I saw several posts about it, but still i have any questions...
I see the backup battery is connected directly to main battery... so, is the backup battery always on charge...There is nothing to cut power on backup battery full charged??
Is not possible that backup battery fryes then, or main battery get discharged, if you have the car's engine off any days?
I saw ricky's relay circuit too, but i can't understand how relay is connected.
I see positive of relay's coil connected to after the started key +, and negative of the same coil to ground... but, how is connected main and backup battery?Is that a relay or a commutator?
And in this circuit...backup battery is not discharged to main battery across the resistor when crank?
Check the FAQ Emporium on this site for a section on Tank circuits. One bit of advice, though. If you don't understand the circuit, reconsider whether you have the skills to install it.