Automatic Battery Discharger Design

6 Jan 01 - Enhanced design produced

This was built to discharge packs of between 4 and 8 cells. The operation is very easy - set the cut-off voltage, connect the battery and press the start button. With 1 lamp I found the discharge current for 6 cell and 8 cell packs was about 550mA and 750mA respectively.

Automatic Discharger Schematic

The relay can be of any type, but needs to meet the criteria on the right. Must be able to operate for several hours on the maximum battery voltage you want to use.
The release voltage must be below the lowest discharged voltage you want. This can be tested by connecting the relay coil directly to a discharged battery. The relay must be able to maintain the contacts at this voltage.
Must be able to switch the current drawn by the lamp(s).

I used an automotive (12V) relay with a coil resistance of 80 ohms. This would start when the button was pressed at 5v but it needed a slight shock to start at 4v.

The value of the variable resistor needs to be worked to suit the relay by experimentation. The resistance range required for my relay was 44 to 58 ohms, for 4 cells and 8 cells respectively, and is probably fairly typical. This range can be achieved using a 39 ohm fixed resistor and a 20 ohm wire-wound loudspeaker volume control potentiometer. The voltage test point is used to calibrate the cut off voltage over a series of discharges.

The load can be of any type you wish. I find that 21W automotive lamps are the most convenient and the number can be varied if required. I normally operate with a single 21W lamp. If wire of a suitable current rating is used for the the voltage test point it can be used to connect an additional lamp when required.

This design is easily modified to accommodate higher numbers of cells. If more than 14 cells are to be used you would need to use 24v lamps. You could get away using a 12v relay if a resistor is inserted between the coil and the variable resistor on the above circuit.

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