Charging a 24V Battery From a 12V Source

One thing that sometimes puts people off using a more efficient 24v system is the difficulty and expense in getting 24v chargers. So the question arises how can we go about charging a 24V battery from a 12V source? The circuit below shows how to arrange switching so that two 12v batteries can be connected in series to supply 24v, yet can be charged in parallel from a 12v charger.

Two 12v automobile relays are used for a 24v system. These relays are available with a 30 amp continuous rating. You could of course use a single double pole relay instead of two of single pole ones, but these are not generally available with more than a 10 amp rating. The 30 amp relays we suggest have contacts capable of carrying well over 100 amps for short periods so are fine for most controllers in this application

Consider the 24v system above. When the relays are not operated, the two batteries are connected is series through the normally closed contacts (solid black). When both relays are operated the batteries are in parallel. The relays are operated by a third contact, B, and are energised automatically by connecting the 12v charger.

It is tempting to connect the NC contacts effectively in parallel instead of series as here: this would give better current handling – but there is a danger that, is ever a relay contact stuck, one battery could be shorted out, destroying the other relay as well.

With this system you must make certain that the 24v (or 36v) which will, for an instant, be applied to the 12v charger, will not damage it. Alternatively you must arrange that contacts B and C make first, energising the relays before the charger is connected.

Other versions of this system are possible, this diagram shows a 36v system which uses 4 relays.