Trickle charge trailer battery

Morning all,

Been thinking for a while how to solve a problem, my old man regularly tows a tipper trailer that has a battery to power the hydraulic pump. The only way the battery gets charged is when someone actually remembers to hook a battery charger up to it. Tows it with his stock standard navara

Im trying to work out a way to maybe just trickle charge the battery through the spare pin on the trailer plug. Basically I need something that is simple, automatic and won't require any work on his behalf.

A mate told me about a setup he had seen for similar ideas, boat battery. This setup was just a fused wire with a diode, nothing special and only a small guage wire, something similar to the lights or trailer 5core wires. This seems a little over simplified to me.

Anyone have any ideas?

Cheers, Brett

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A basic setup would be, run at least 6mm2 wire (10mm2 would be better) from the battery in the Navara through an anderson plug to the battery in the trailer.

You will also need to use a couple of 50 amp circuit breakers mounted as close as possible to each battery.

If the vehicle will sit (engine not running) for a while with the trailer connected and you are concerned that the Navara's battery may be flattened by the trailer you will also need to add a constant operation solenoid that will isolate the two batteries when the engine is not running.

If you have some spare coin mount a Redarc DC-DC charger near the trailer battery and you will have a good setup.

Your mates setup is not ideal because of VD (voltage drop) the wire size is too small and standard trailer plugs aren't designed to handle large current. A diode will also have a minimum of .6 voltage drop across it.

With your mates setup you would be putting minimal (if any) charge into the trailer's battery and would run the risk of overheating the wiring or melting the trailer plug.

What a great responce Rodney :thumbup:

does the trailer spend much time hooked up to the Navarra? If only rarely hooked up, I wonder if enough time will be available to allow a charge?

What about a bit of lateral thinking? If the battery is not needed to operate a breakaway braking system, why not run a heavy enough wire to the rear of the tow vehicle to run the pump, anderson plug, and then from trailer anderson plug to the pump motor. Do you know what current the pump draws?

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Originally Posted by Yendor

A basic setup would be, run at least 6mm2 wire (10mm2 would be better) from the battery in the Navara through an anderson plug to the battery in the trailer.

You will also need to use a couple of 50 amp circuit breakers mounted as close as possible to each battery.

If the vehicle will sit (engine not running) for a while with the trailer connected and you are concerned that the Navara's battery may be flattened by the trailer you will also need to add a constant operation solenoid that will isolate the two batteries when the engine is not running.

If you have some spare coin mount a Redarc DC-DC charger near the trailer battery and you will have a good setup.

Your mates setup is not ideal because of VD (voltage drop) the wire size is too small and standard trailer plugs aren't designed to handle large current. A diode will also have a minimum of .6 voltage drop across it.

With your mates setup you would be putting minimal (if any) charge into the trailer's battery and would run the risk of overheating the wiring or melting the trailer plug.

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Thanks mate, that was pretty much exactly my thoughts. I was hoping to get away with something simple, but I don't think I am.

I understand how a proper system should be wired up. Have not long since finished wiring in my own dual battery system and one for a mate. Just a simple one with VSR isolators.

Whilst the Anderson plug idea is how I would prefer to do it, unfortunately its just something else my old man will forget to do. Hence the idea of using the trailer plug, being something he knows, and has to do anyway.

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Originally Posted by Silver

does the trailer spend much time hooked up to the Navara? If only rarely hooked up, I wonder if enough time will be available to allow a charge?

What about a bit of lateral thinking? If the battery is not needed to operate a breakaway braking system, why not run a heavy enough wire to the rear of the tow vehicle to run the pump, Anderson plug, and then from trailer Anderson plug to the pump motor. Do you know what current the pump draws?

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I like what your thinking, unfortunately I don't think it would quite work. The trailer does sometimes get used (hydraulic pump run) without being connected to a vehicle, as well as being used on multiple different vehicles.

The trailer is connected to the Navara for a fair amount of time. Being a builder, he uses the trailer almost daily.


Im trying to think of some sort of dc-dc converter, or current restriction device. Fused from the starting battery, throught a relay controlled by the ignition circuit, maybe get carried away with a VSR set up. through the current restricting device, then on to the trailer plug, to the battery.

The only hold up is the current restriction, or dc-dc converter.

Im not interested in spending a great deal of money on this either which doesnt help.


Thanks for your input and ideas.

Cheers, Brett

an alternative is to change the trailer plug on the both trailer and navara over to 12 pin flat and use the high current terminals dedicated for a hot wire. is it by chance a 7 pin flat now?

Wack a solar panel on it, doesn't need to be a large one if the trailer is being used almost every day

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Originally Posted by philfree

an alternative is to change the trailer plug on the both trailer and navara over to 12 pin flat and use the high current terminals dedicated for a hot wire. is it by chance a 7 pin flat now?

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I did think of that. What sort of current are they rated to do you know?

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Originally Posted by itchyvet

Wack a solar panel on it, doesn't need to be a large one if the trailer is being used almost every day

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Not a bad idea that... I wonder how well those small $50 solar panels from auto barn would go...

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Those small panels aren’t really for charging only for maintaining full charge. the 12pin flat are rated for 35amp(the 5 larger pins) constant load and the 7 pins are 15 amp, you would be better off going a large 7 pin round made in diecast as they are rated 40 amp continuous but im not sure of the plastic constructed large 7pin round though. but again it depends on the frequency the trailer gets used, a ctek250s dual dcdc charger will be 1/2 the price of a redarc and you can whack a panel on that also

I fitted a small solar charger to my forklift and it was enough too keep the battery fully charged. Before I fitted it I was jumping the thing every third or fourth day. Might be a cheap and easy alternative?