Battery Refresher

idea: Burkhard Kainka (Germany)

In the past, a sawtooth was often generated using no more than a small neon lamp and a capacitor, and the resuting circuit was called 'relaxation oscillator'. Less well known, the effect can also be achieved with an ordinary NPN small signal transistor like a BC547 as sketched in Figure 1 below. It is too much to explain here in detail how this can work – let it be sufficient to state that it works and we refer you to the original publication in Elektor's Project Generator Compendium 2011 (page 80) for the details.

 
Figure 1.

Lead-acid gel batteries have the undesirable characteristic that they go 'high impedance' i.e. lose their capacity if they haven’t been used for a (very) long time. And while it is possible to revive such a ‘dead’ battery, this is a tedious and lengthy process. It is, of course, much better to avoid this state in the first place.

A tried-and-tested method to prevent the above is to load the battery continuously with very short but reasonably large current pulses. This is achieved with the circuit of Figure 2; the alert reader will recognise the transistor relaxation oscillator from Figure 1.
 

Figure 2.

The oscillator around T1 drives the base of power transistor T2, which will discharge the battery with 1-amp pulses having a duration of about 2 ms when using the component values shown here. This corresponds to an average current of about 1 mA, which is of the same order of magnitude as the typical self-discharge current of a lead-acid gel battery.

The combination of T3, D1 and R2 gives a rough visual indication of the state of charge of the battery; the less frequently the LED lights up, the lower the battery voltage.