Letting Batteries Discharge?

We put this to Patrick Palmer, University of Cambridge, Department of Engineering:

This is a very good question that exercises me most mornings after I've cleaned my teeth and I don't know whether to put my toothbrush back in its holder and charge it or whether to just leave it on the side. There is some truth in the fact that the nickel cadmium battery which is the light-weight one occasionally needs to be helped by being deep-discharged. Most of the time just discharging it 20% and recharging it is okay but it needs to be reset once every month or so, something like that. The lithium battery that's popular in telephones is also light-weight. These, however, do need considerable care. That's why you find lithium batteries in mobile phones and in laptop computers. Their charging and recharging has to be monitored very carefully. They have protection circuits in them usually. So occasionally it probably is a good idea to let your laptop run flat. Do that occasionally because that allows the computer to recalibrate itself and be up and running for the future. The other main type of battery is your lead acid battery in the car. In fact we know very well that lead acid batteries can work very well if they're just kept basically topped up the whole time. Care is required and occasional deep-discharge of nickel cadmium, and for that matter probably nickel metal hydride - probably less often - just by using it in the equipment 'til it's flat is probably not a bad idea.