BATTERY CHARGING – GENERAL INFORMATION.

The majority of batteries used today are of the sealed or semi sealed variety. This applies to lead acid as well as nickel cadmium or other rechargeable cells.

Lead acid cells

They cannot be charged properly by simply supplying a D.C. voltage to the terminals and hoping for the best. If batteries are to be kept in good order, it is desirable to observe the directions provided by the battery maker. Lead acid cells are mainly used either for cyclic use, when they get used and discharged on a fairly regular basis, or on standby power systems where it is essential to ensure that they are needed (e.g. emergency lighting). In either case it is essential to control the charge voltage and current to avoid battery damage or reduction in life. These facts govern the way that a good battery charger should work.

Cyclic batteries

They are likely to be partially, fully, or maybe deep discharged where the cell voltage has fallen to a very low value. It is usually desirable to provide a high current in the early stages of recharge in order to keep the recharge time down to reasonable limits. The value of the current depends on the size of the battery. At this stage it is best to supply a current limit to the charging to avoid overheating of the battery and undue load on the charger. For example one can choose a current limit of C/10 which is one tenth of the amp Hour capacity © and means the recharge will take somewhat more than 10 hours (e.g. a 1 Amp Hour battery would be charged at 100mA for 10 hours). Other current limits may be chosen such as C/3 if desired. The voltage applied needs to be approximately 2.45 volts per cell depending on the battery suppliers instructions. Once the charge current has fallen to a low level then the voltage must be reduced to a ‘float’ level in order to avoid damage to the battery plates. Generally speaking the float voltage, which is about 2.27 volts per cell, can be supplied almost indefinitely. The current then falls to a very low level. In the above example it falls to a few milliamps. This is of course necessary in the case of standby batteries, which may be float charged for weeks or even years.

Nicads.

In the case of nickel cadmium cells the principles are different. It is still necessary to control the charge current to a known value, and to provide a known charge into the cell or cells. In order to do this it is desirable to discharge the cell to a known voltage, then recharge for a fixed time at a fixed current. There are also systems which analyze the pattern of the recharge voltage and use that to determine the end of charge. Protolink design and manufacture a variety of battery chargers operating on the above principles.