12 or 24 volt M37's
What a subject. First, lets discuss hooking up a 12 "Load" to just one of the two batteries that make up a 24 volt system. Bad Idea. When you use just one of the batteries which are in series, you drain one more than the other. The 24 charging system (which actually runs around 27.5 to 28 volts) compensates and increases the charge rate. This will overcharge the battery you're not draining the 12 volts off of. A constantly overcharged battery dies very quickly. Very common to use what are called "Dropping Resistors" to compensate for the 24 volts on a 12 volt load. While motors such as fans, starters, and wiper motors have what is called dynamic resistance because on their inductive load, items such as bulbs, regardless of size, are resistive in operation and can be measured for their total resistance with a multimeter. Fan and wiper motors are so small a simple resistive check using ohms on a multimeter will give you a quick and close enough measurement of their resistance to calculate dropping resistors for them. The reason you would measure them is to determine their resistance and calculate the size wattage and ohms of the dropping resistor needed. Headlamps are two element items so the resistor would be best installed in the ground leg. This would provide the dropping needed for either low or high beam. Generally headlights are 50 watts at roughly 13.8 actual volts. This means they generally draw a little over 3 amps per headlight. Watts being determined by amps times voltage. You would need two 50 watt 4 to 5 ohm resistors to operate your 12 volt headlights on a 24 volt system. I would recommend 75 watt as they will last longer under the load. I have seen folks use snow plows and other heavy or high current loads at 12 volts by running a wire to one of the batteries. Nothing but dead batteries, won't start because of low batteries, etc. etc... I really don't recommend it because of all the problems to the truck and batteries. My early background is electrical and electronics, if I can help let me know what your situation is.