Battery Charging Technology

For rechargeable batteries, great care is required in the design of charging circuits and especially the Fast charging circuits. These circuits are dependent on the battery chemistry. On the other hand slow charging circuits – charging time more than 12 hours – are simpler than fast charging circuits.

Fast charging circuits have in-built means to terminate the charge because, overcharging the batteries may cause reduced battery life and sometimes even destroy the battery. Overcharging the battery may also lead to over-heating and emission of dangerous corrosive gases.

Below is a figure which shows the specifications of various types Rechargeable batteries:
Li-ion batteries are better than NiCd and NiMH in regard of self discharge.
The discharge rate is defined as the Max. allowable load or discharge current and is expressed in units of C-Rate.
The no. of charge and discharge cycles is the avg. no. of times a battery can be discharged and then re-charged and is measure of battery’s service life.
Memory in case of NiCd batteries usually disappears if the cell is almost fully discharged before recharging. However it is very rare that NiCd battery are discharged to that level before re-charging.

Below is the Battery Charging generalized circuit:
The battery is charged to full with a constant current supply. Voltage across the current sense resistor is used to maintain constant current supply. Control circuits and microprocessor is used to monitor voltage. Temperature sensors are used to monitor battery temperature and ambient temperature.

The above type of circuit is used for Fast charging applications.

Most difficult thing is to determine correctly that when to terminate the charging, since overcharging can damage the battery.

The charge may be terminated by monitoring battery voltage, voltage change vs. time, temperature change, temperature change vs. time, min. current at full voltage, charge time and various combinations of these depending upon the battery type.

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