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Why Won't NiMH Batteries Work With Older Gadgets?

NiMH batteries are rechargeable batteries that behave somewhat differently than zinc-carbon or alkaline single-use batteries. Older gadgets that were not designed for rechargeable batteries may not work with NiMH batteries. The voltage and internal resistance is different, and the batteries tend to lose charge over time. Some gadgets may be dead after a few months of storage, when they would normally work fine with single-use batteries. Others may not start at all, even with new batteries. Newer gadgets compensate for these different characteristics, but many older gadgets require the voltage and sustained charge of single-use batteries.

Voltage
- The voltage of NiMH batteries when fully charged is less than that of new alkaline or zinc-carbon batteries. NiMH batteries deliver a voltage of 1.2 volts for most of their charge. While the voltage of single-use batteries may drop from 1.5 volts to about 1.25 volts after brief use, older gadgets that don't draw much current often require close to the full 1.5 volts to start. The NiMH batteries don't provide that voltage.
Discharge Characteristics
- Older gadgets expect batteries to deliver less and less voltage with use. Single-use batteries start with 1.5 volts and drop steadily until they are discharged. NiMH batteries start at 1.2 volts and deliver that voltage until suddenly going to zero at full discharge. This means that, for NiMH batteries, charge indicators or circuits that monitor battery charge initially show a partly discharged battery, even when the NiMH battery is fully charged. After a period of use, these circuits show a good battery, even when the NiMH battery is almost fully discharged. Such discrepancies can lead to faulty operation of older gadgets.
Internal Resistance
- Older gadgets are designed with an internal resistance matching the high internal resistance of single-use batteries. NiMH batteries have a low internal resistance. Gadgets that need small amounts of power have a high internal resistance while devices like cameras, which need short bursts of power, have a low internal resistance. The circuits of older gadgets may be sensitive to the internal resistance of the battery and may malfunction if it is lower than expected.
Self-Discharge
- NiMH batteries lose a fixed percentage of their charge every day, while single-use batteries hold their charge very well over time. The latter can be used in devices like smoke detectors that need very small amounts of power for extended periods, or emergency lights that are stored for months before use. Such devices, when designed for NiMH batteries, have a provision for recharging. Older gadgets without such a provision run the risk of a dead battery since the NiMH battery loses its charge very quickly.

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