The author found that a long-neglected gel battery in a hand-held vacuum cleaner had gone high-resistance. It took some effort to make it usable again, by alternately applying voltages of opposite polarities to it. A reverse voltage can help to break down the internal non-conducting layers which can form when the battery is left idle. The battery is now back in action and charging and discharging as normal. Unfortunately, however, the battery will probably start to fail again if we once more leave the appliance lying around unused for a while. To prevent this happening the author applied a well-known technique: the battery is intermittently presented with a very brief high-current load.
The circuit shown here does the job: every two seconds it draws a current of about 1 A for 2 ms. This corresponds to an average current of about 1 mA, which is of comparable magnitude to the self-discharge of the battery. Although the circuit does not consume much energy, it can keep the battery fresh.
The circuit is based on the NPN relaxation oscillator from the 2011 Project Generator Edition of Elektor (www.elektor.com/110195), here delivering the base current for the power transistor. In the prototype the current was measured at around 1 A: to be on the safe side, you can add an extra load resistor to the circuit.
The LED indicates when each current pulse occurs, which also serves as an indication of the battery’s charge state: the less frequently the LED flashes, the lower the battery voltage.