ok here it is...Originally Posted by gyro
first of all the idea is this (i may not be 100% accurate on the small things as i had this setup for quite some time now, if you found some plz. inform me. also i made this possible with the help of robiewp, another member):
1. tank batt. will be only connected to car batt. if the engine is running (for charging).
2. and if the engine is not running, unless i turned on car key to acc. the two batt. should not be connected to each other.
...your 2nd batt. should be deepcycle as it is designed to be drained and charged again and again. the normal car batt. is made only to crank an engine.
a typical relay such as horn relay has 4 or 5 pins (30, 85(ground), 86, 87a, 87):
my relay (30amp./5pin) setup is connected like this:
main (car) batt. + ---> 30
main (car) batt. - ---> ground
+12v present ---> 86 (ex. car key is in acc. or from cig. ligther)
2nd (tank) batt> + ---> 87
2nd (tank) batt> - ---> ground
since i don't have a solenoid or diode, i control the relay manually, check the pics, it is in the middle of the 3 socket cig. lighter, remember, 2 batt. in parallel always goes equilibrium when connected. if your 2nd batt is very low while running the car and you stop on red light and your main batt is below 13.5v, your engine will hesitate. if you're on an uphill it may even stop, so some kind of relay cutoff is very important.
i can't use diode for two reasons: first it reduces voltage comming in and 2nd bec. of the high amperage involve.
rather than spending money on some very expensive off-the-shelve technology, i spend more on research and ideas. i have found a small kit that signals when voltage goes low. i can use it to control the relay so when main batt goes low to say 13.5 (in my case) it will cut the supply of +12v to the relay, therefore cutting off the supply of power to the 2nd (tank) batt.