Simple battery isolator
Over the years there have been a number of circuits used to
charge and isolate a second 12V battery in a vehicle. The most recent of these
was featured in Circuit Notebook in the March 2007 issue.
This circuit is even simpler and employs a 6V feed from one of
the stator connections on the vehicle’s alternator. This is connected to a 6V
automotive relay (RLY1) which controls a Continuous Duty Solenoid (RLY2). This
solenoid electrically connects or isolates the batteries.
When the engine is started and the alternator stator voltage
rises, the 6V relay turns on. This turns on the Continuous Duty Solenoid to
connect the two batteries in parallel.
As long as the engine is running, the vehicle’s alternator will
maintain charge in both batteries. When the engine is shut down, the alternator
stator voltage drops and the Continuous Duty Solenoid switches off, thus
isolating the second battery from the vehicle’s electrical system.
Provided that camping accessories are only connected to the
second battery, the main battery should never discharge. Because the concept is
entirely dependent upon the alternator’s stator output voltage, you cannot
forget to turn the system on or off as it happens automatically.