![]() If you charge several cells in series, you need 1.5 V times the number of cells plus 3 V. If you charge only one cell, a supply voltage of 4.5 V is adequate. The maximum voltage across a cell during charging is 1.5 V, and the current source needs around 3 V. In order to allow a current of 180 mA to flow, we require a certain voltage. This LED is illuminated only when current is actually flowing, so it can be used to verify that the batteries are making good contact. For convenience, an indicator LED has been added to the charger. ![]() Since in practice you cannot buy a resistor with this value, we have chosen a value of 6.8 Ω, which is available. If we chose a value of (1.25 ÷ 0.180) = 6.94 Ω for R1, then exactly 180 mA will flow. 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. Here we have ‘misused’ a type LM317 voltage regulator as a current source. The most elegant and accurate solution is to use a current source. The question is thus how to obtain a current of 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. 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.Ĭurrently, penlight cells (AA) commonly have a capacity of 1500 to 1800 mAh (milliampère–hour), so the charging current should be 150 to 180 mA. 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. 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. With the advent of nickel–metal hydride rechargeable batteries, capacities have increased and it is no longer necessary to worry about the memory effect. 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. 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.
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