Naked Science Forum
Non Life Sciences => Technology => Topic started by: paul.fr on 05/12/2007 14:36:29
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why, when my phone is fully charged does it lose charge just by turning it off? IE. the battery meter give you a full charge level, you turn it off, and some time later when you turn it back on the meter is only 2/3 full!
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All batteries have an internal current leakage. Depending on the technology, the rate at which the battery leeks current can vary. For alkaline dry cells, the current leakage can be so small that it may be a couple of years that the battery will last until it runs down (if not used), while most rechargeable batteries will run down very much faster (the lead acid batteries used in most cars can run down within a couple of weeks of not being used).
In general, the higher the current the battery will provide, the greater its leakage current, and so the shorter its shelf life.
There are a number of aspects of this, but the primary one us, if my understanding is correct, that the way batteries work is they have a chemical reaction that generates electricity; and the ideal is that when no electricity is required, then the chemical reaction will stop; but in the real world, it is not possible to completely stop that chemical reaction, so the electric current that could be generated while that low level chemical reaction continues is wasted because the circuit does not need to draw current at that time.
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why, when my phone is fully charged does it lose charge just by turning it off? IE. the battery meter give you a full charge level, you turn it off, and some time later when you turn it back on the meter is only 2/3 full!
There's possibly at least two factors here:
- the battery may really be self-discharging
- you may be seeing a fault/limitation of the method used to estimate the amount of charge-remaining.
Batteries, especially rechargeables certainly do self-discharge; older NiCads were quite bad (self discharge in 4-3 weeks), newer NiMH better (a few months to self discharge at room temperature, I believe), I'm not sure how Li-Ion used in many mobiles fare. If cells are "old" then they may develop internal whiskers or other faults which accelerate their rate of self-discharge.
It's not straightforward to gauge remaining battery capacity; parameters like the cell voltage depend not only on the state-of-charge, but also on the battery age and temperature. More sophisticated charge-indicators actually measure the current drawn from the battery and integrate it over time (I assume Sony's InfoLithium batteries used in digital cameras and camcorders do something like this).
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why, when my phone is fully charged does it lose charge just by turning it off? IE. the battery meter give you a full charge level, you turn it off, and some time later when you turn it back on the meter is only 2/3 full!
There's possibly at least two factors here:
- the battery may really be self-discharging
- you may be seeing a fault/limitation of the method used to estimate the amount of charge-remaining.
Batteries, especially rechargeables certainly do self-discharge; older NiCads were quite bad (self discharge in 4-3 weeks), newer NiMH better (a few months to self discharge at room temperature, I believe), I'm not sure how Li-Ion used in many mobiles fare. If cells are "old" then they may develop internal whiskers or other faults which accelerate their rate of self-discharge.
It's not straightforward to gauge remaining battery capacity; parameters like the cell voltage depend not only on the state-of-charge, but also on the battery age and temperature. More sophisticated charge-indicators actually measure the current drawn from the battery and integrate it over time (I assume Sony's InfoLithium batteries used in digital cameras and camcorders do something like this).
My experience with NiMH is that a couple of months at the outside and they are dead (at least as far as my camera is concerned). NiCad were even worse, but NiCad are pretty much obsolete these days, mostly on environmental ground.
Lithium batteries are far superior in terms of holding charge (as well as the energy density of the battery). The big problem (although this can also be a benefit in some ways) with Lithium Ion batteries is their almost totally flat voltage curve right up until almost the very end of their discharge - which means that the common way of measuring the health of a battery, by measuring its output voltage, is pretty much useless for a Lithium battery. As you say, in these cases, the only way of actually measuring charge levels in the battery is by coulomb counting (i.e. measuring the current going in during recharge, and then measuring the current coming out during discharge, and assuming the difference between the two reflects the remaining charge in the battery).
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why, when my phone is fully charged does it lose charge just by turning it off? IE. the battery meter give you a full charge level, you turn it off, and some time later when you turn it back on the meter is only 2/3 full!
= in this "modern" world many things are made to make us spend more money...
Did you ask yourself why electronics technology has improved so much but batteries tecnology not?
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= in this "modern" world many things are made to make us spend more money...
Did you ask yourself why electronics technology has improved so much but batteries tecnology not?
On the contrary, batteries have improved enormously. NiCad technology of a decade ago gave us about 650mAh in one AA cell; modern NiMH of the same size give about 2500mAh. They are also cheaper in real terms. Li-Ion have an even greater charge (or capacity) to size and weight ratio.
Half the problem is that modern gadgets (mobile phones with cameras and video-screens and mp3 players for example) keep upping other aspects of their "performance" and thereby increase their power demands to match.
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On the contrary, batteries have improved enormously. NiCad technology of a decade ago gave us about 650mAh in one AA cell; modern NiMH of the same size give about 2500mAh. They are also cheaper in real terms. Li-Ion have an even greater charge (or capacity) to size and weight ratio.
Half the problem is that modern gadgets (mobile phones with cameras and video-screens and mp3 players for example) keep upping other aspects of their "performance" and thereby increase their power demands to match.
The other problem is miniaturisation - modern devices keep getting smaller, so the batteries keep getting smaller, so even as they increase energy density, so a higher density in a smaller package may still leave you with less energy to play with.
And yes, energy densities are definitely increasing - which is why we have Lithium batteries catching fire.
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Batteries will behave fine up to the end of the warranty, usually.
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Battery University.. Detailed info on each type of battery and prolonging life.
http://www.batteryuniversity.com/