My cell phone had been on for days and was down to one bar of power showing in the battery meter. I turned it off for the morning. When I turned it back on, it was back up to 2 bars...
Battery level meters are black art, not science. Many batteries will recover slightly when rested after a period of use, so your result isn't too surprising.
Yep. It's the reason that cars often won't start if you've left the lights on and drained the battery to a very low level, then try to start it immediately. But if you turn off the lights (or whatever is draining it), then wait a couple of hours, it may well start. Then the trick is to drive it around for awhile so the battery fully recharges.
Most battery meters are simply measuring the voltage output of the battery.
What's that you say? A AA battery is 1.5 volts, isn't it always just 1.5? Well, no.
When a battery is fully charged, it will have a voltage at or above it's listed output voltage. As it drains, it's internal resistance rises, meaning essentially that it's voltage output drops. If it drops too far, then the battery doesn't have enough power left and it's time for a recharge. This is how the voltage meter is calibrated. It's also why it may stay at 5 bars for a long time then drop quickly down to one -- the voltage doesn't drop linearly over time, but drops much more quickly as the battery is drained.
Anyway, when the battery is in use for a long time, that internal resistence builds up and eventually the voltage may drop to very low levels. If you turn the battery off, that internal resistance will lower again as it rests. This has something to do with the chemistry of batteries, which I don't quite understand. But after a rest, when you turn it back on the internal resistance is less and the output voltage is higher. It doesn't mean that the total energy remaining in the battery has increased, but some of the chemical changes that caused the voltage output to drop have reversed while the battery was resting.
I often see the same thing with a flashlight when the batteries are getting old.
Anyway, that is probably way more explanation than you wanted or cared for, but it's not often I get to dust off that EE hat! :-)
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Most battery meters are simply measuring the voltage output of the battery.
What's that you say? A AA battery is 1.5 volts, isn't it always just 1.5? Well, no.
When a battery is fully charged, it will have a voltage at or above it's listed output voltage. As it drains, it's internal resistance rises, meaning essentially that it's voltage output drops. If it drops too far, then the battery doesn't have enough power left and it's time for a recharge. This is how the voltage meter is calibrated. It's also why it may stay at 5 bars for a long time then drop quickly down to one -- the voltage doesn't drop linearly over time, but drops much more quickly as the battery is drained.
Anyway, when the battery is in use for a long time, that internal resistence builds up and eventually the voltage may drop to very low levels. If you turn the battery off, that internal resistance will lower again as it rests. This has something to do with the chemistry of batteries, which I don't quite understand. But after a rest, when you turn it back on the internal resistance is less and the output voltage is higher. It doesn't mean that the total energy remaining in the battery has increased, but some of the chemical changes that caused the voltage output to drop have reversed while the battery was resting.
I often see the same thing with a flashlight when the batteries are getting old.
Anyway, that is probably way more explanation than you wanted or cared for, but it's not often I get to dust off that EE hat! :-)
--Scott