Run at least a 10 gauge wire directly from your truck battery, all the way to your trailer battery... but keep reading, there’s more to it.Now, of course, this circuit must travel through your trailer’s electrical plug (the plug connecting your trailer to your towing vehicle). Your male/female trailer plugs probably have one large connector out of maybe four, five, or seven. It’s tempting, but do not use this single large connector in your plug for this trailer battery charging circuit. This large connector should be used for the needed ground. Don’t trust the ball to coupler’s (or pintle ring to hook) metal to metal connection for the ground. If you do so, your lights will probably flicker, and there may be excessive impedance created between the ball and coupler. The resulting voltage drop will not allow the trailer’s battery to fully charge. Remember, any electrical energy sent to your trailer must return to it’s source. That’s why it’s called a circuit, and that’s why your battery has two connections. So, at any one time, you may have low trailer battery voltage (maybe 8 to 10 volts), you may have your marker lights on, and you may have the brakes applied (that means the brake lights are on). That’s a lot of current (or amperage) being drawn by the trailer over three separate circuits simultaneously. Now, all that current going out (+) needs to return to the towing vehicle over one single circuit. That one circuit is the ground (-). Three small wires out, one large wire in. Keep reading... there’s still more. Running the 10 gauge line directly from your towing vehicle’s battery to your trailer’s battery will only serve to equalize voltage from one battery to the other. If your towing vehicle’s engine is running (and the alternator is charging), there should be about 14.7 or so open circuit volts in the system. If the trailer’s battery is down to anything less than this 14.7 value, there will be a flow of current to the lower voltage battery. It’s a lot like water seeking its own level. Eventually, if all is correct, both batteries will become fully and equally charged. But the problem is this: If you sit for a time with your towing vehicle’s motor off, and you draw down the voltage in the trailer’s battery, then that 10 gauge line will draw the voltage down in the tow vehicle’s battery too. You will then have two dead batteries. Therefore, when the tow vehicle’s motor is not running, you need to isolate its battery from the trailer’s battery. You can build (buy) a very simple, small, and cheap battery isolator -- and one that does not cause any voltage or current drop. Simply install a relay in the 10 gauge line. This relay goes anywhere between the tow vehicle’s battery, and the female trailer plug (located at the back bumper of the tow vehicle). Best place is somewhere on the wheel well or fire wall. An 18 or 20 gauge control line should work well. And, make sure the relay is a 100% duty cycle type. This relay is activated by the accessory circuit in your tow vehicle (same circuit that turns the radio on and off when you turn the motor on and off). A relay draws very little current, but will pass a lot of current. A starter solenoid is a relay, and starters draw 300 plus amps. That’s more than enough to weld with -- but a starter solenoid is activated with just a little switch, and light little control wires. Finally, you will need to install a fusible link in the 10 gauge. Anything less than 60 amps will probably not work very well. You may even need more than 60. You can use two or three 30 amp fuses connected together -- just make sure you connect them in parallel (all positives connected together, and then all negatives connected together, instead of positive to negative to positive to negative etc.). If you start the motor in the tow vehicle, and its battery is fully charged, and the trailer’s battery is very low, most all the current from the alternator will go to the trailer battery. There you have it.
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