A Dirt-Cheap Battery Tester

RARS Meeting – April 2011

You located a 12 volt battery at a hamfest. You were promised that the battery was good and you bought it at a very good price. Or, perhaps you have a 12 Volt battery lying around the shack.

To learn how good the battery is, one will typically put a voltmeter to it. A measurement of a battery’s open circuit voltage will indicate that a battery is possibly good, but a better test is to measure the battery terminal voltage with the battery under load.

But, for how long will the battery carry the load? One could leave the load on the battery and then go shopping, but how would one know when the battery was effectively discharged; and batteries do not take too well to being severely depleted. Here begins Alan’s and Bernard’s method of charging, and conditioning, 12 Volt batteries.

First, one should fully charge the battery. The recommended charger is one that senses a full charge and automatically switches to a maintenance trickle charge.

The fully charged battery is then discharged. But instead of putting a load on it and then going shopping, Alan and Bernard use an inverter to do the work. The amount of load on the inverter determines the rate of battery discharge. But, how do they know that the battery is fully discharged?

A timer is put on the inverter output. The timer stops when the inverter automatically turns itself off. If one knows where the timer was set at the start of battery discharge, one simply reckons the interval between timer start and stop. But batteries do not like to be left in a discharged condition and should be immediately recharged.

To that end, Alan and Bernard added a control relay for an automatic battery recharge. The relay coil is energized by the output of the inverter. The relay normally open contact is placed between the battery and the inverter. The normally closed contact is placed between the charger and the battery.

When the inverter turns off, the relay drops out. A normally open contact disconnects the inverter from the battery and the normally closed contact connects the charger to the battery.

For control, pushbutton switches are provided. The Start pushbutton switch is pressed to apply battery voltage to the inverter. The inverter energizes the relay. The normally open relay contact closes to parallel the Start contacts to effectively seal-in the relay, that is, to keep the relay picked up when the Start switch is released. Pressing the Cancel pushbutton switch turns off the inverter and de-energizes the relay to start charging the battery. This is the simplest design of the battery charger/conditioner.

Alan and Bernard have taken the basic charger/conditioner design and embellished it as required to allow the use of whatever components that they had on hand. This schematic shows use of a wall wart to provide d.c. to the control relay and a second set of contacts on the control relay to provide a.c. to the timer. Notice, too, the lamp dimmer to control the load on the battery.