HE SIMPLE LEAD-ACID battery zapper we described in the July 2005 issue of SILICON CHIP has been very popular with readers but a few shortcomings did become apparent as people started putting it to work desulphating their batteries.

For a start, when the zapper was connected to a battery with a high level of sulphation, the high voltage zapping pulses could rise in amplitude above the 100V rating of the switching MOSFET, causing it to suffer breakdown. A circuit modification to limit the maximum pulse voltage was published in the Notes & Errata section of the September 2005 issue (page 107).

This photo shows the same setup as depicted in Fig.6. A battery charger is needed to provide current for the zapping function.

We also showed how to connect a switch in series with one of the leads between the zapper and battery, to avoid dangerous sparking at the battery terminals when the connection was made or broken. A number of readers also enquired if they could fit an indicator to show when zapping was taking place, as it wasn’t easy to be sure of this unless you connected an oscilloscope across the battery terminals.

Another complication arose regarding the power MOSFET’s over-voltage protection, because the MOSFET used in the July 2005 design became unavailable and the only replacements we could find were rated at just 60V. So the over-voltage limiting had to be changed again.

It also became clear that batteries needing desulphation must be connected to an external charger at the same time, because they couldn’t provide the zapper with sufficient current. Although we had shown how this could be done, it did involve the use of an external "floating" inductor in series with one of the charger leads. Now we have incorporated the extra inductor inside the box.