Here we have yet another excellent universal battery charger that is easy to build and can be used to safely charge practically all commonly used NiCd and NiMH penlight cells. The only downside of the universal approach is that it is not a fast charger since it works with the well-known standard charging current of one-tenth of the battery capacity in combination with a charging time of 10 to 14 hours. With the advent of nickel–metal hydride rechargeable batteries, capacities have increased and it is no longer necessary to worry about the memory effect. This means that a topping-up charge can be used at any time, and if this is done using the above-mentioned current of one-tenth of the battery capacity the charging time is not critical. In other words, the battery is guaranteed to be fully charged after 10 to 14 hours and there is no danger of overcharging, so it does not matter if you accidentally charge for 20 hours. If you are certain that the battery is only half empty, you can restore its full capacity by charging for around 6 to 7 hours.
Currently, penlight cells (AA) commonly have a capacity of 1500 to 1800 mAh (milliampère–hour), so the charging current should be 150 to 180 mA. If you want to charge several cells at the same time, you can simply connect them in series, since the same charging current will then flow through all the cells and they will all be charged simultaneously. The question is thus how to obtain a current of 180 mA. The most elegant and accurate solution is to use a current source. Here we have ‘misused’ a type LM317 voltage regulator as a current source.
The well-known LM317 three-lead regulator is designed to adjust its internal resistance between the IN and OUT leads to maintain a constant voltage of 1.25 V between the OUT and ADJ leads. If we chose a value of (1.25 ÷ 0.180) = 6.94 Ω for R1, then exactly 180 mA will flow. Since in practice you cannot buy a resistor with this value, we have chosen a value of 6.8 Ω, which is available. For convenience, an indicator LED has been added to the charger. This LED is illuminated only when current is actually flowing, so it can be used to verify that the batteries are making good contact. In order to allow a current of 180 mA to flow, we require a certain voltage. The maximum voltage across a cell during charging is 1.5 V, and the current source needs around 3 V. If you charge only one cell, a supply voltage of 4.5 V is adequate. If you charge several cells in series, you need 1.5 V times the number of cells plus 3 V. For four cells, this means a supply voltage of 9 V. If the supply voltage is too low, the charging current will be too low.
A supply voltage that is greater than necessary is not a serious problem since the circuit ensures that the charging current cannot exceed 180 mA. The required voltage can be conveniently obtained from a standard unstabilized mains adapter (or ‘battery eliminator’), with a 300-mA type being highly suitable for supplying the required 180 mA. It is usually possible to select several different voltages with such an adapter, and it is recommended to choose the lowest voltage for which the indicator LED of the current source still lights up well. We should mention a couple of practical points. First, any desired color of LED may be used, but it must be a high-efficiency (low-current) type since such LEDs are brightly illuminated with a current of 2 mA as used here. When charging several cells in series, the cells should naturally be placed in a battery holder. Although it is not all that important for use with this charger, we would like to point out that the quality of most battery holders is very poor. The interconnecting springs sometimes have a resistance of as much as 1 Ω (!), which can result in considerable losses (a cell loaded at 1 A will then provide a voltage of only 0.2 V…). Finally, note that the LM317T (the ‘T’ refers to the package type) must be fitted with a heat sink. Although it will not be destroyed by being overheated, it’s no fun to burn your fingers and it’s naturally not particularly good for the charger to become so hot. A Fischer SK104 heat sink (approximately 10 K/W, available from Dau Electronics) is a suitable type.