Circuit Description

This device has three sub-assemblies. The bottom plate has the battery, the charging connector, and the high-current components. The connector sub-assembly connects to the battery with two permanently-connected concealed wires through a groove, permitting the top to be removed. The circuit board is connected by a header to the bottom plate,

The charging and load current is throttled by a PNP transistor for minimum charging voltage. Its base current is proportioned by a pulse-width modulated output, at 37 KHz, which is smoothed by a R-C filter. The PNP transistor is by-passed by a resistor which is necessary to get the minimum charging current. The base current provided by this resistor is added to the control circuit output.

Battery current is sensed by a shunt resistor in its negative lead. The voltage drop is biassed to allow discharge readings and amplified by an amplifier. It has a range of +/- 500 MA.

A resistor divider measures the battery voltage, plus the blocking diode drop, about 0.3 V. A three-color LED gives an indication of the battery state when charging. It is visible through a hole in the cover.

The PICAXE is programmed in-situ via a 3-pin header and the usual protection circuit. The cover must be removed to program it.