Battery charge circuit
The battery is trickle charged through resistor R9, diode D3 and thermistor RT1. D3 is used to
stop the battery from discharging back into the power supply if the mains voltage is lost.
Battery charge current is controlled by R9 and RT1. If the battery charge is low then it will
demand a higher charging current from the power supply. This will cause RT1 to heat up
resulting in an increase in the resistance of the thermistor and a reduction in the current
supplied to the battery. This circuit limits the maximum charge current to the battery to 3 Amps.
Using the 2022 power supply with a backup battery
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