General Principles
- The main criteria for effective charging are choosing the appropriate
rate, limiting
the temperature and selecting the appropriate termination technique.
- Ni-MH is more sensitive to overcharge than Ni-Cd batteries,
therefore, the requirements for charge control are more stringent.
- The most common charging method for Ni-MH batteries is a constant
current charge with the current limited in order to avoid an excessive
rise in temperature.
- Temperature rise during Ni-MH charge is more pronounced because
the charge reaction is exothermic while Ni-Cd is initially endothermic.
At 80-85% charge, both batteries heat due to the exothermic oxygen
recombination reaction.
- Both the voltage drop after peaking (-
 V)
and the temperature rise (T/t)
are used as methods to terminate the charge. (See following figures
for voltage and temperature profiles during charge.)
Recommended charge termination for DURACELL Ni-MH batteries is shown
in the section "Recommended Charge Procedure."
The voltage of the Ni-MH battery during charge depends on
a number of conditions, including charge current and temperature
(see following figures).
A rise in temperature and pressure at high charge rates underscores
need for proper charge control when "fast charging."
Charge efficiency decreases at higher temperatures due to increasing
evolution of oxygen at the positive electrode.
Proper charging is critical to obtaining maximum capacity and
assuring optimum battery life.
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