Constant Current Charger

A server power supply was used to create a constant current output to charge the new ‘Tennant’ battery. The supply has constant voltage with short circuit protection, because the battery is such a low resistance the charger’s voltage rail would crash out when connected. So a circuit was added to its output to make the supply a freewheeling constant current supply. This was done by connecting a current sensor to a comparator circuit that controlled the gate of a FET. Power is supplied to the battery, when the current sensor detects zero current it would drive the power FET’s Gate high and charge the battery, when the current sensor detected this power it would turn off the supply. This made PWM-like output to the battery that would continually charge tenant with constant current. Multiple large heat sinks were added to the components including the voltage regulators, the FET, and the diode. The diode was added to the design for the inductive kick that will come off of the battery when the voltage supply goes off. The diode allows that kick to be recirculated through the battery so that it does not damage the power source or electronics. A copper wire was placed under the solder traces on the proto-board and so the traces could carry more current.