relay / battery isolator dilemma

I am working on a trailer with a 12V battery and an electric-hydraulic brake actuator (pump).

The charge wire from the truck powers up the brake actuator only and does not connect to the trailer battery for charging. The manufacturer wired the trailer this way to prevent the actuator from draining the trailer battery when parked (.3A draw when actuator is energized)

I wired two SPST relays on the trailer to split this hot wire (from the truck) and power up the actuator and also charge the trailer battery. This would keep the actuator from draining the trailer battery.

However, I did not foresee that because I used the incoming wire for both the relay coil(87) and feed through(86) that once the relays were closed, my trailer battery would backfeed and prevent the relays from opening.

I haven't figured out a way to resolve this problem without buying a dual battery isolator.

Any ideas would be much appreciated

You can't - you need to separate the signal (charging) wire from the +12V supply wire.


The single-wire solution is intelligent signalling (ie, a BUS system), else a diode to the second battery that is full-load rated - but that will cause a 0.5-1.0V or higher voltage drop to the battery.


Now if you could run either an extra +12V power wire else another signal wire (meaning from the alternator charge lamp or charge sensor), you could avoid the diode.