Dual battery, single alternator charging circuit.

The use of a 'battery isolator' (double diode) to charge two batteries from same alternator has been suggested. An EAA article put down the idea as not suitable for aircraft use because of 1) a problem with only one battery providing the current for the alternator field - a failure of that battery means the other battery will not be charged and 2) isolators waste about 6% of the energy into heat. The first problem can be easily remedied with a simple modification to make a symmetric circuit. The other "problem" of 6% loss is hardly a problem. Any circuit looses energy into heat before it does work (second law of thermodynamics). And even if the loss is more than the complex circuit suggested by the author, the simplicity of the battery isolator translates into greater reliability and ease of maintenance. The simple addition of a second smaller double diode solves the symmetry problem. If one side fails the other circuit side continues working independently. The circuit I used is shown for an alternator with built in regulator, independent panel switches to feed the regulator and pullable regulator and charging breakers. This could be easily adapted for a external regulator setup. Two switches are not absolutely necessary - one switch could go after the smaller double diode, or it could be typically incorporated into the ignition switch - but I like to have more control of the situation.

The required double diodes:
1) the 'battery isolator', available from Gateway Electronics - $25 and

2) the hi duty Roadmaster, Inc., 'wiring diode', available from Camping World - $11

The Batt(F) and Batt(R) refer to the front and rear batteries as located in my setup.

The dual battery setup add a few pounds weight, but also save some weight since no heavy cables are needed to go from front to back. The back battery is for one fuel pump, ignition circuit and engine starting and has very short cables to the starter. The front battery runs the second fuel pump, second ignition and avionics/controls. There is no need even for a heavy ground cable from front to back, since the front carries only minimal current to the back (second fuel pump/ignition, nav/strobe lights and engine sensor wires).


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