No battery abuse here

No battery abuse hereCareful circuit design around NiCd batteries can be difficult, but not as troublesome as problems caused by early cell death, writes Steve Bush
Simple precautions can extend the working life of rechargeable batteries dramatically. “You musn’t forget that most of the public don’t have a clue about battery technology. If they are going to operate your product, it needs protecting from unintended abuse,” said John Hutchinson, engineering director at BayGen, the wind-up radio company.
It has just released a product with an internal NiCd battery pack and Hutchinson has made it as idiot-proof as possible.
“There are essentially two ways to damage a NiCd battery, overcharging and over-discharging them. Most commercial products with NiCds, like hand vacuum cleaners and torches, do both,” said Hutchinson.
Overcharging , according to Hutchinson, results in electrolyte evaporation and dendrite growth – dendrites are metal crystals that short out the cell internally.
“The majority of simple products charge their batteries continuously at C/5. This abuses them once they are fully charged and they don’t last long,” said Hutchinson.
Conventional wisdom is that C/10 is a safe rate for continuous charge, but Hutchinson has come to differ: “C/10 may be all right for some hours, but charging at this rate for hundreds of hours damages cells.” He has found C/30 completely safe as a continuous charge current. “I arrived at this figure initially by talking to a number of cell suppliers. But it is hard to get a consistent story. The most that the makers agree on is that C/10 is worse than C/20.”
C/30 takes around 40 hours to fully charge a flat cell. This is an unacceptably long time for BayGen’s application, so some form of controlled fast charge was called for.
It chose C/5 as a fast charge rate, terminated by a six hour timer. “A C/5 current completely charges the battery in six hours, then the charger switches to a C/30 trickle charge.”
Using a timer is not the ideal solution, plugging the equipment in with an already full battery results in it being somewhat overcharged for six hours.
“Sensing battery parameters like temperature and pressure to determine when charge is complete is one way, but the sensors add a lot of cost. Other schemes use voltage sensing which isn’t too expensive in hardware, even though you need a microcontroller.”
Problems also occur during discharge when more than one cell in a series-connected battery runs out of charge before the others. “We detect the battery voltage and disconnect the load with a transistor when it drops below 1V per cell,” said Hutchinson, “this means that no cell ever gets reversed.”
By limiting overcharge and overdischarge Hutchinson has made a product that is unlikely to be returned because its battery expired within the warranty period.


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