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Super Simple NiCd/NiMH Charger

 

 

 

 

A few years ago the charging of Nickel Cadmium cells was a fairly standard procedure. An AA sized NiCd cell used to have a capacity of 500 or at most 600 mAh. You bought or built a charger that was rated for that capacity and you could use it to charge virtually all AA NiCd cells. The capacity of these popular rechargeable cells has increased enormously, especially since the advent of Nickel Metal Hydride cells. Nowadays AA cells are available in any imaginable capacity up to 1800 mAh (at the time of writing). This is of course a welcome development, but it also introduces a few problems. When you already have a charger for 600 mAh cells it is irritating to have

to buy a new charger when you buy some new 1800 mAh cells. To avoid the need to purchase two or three expensive chargers, we’ll describe a simple charging process that is suitable for any NiCd or NiMH cell and which can be built in only quarter of an hour. For a safe charging process we choose a charging current of 1/10 the value of the cell capacity, C. Why? At this current the cell can never be damaged, even if it is accidentally left in the charger for several days. At higher currents it can be fatal for the cell if it is overcharged. This can occur when we forget to remove the cell from the charger, but also when the cell is still half full when charging starts. The normal charging period will then be too long as well. At a current of 0.1 C these problems won’t occur. We don’t have to worry about the notorious memory effect; new NiCds haven’t had this for several years and NiMH cells never suffered from this in the first place. It is therefore no longer necessary to discharge the cells first. (There are some exceptions to this rule: when large peak currents are drawn from the batteries, such as in model cars and cordless drills, the recommendation is that the cells should occasionally be discharged and charged at high currents.) After following the next example, anybody should be able to make his or her

own charger: We’ll assume that the cell has a capacity of 1800 mAh. That means that the cell will be able to deliver a current of 1800 mA for one hour, or 900 mA for two hours, and so on. It will be charged with a current of 1/10 the current it can deliver for one hour, making 1800/10 = 180 mA. After 10 hours the cell should be fully charged, but because there are always losses and we want to be completely sure that the cell is fully charged, the charging period should be 14 hours. As we said previously, the cell may be charged for

longer, but if we charge it for a shorter

 

 

 

period it may not be fully charged. For the power supply we’ll use an ordinary 12 V mains adapter. Keep in mind that an unregulated adapter usually has a voltage of 13 V or more; if you want to know the exact figure you should measure it with a multi-meter during the charging process. In this example we’ll assume 12 V. To draw a current of 180 mA (= 1/10 C) at a voltage of 12 V Ohm’s law tells us that the value of the required resistance is 12/0.18 = 66.7 Ω. But there is also a voltage drop across the cell of about 1.4 V when it is being charged. Keeping that in mind, the resistor should be (12–1.4)/0.18 = 58.9 Ω. The resistor will become rather warm, so we should choose one that can dissipate at least (12–1.4) × 0.18 = 1.9 W. In practice we’ll choose a 5 W or even a 10 W type, otherwise it could become too hot.

 

It is of course not possible to buy a 58.9 Ω resistor, so we have to choose one of the nearest standard values of 56 Ω or 68 Ω. It is always best to choose the slightly higher value (in this case 68 Ω), because there will be a reduced current and that is safer. The calculations don’t have to be exact, because the capacity of the cell will most likely differ from that stated by the manufacturer. The cell is put in the battery holder, the resistor and plug are connected and Bob’s your uncle. Do take care that the polarity of the connection is correct: the negative terminal of the cell goes to the negative output of the adapter and the positive terminal of the cell goes to the positive output of the adapter via the resistor. If the cell is connected the wrong way round it will be discharged instead, and could well become damaged! It is also possible to recharge more than one cell at a time. Take a battery holder for the correct number of cells and calculate a new value for the resistor. For two cells you should subtract 2 times 1.4 = 2.8 V from the 12 V supply, for three cells 3 times 1.4 = 4.2 V, and so on. The maximum number of cells that may be charged at a time is six; the value of the resistor is then (12–8.4)/0.18 = 20 Ω and the heat dissipated is 0.65 W. In this case we should choose a 22 Ω resistor rated at 5 W, so it won’t become very hot. You may think that you could charge a few more cells, such that the resistor becomes unnecessary; after all, the resistor only wastes energy. But if you try that, you will find that the charging current becomes overly dependent on factors over which you have no control, such as the value of the mains voltage and the charging voltage. We don’t mind sacrificing some energy to obtain a stable charging current. That is what the resistor is for. And finally a warning: Lead-acid batteries and Lithium-Ion cellsshould absolutely not be charged with this charger!

From Elektor 2002