Well, we’ve seen how simple it is to resuscitate the batteries in your "cordless" power tools elsewhere in this issue. Now it’s time to ensure you don’t kill them all over again.

While those battery-powered tools have many virtues, we are not so enthusiastic about their battery charging systems.

As we discussed, most low-cost power tools include a very basic charger: a plugpack to supply power and a resistor to limit the current flow into the battery pack. There is nothing to prevent overcharging: no timer to switch off charging when the time has elapsed and no full-charge detection.

Fig. 1: typical charging curves for Nicad batteries, as supplied in the majority of cordless power tools. Cell temperature (green) and voltage (red) are most often used to detect the "end point" or 100% charge.

At best, this type of basic charger will shorten the battery pack life so that it will require replacing after only relatively few charges. At worst, the basic charger can cause destruction of the battery pack the very first time it is used!

Destruction of the battery pack can happen if the charger is left on for too long after the battery pack has reached full charge. And it is all too easy to forget to switch the charger off at the required time. The result is serious overcharging

You cannot even rely on the fact that charging requires a certain time period and the charger can be switched off after that because the time period required to reach full charge depends on the state of charge for the battery pack at the start of charging.