Q: I bought a toy recently which requires batteries. It has used six so far. On the instruction sheet it says not to use rechargeable batteries. Why not? What harm would rechargeable batteries do?
A: There are two possible explanations for this advice, although without knowing the toy鈥檚 characteristics it is impossible to be precise.
The first is that the nominal voltage of a primary (non-rechargeable) zinc or alkaline battery is 1.5 volts, whereas the nominal voltage of a secondary (rechargeable) nickel cadmium battery is 1.2 volts. The toy may require a minimum voltage, below which it will not operate.
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If, for example, the toy requires two batteries in series and fails to operate if the voltage falls below 2.5 volts, the use of primary batteries would make 3 volts available, whereas secondary batteries would only provide a maximum voltage of 2.4 volts and would not power the toy.
The second explanation is that the impedance (internal resistance) of a secondary battery is generally lower than that of a similar primary battery. When a secondary battery is discharged it can deliver a higher current than that achievable from a primary battery. This higher current may damage the electronic circuitry of the toy so rechargeable batteries would obviously not be recommended.