Why do I have to charge my new rechargeable batteries before use?

It may be a familiar story—Christmas morning and there is a new remote-control car or perhaps even a drone. But the instructions explicitly say not to use the device until it's been on charge for hours. Why can't they just come pre-charged?!

The issue is, the very chemistry that makes batteries rechargeable also makes it easier for other side reactions to occur. These side reactions are not always detrimental, but it does mean that instead of all the energy being stored indefinitely inside the battery cell, some can be 'lost' in powering these reactions.

The end result is a battery that has lost some of its chemical energy over time without even being plugged in. Depending on the battery chemistry, this can reduce the future capacity of the battery if it isn't completely recharged before use. Some batteries can even be irreversibly damaged if they are over-discharged (such as being used in a remote-control car after the long trip from the North Pole). The end result—manufacturers recommend you charge the battery before use.

Not all batteries are the same though. The lithium-ion batteries in our mobile phones have a pretty good self-discharge rate of around 2–3 per cent per month, and our lead-acid car batteries are also pretty reasonable—they tend to lose 4–6 per cent per month. Nickel-based batteries lose around 10–15 per cent of their charge per month, which is not very good if you plan to store a torch for a whole season when you don’t need it! A non-rechargeable alkaline battery only loses around 2–3 per cent of its charge per year.

While it can be frustrating, it is probably a good idea to follow the manufacturer’s instructions.