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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