Voltage matching

Some time back a circuit was published in one of the electronics magazines that allowed a 12 volt battery to be charged from another. This was used for charging small sealed-lead acid batteries from a car battery, and involved a switchmode step-up circuit to provide a regulated 13.8 volts or so from 12 volts from the car battery.

I have built several of these from various suppliers, and they work quite well. The one I used I already had from another disused project, so I tested it and found that it would output the required 13.8 volts from as little as four volts in. This kit was originally bought from Jaycar Electronics, who no longer sell it, but they do have a pre-built module called a solar power converter, cat# AA-0259, that will take an input voltage from one to ten volts and charge a 12 volt battery—perfect for uses such as this.