Split Charge Trailer Battery Isolator Install

How to Isolate a Trailer Battery from the Tow Vehicle When the Engine is Off

Travel trailers, toy haulers, dump trailers, trailers with winches, and other trailers that contain their own power source typically rely on an auxiliary charging circuit so that the tow vehicle will recharge the trailer's battery supply while the two are hitched. The process is known as split charging. This maintains the trailer's battery charge and, in some instances, eliminates the need to frequently recharge the trailer battery with a standard battery charger. However, a bad or weak trailer battery may rapidly deplete the charge on the tow vehicle's batteries while the vehicle is off. To prevent this, a relay can be used to turn the 12 volt auxiliary power circuit to the trailer ON when the vehicle is running and OFF when the vehicle is parked. The circuit is rather simple and the modification is relatively inexpensive.

Split Charging Battery Isolator Circuits

All that is required to build this circuit is a SPST (single pole, single throw) 12 volt relay, some automotive wire (14 AWG will suffice), and an assortment of terminals. The switched terminals of the relay are spliced into the 12v auxiliary power circuit of the tow vehicle and the coil of the relay is wired to a 12v source on the tow vehicle that is hot only when the key is in the "RUN" position. All vehicles have empty circuits in their fuse box and thus is not particularly difficult to find a power source with a multimeter or probe light. A typical circuit looks something like this:

split charge circuit diagram connected to battery

Split charge circuit wired directly to the vehicle battery

Keep in mind that on a dual battery system (all diesel vehicles) the 12v+ recharge circuit only needs to be connected to ONE positive battery terminal since the batteries are connected in parallel. Alternatively, the auxiliary charge circuit may be wired to the 12v+ output terminal of the alternator (this is common). This is recommended if you are running a new circuit to the trailer plug or are replacing an existing circuit. An example circuit would then be represented in the following figure:

split charge circuit diagram connected to alternator

Split charge circuit wired to 12v+ alternator output terminal

We're using a 12 volt, 120 amp marine grade relay. This is extremely overkill for this application but will make for a very reliable system. Our 12v auxiliary power circuit is protected by a 20 amp fuse. In selecting a relay, you need to find out what size fuse is on this circuit and select an automotive/marine relay that reasonably exceeds the fuse size. I recommend sourcing a relay that is at least 10 to 20 amps higher than the circuit's fuse rating. This way, a larger fuse could be used on the circuit if ever necessary without needing to upgrade the relay. The relay you select must be a normally open SPST. A normally closed relay will not allow the trailer to be charged while driving and may drain the batteries when the tow vehicle is off. We recommend one of the following relays for split charging:

Part Number


Voltage Rating

Current Rating (amps)


SPST, normally open

12v DC

120 amps


SPST, normally open

12v DC

200 amps

As previously mentioned, these are somewhat overkill but will make for a reliable split charge circuit and are similarly priced to automotive relays with a much lower amperage rating. They also feature 1/4 inch studs for the switching side of the relay, which makes for a very reliable connection compared to the spade connectors that lower rated relays often use. With regard to fuse size on the charging circuit, refer to the vehicle, trailer, or brake controller manufacturer for recommendations. If in doubt, select a fuse by the ampacity (max amperage rating) of the circuit's smallest wire size.

Note that a numerically higher wire gauge is physically smaller. For example, a 14 AWG (American Wire Gauge) is smaller than a 12 AWG. If the wire at the truck/trailer connector is 12 AWG but the circuit running from the tow vehicle's battery to the connector is only 14 AWG, the size of your fuse must not exceed the maximum amperage rating of the 14 AWG wire. To eliminate confusion, a circuit cannot be protected by a fuse that exceeds the wire's maximum amperage rating. Ampacity ratings vary by wire/insulation type and the length of the circuit.