Charging Rechargeable AA Batteries with a 5.7 Volt Charger: Myths and Facts

When it comes to charging rechargeable alkaline AA batteries, many believe that real chargers are rated by voltage. However, this assumption is often misleading and can lead to confusion about whether a specific voltage can be used to charge certain batteries.

The Reality of Charger Ratings

Real chargers are not rated by voltage but by the type and number of cells that they are designed to charge. This means that a charger is designed to handle specific types of batteries (such as NiMH or NiCd) and the exact number of cells in the battery pack. For example, a charger for four AA NiMH cells might be suitable for a 5.7-volt charger, but this does not mean that the charger can be used to charge the batteries directly at this voltage.

The Concept of Open Circuit Voltage

It is indeed possible for a 5.7-volt open circuit to charge 4 NiMH cells, but this voltage alone is not sufficient to initiate charging. The key factor in charging is the charging voltage, which must exceed the full charge level of the battery. Common NiMH and NiCd cells have an open circuit voltage of around 1.45 to 1.54 volts under charge. To charge a series of these cells, you must apply a voltage higher than the sum of their individual voltages, which is why a 5.7-volt charger might not be enough to charge them directly in series.

The Role of Resistor and Diode Circuits

My experience designing a product used a 12-volt input which was divided by a suitable resistor into a 5.7-volt output. This output was then connected through a diode to the output of a 5-volt regulator and the load, ultimately providing power to the 4 AA NiMH cells. While the company manufactured thousands of such products, it is important to note that these circuits were not intended for direct charging but rather for providing a regulated power supply to the cells.

Charging in Series and Parallel

The standard method of charging a series of 4 NiMH cells is not to connect them directly to a 5.7-volt charger. Instead, you should exceed the charging voltage of each cell. For NiMH cells, this typically means applying a voltage higher than 5.8 volts to initiate charging. This can be achieved by using four separate charging circuits or two sets of circuits to charge pairs of batteries at a time in series.

Conclusion

To summarize, a 5.7-volt charger is not generally suitable for charging 4 NiMH rechargeable AA batteries in series directly. The voltage must exceed the full charge level of the batteries for effective charging. For practical applications, using separate charging circuits or charging in parallel can provide the necessary voltage to charge the batteries efficiently.

For more detailed information on charging rechargeable batteries, please refer to the guide on our website.