Back when mobile phones just came into fashion my friend had a Nokia and I remember it turning off because the battery ran out. He then turned it back on and at the time we were amazed that the battery bar had filled up nearly half way meaning it had charged itself and had more power to be used. Ever since then I always assumed batteries in devices such as mobile phones recharged a little when turned off or somehow gave some power back. So my question is, is this true or what is it that happens to give more power, and if it is true does it work with other gadgets like iPod's? Thanks!.
No they don't recharge themselves. The battery indicators are quite crude, not perfectly exact at all. If you continually keep the device on and charge and recharge the battery without it being totally depleted in the first place, it loses more accuracy. By totally charging the battery then using it until it's almost totally depleted every month or so, the accuracy will be higher and you'll get better charge capacities from the battery.
You could liken the old ni-cad batteries to a sponge. Continued use would 'suck' the charge from the closest points to the terminals, sucking the surrounding area dry and indicating an empty battery. Leaving the battery for a while would re-distribute the remaining charge throughout the cell and show up as having some power left.
To revitalise an aging ni-cad battery you should completely discharge and fully re-charge it two or three times in succession. Some of the better battery chargers have a discharge-recharge option for this very reason.
The newer Ni-MH do not have this memory problem and can be 'topped up' with a charger at any stage. They should never be left fully discharged though.
Not sure how true this is but i have heard if you dial *3370# on a nokia phone when the battery is just about to run out it actually activates a reserve charge which will give you 50pc battery life. Supposed to be for emergency only.