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Q: My children have many small electronics that operate on rechargeable batteries (MP3 players, laptops, handheld video game consoles). They play them until right before the charge runs out. They then plug them in and keep playing. Does this negatively effect the life or operation of the battery? —Patrick Madigan, MO
A: MP3 players and handheld video game consoles typically use AA or AAA batteries with nickel-metal hydride (NiMH) chemistry. Laptops use Lithium Ion (Li-ion) chemistry for their batteries. With both types, it is OK to run them until there is a little charge left then recharge them. (Also, there's little chance of overcharging them, since today's smart chargers will automatically stop when the battery is full.)
Just don't let the batteries become fully discharged and then leave them like that for multiple days or longer, which can damage them. If you need to store your mobile device for a long period of time, it's best to discharge the battery about 50 percent and remove it (and keep it at room temperature), as its lifespan is extended somewhat if stored at less than full charge. The risk is that a battery's self-discharge rate (about 1 percent a day for Li-ion and 1 to 4 percent a day for NiMH) will cause it to "over-discharge" and kill the battery.
—Rich Fisco, Senior Project Leader