So here is how to do it the safe way using only a single pole single throw switch.In the circuit diagram, you will see that the auxilliary battery is connected in parallel to the 12 volt battery through a pair of headlamps, one in the positive lead and one in the negative. With the "24 VOLT" switch open, the charge on the additional battery will be the same as the 12 volt starting battery because the headlamps, which carry a couple of amps when lit, will trickle charge the additional battery to the full voltage. Under normal circumstances, the headlamps will only have a fraction of a volt across them so except under heavy charging/discharging conditions they will stay off or have a very dim glow.
There will be 12 volts going to the 24 volt load when the switch is in the 12 volt position but since the headlamps are in series with the circuit, if the bow thruster were turned on, only a couple of amps would flow and the headlamps would light. When you close the 24 volt switch, the batteries are now in series and 24 volts is available for the bow thruster. While in the 24 volt mode, a couple of amps will flow through each headlamp and they will come on full brilliance. The amount of energy wasted, however, is small compared to the battery capacity and the few amps through them does not materially diminish the high current available for the thruster.
COMPONENT SELECTIONThe BATTERY capacity of the auxiliary will be a function of how long you think you will need the bow thruster in one session, and how often sessions will occur. Refer to the bow thruster current requirements to determine this figure. For example, if the bow thruster draws 50 amps (about 1.5 horsepower) and you need to be able to run it for up to 30 minutes, that comes to 50 x 0.5hr = 25 amp hours. You should double this figure to provide a safety factor and reduce cycling the battery below the 50% charge level. So a 100 amp-hour automobile battery for about $35 would be ideal. You should match the chemistry of the battery to that of the starting battery - don't mix an AGM or GELL battery with a WET lead acid. Otherwise the batteries can be of different ages, manufacturers or style. You don't need a deep cycle battery here - the usage is more like that of a starter motor battery in an automobile.
Charging time after a typical use can be calculated by dividing the amp-hours used, say 25 in a 30 minute period, by the charging current, say 4 amps = 6 hours, or 12 times the discharge time.
The HEADLAMPS should usually be the highest wattage you can find. In fact I use a high/low beam headlamp and wire the high and low terminals together so both filaments are in parallel. The high temperature will reduce life but since they are only going to be on for a few minutes a day maximum, who cares? A major advantage of using headlamps to limit the current is that the resistance is non linear. As they cool down the resistance goes down dramatically and they tend to draw a constant current for charging even though they are not brightly lit. You should consider placing the headlamps so the light is visible from the bow thruster control panel location so you don't forget to switch back to CHARGING when the bow thruster is no longer needed, otherwise the headlamps will eventually discharge the auxiliary battery.
The SWITCH should be sized for the maximum current of the bow thruster plus a safety margin. For an economical installation, a simple battery disconnect switch is ideal. If you want a remote control, then you should use a simple single pole, normally open relay instead of the switch. We have a 130 amp relay in our Parts and Kits catalog that will handle many thrusters.
The CABLES should be sized as recommended by the bow thruster manufacturer with regard to the length of the run. Note that only the cables to the switch have to be this size. The cables to the headlamps only have to carry a few amps so a 12 or 14 gauge wire would be adequate.