I have a switch on my boat which has four positions: Off, 1, Both, 2. 1 refers to battery bank 1, 2 refers to battery bank 2. The batteries are charged via the engine alternator. The centre of the switch main cable goes to the starter and all the other circuits, i.e. either bank 1 or bank 2 can be used to start the engine and power the circuits. Another main cable goes to the earth of the engine block. Another main cable is attached to the alternator and to bank 1. There is a battery combiner between bank 1 and bank 2, i.e. amps coming from the alternator will go to both batteries or which ever battery needs charging. My usual mode of practice is to use bank 1 for starting the engine, then turn off the engine and start sailing, then switch (going through Both) to Bank 2. Bank 1 is a starting type battery (thin plates). Bank 2 is a deep cycle battery (thick plates). My problem is that sometimes I forget to switch to Bank 2 !! This can run down Bank 1, which I need to preserve for just starting. I wondered if a circuit could be designed that would light a warning light all the time I was using Bank 1 ? I could install this warning light in the cockpit.