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Battery Power – Pt3 Don Bradbury continues his look at battery types and their chargers When it comes to charging your batteries, bear in mind that they are probably more sensitive than you think to the way you force charge into them. If the voltage applied is too low, the cell will output current instead of accepting it; if it is too high, then undesirable reactions can take place which can damage the cell. Raising the charging voltage inevitably raises the current, but too high a current input rate can cause the cell to overheat. Further, trying to charge a cell beyond its capacity can result in the production of gases and the risk of explosion, so care has to be taken over the choice of battery charger. Battery chargers are, or should be, relatively
sophisticated electronic devices with safety circuits to protect the
cells. So different battery types require a specific charger
tailored to their needs. Most battery chargers operate,
fundamentally, in one of two modes, constant voltage or constant
current.
Constant voltage chargers are the simplest. They produce a specific voltage and deliver a current that depends on the charge level of the battery. As the battery accepts charge, its voltage increases so there is less difference in potential between the charger and battery. As a result, less current flows through the system. The lead-acid cells, used for cars and some computer backup power systems, typically use constant voltage chargers. In addition, Lithium-ion cells may use constant voltage systems, although these are usually more complex with added circuitry to protect the batteries. Alternatively, battery chargers may maintain a constant current by varying the applied voltage. They switch off when the voltage reaches the level of a full charge, and this simple design you may find used for nickel-cadmium and nickel-metal hydride cells. Calling a halt By one means or another, chargers have to determine the correct time to stop the charging voltage, for a charger can destroy a battery by overcharging it. Depending on the requirements of the battery to be charged and the sophistication of the charger, the charger may use any of several technologies to determine the proper time to turn off. The most straightforward way is by sensing the back voltage. The charger monitors the battery's voltage and switches off when it reaches the correct point. But this voltage-sensing technique will not satisfy all conditions. NiCad batteries, for example, have a linear discharge curve that makes voltage sensing inappropriate. So a more advanced charging system may use temperature cut-off. That is, the charger monitors the temperature of the battery and switches off, or reduces the charging rate, when the battery begins to heat up unacceptably. A battery pack using temperature cut-off will have built in thermometers that relay a signal to the charger circuitry to tell it when it’s unsafe to charge further. More sophisticated chargers still can combine voltage and temperature cut-off. Chargers using this technology may switch from high current charging to a lower charge rate using circuitry that senses both temperature and voltage. A modern battery charger might operate with several charging rates, starting with a high current and then switching to low current as the battery nears full charge. After all, engineers these days design chargers for computer and cell phone batteries to be plugged into their batteries continuously without any detrimental effects. Next time in this series we’ll take a look at my own procedure for maintaining batteries for computer equipment.
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