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| NiCd/NiMH Charging Common values for C for cordless tool and racing cars are in the range from 1000maH to 3000maH. The first step is to determine what C is for your cells. Inspect the cells or contact the manufacturer to determine the cell part number. In drills, the battery packs can often be disassembled easily. The value for C often forms some of the part number and the part number can be searched for on the Internet. For my new battery the value for C was 1700maH. Note that the cell value for C is the same as the battery value for C. Manufacturers data gives that when designing a charger you should first consider how the cells are to be used. For these applications the charge use is termed "cycle use" where the battery is repeatedly charged and discharged. In addition, usually the charge time required is as fast as possible, between 1 and 2 hours. My batteries were capable of taking a fast charge of 100% of C, which equates to 1.7A. Despite this I conservatively selected 1.25A as my charge current, because I wished to be able to charge 1300maH batteries also. This value should be good for most readers, and it doesn't really matter if it is a bit less than 100% of C, because the charger will still detect a peak eventually anyway. However, some readers will want to adjust the maximum current, and this is described a bit later on. For "cycle use", there are two recommended methods of detecting charge termination, either using a temperature sensor in the battery pack or using a "negative delta V" cutoff system. The temperature technique relies on detecting the sudden rise in battery temperature to shut off the charge. There is nothing wrong with doing this but battery packs do not always come with temperature sensors built in. Furthermore ones that do, often sense the temperature of only one cell. The negative delta V system relies on the electrical characteristic that the NiCd/NiMH battery voltage peaks and drops about 20mV per cell when fully charged. This charger in its basic configuration will detect a peak of 84mV (per battery) from 2V to 21.5V, and thus will charge any battery pack in this range (i.e. 6-12 cells or 7.2V to 14.4V). Options to alter this range are described in Figure 10. Hence, no matter how discharged the battery is, this technique will give enough charge to restore the battery to its full state, and then the battery is continually "topped up" with a trickle charge to prevent slow leakage through internal resistance. Other things to consider are the requirement to let a battery cool down, so a better charge can be applied. This battery charger waits for the battery voltage to stabilize for about 30 seconds before starting to charge. If the battery has just come off discharge and is hot, it may take a minute or so for the charge to begin to start. Additionally new batteries may show false peaks in the first 4 minutes of charge. For this reason the charger starts with a slow "soft start" charge for 4 minutes to allow the battery to cool and get past this point. The charger uses a threshold of 2V (open circuit voltage) to recognize that a battery has been connected. In practice even a very old battery that has been actually shorted out for some time, will recover above this value when unloaded. The charging algorithm used by the PIC is shown in bargain-batteries.com. Note that the first Light Emitting Diode (LED) comes on continually whilst battery is undergoing the bulk charge process, while the second LED gives an indication of the particular type of charge being applied. Normal operation of the charger is fairly straightforward. Normally the Charger is switched on and both LEDs will flash once. The charger will then wait in mode 0 (standby) until a battery is connected. Once a battery is connected, the charger will progress though mode 1 (cool), 2 (soft), 3 (fast) and 4 (trickle). The battery can be left in mode 4 (trickle) indefinitely, or removed at this point. When the battery is removed, the charger will revert to mode 0 (standby). |
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