stress to a minimum.

The runtime is reduced to ~4 hours ( in the case of the Clip+) but that is a decent duration between recharges.



--- Quote from: torne on February 24, 2012, 11:45:58 AM ---Anyway, if you feel like writing a patch to control charging behaviour in this way, then go for it; I'm just noting that the effect is likely to be much smaller than you imagine, and may in fact be unmeasurable.
--- End quote ---
Heh, no, i cannot do this. I've posted to stimulate a discussion and hopefully generate interest in allowing such a feature into RB. Way i see it it such a feature will benefit any portable li-poly device RB is installed on to the tune of 2-3x longer battery life if BU's figures are correct.

I absolutely detest how i have to throw out a much loved device because its impossible for the user to change the battery  ::)


--- Quote from: FaN ---I was wondering why I've never seen any Li-based device having this kind of parameter. But is i quite obvious: for manufacturer, the only interest in battery management is to show that a brand new device has a big autonomy. If the battery life is short, it's even a plus, as the customer will buy a new one!
--- End quote ---
Because the consumer wants a device that will run the longest. Run time is more important than battery longevity.

saratoga:
If batteries are really good for so many charge cycles as that chart suggests then I don't think this setting makes sense.

torne:
The information you link to does not support your conclusions (in fact, they agree with me) :)

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