Why does a battery go flat, and why can't you recharge a normal one?

A battery goes flat because the chemicals that make its current get used up. Inside an ordinary AA alkaline battery, zinc metal reacts with manganese dioxide, and the reaction pushes electrons through your device. When there is little zinc and manganese dioxide left in their original form, there is nothing left to push, and the battery stops producing current. The material is still in there, but the chemicals have changed into something that has nothing left to give.

Why can't you just recharge it? Ordinary batteries are called primary batteries, and their reactions are generally not reversible. Rechargeable ones, called secondary batteries, are built so that applying an electric current runs the reaction backwards and regenerates the original chemicals. Standard alkaline batteries were not designed for that. Strictly, the reaction is not impossible to push backwards: they are reported to survive a few recharges, typically not more than ten, but they lose capacity each time, and attempts to recharge them may cause them to rupture or leak hazardous liquids. In a test by the UK consumer organisation Which?, chargers for alkaline cells left them with only around a tenth of their original capacity after two recharges.

Even a battery you never use slowly fades. Disposable batteries typically lose 8 to 20 percent of their charge each year on the shelf, because side reactions run inside the cell even when nothing is connected. And as a battery dies, the chemistry inside it changes and produces a little hydrogen gas. Pressure builds until the seal gives way, which is why forgotten dead batteries sometimes leak a caustic liquid that corrodes the device they sit in.

The modern alkaline cell came from the Canadian engineer Lewis Urry in the 1950s, and it became the battery most people have in a drawer.