Charging battery from tail light lead?

[Hennesseystealth]

Is it possible to tap my camper's tail light wiring to charge my battery? What I can't find is a controller that would go between the 12v source and the battery to prevent overcharging. My idea is that the battery would charge when I was driving and had my headlights turned on. I only have a flat 4 connector for the lights since I don't have brakes.

[KennethW]

Yes and No , You can use the tail light lead to power a small relay (like the ones in your cars fuse box)using a power wire from your car(power outlet). The tail light lead does not have the power to charge a battery. If you really use up the camper battery One should run a wire from the car's battery to the relay. The relay can be wired in to the trailers wiring so it can work on any car that is towing it.
I have done this with a power outlet wire(cigarette light end) that lead thru the back hatch and it works.

[Dale M.]

If the trailer battery is in good shape, and the tow vehicle charging system is good, there is very little chance of overcharging the battery..... The charging while towing concept is good, a battery will only accept a charge to where it chemically reaches a saturation point of energy it can store according to VOLTAGE APPLIED BY CHARGING CIRCUIT.... Only thing I would be concerned with is having both batteries connected while engine is off and over extended period of time and two batteries connected together and BOTH batteries drained to a point where you can not start tow vehicle..... Simply pulling connector (between car and trailer) while parked or having a relay (disconnect) in tow vehicle system that disconnects trailer battery from TV while engine is off is all you really need for control circuit.....

To properly charge trailer battery you need a circuit that can handle 30-40 amps between tow vehicle battery the includes a some sort of cut off and a fuse of large enough capacity to hand initial inrush of current and batteries try to equalize when first connected....

It has been covered in extreme depth here....

viewtopic.php?f=30&t=28620

Dale

[MtnDon]

Consider installing a separate positive and negative wire using a 2 pole connector. They are available in #10 wire good for 30 amps. Couple that with a continuous duty relay that only closes when the engine is running. Add a fuse in the dedicated #10 wire from the battery, as close to the battery as possible.

[KennethW]

Am I wrong in thinking that connecting a trailer battery to a TV the battery in the trailer will only charge at the rate that the TV's battery requires. So unless your TV's battery is very low the trailer battery will be charged at a low (amperage) rate.

[Hennesseystealth]

Thanks for all the information. I think I will postpone this until I upgrade to brakes and the 7 pin connector. That will give me the extra 12v lead to run from the battery.

[H.A.]

As others said, taillight as a battery charging circuit could work, sort of...
One thing could be a problem. Without manual or automatic isolation, when the car is switched off, the trailer would put power back into the lighting circuit of the car. Depending on wiring configuration of the car, could illuminate all or a portion of the cars lighting as well as the trailer lights.

Possibly create additional problems to the car as well.
(I say "possibly" as I am old-school in regards to vehicles. Cars I have good expertise of are 20 years plus...)
Some cars nowdays have electronic supervision of taillights and such, Back feeding power into a lighting circuit might be bad for the cars electronics. (Rumours too of CAN BUS type controls of some lighting systems ??)
Lastly, Many new cars are LED taillamps, At the least, I expect their wiring would be sized only to accommodate the LED.

There is something to said for keep it simple & old-school regards to trailer charging...

[Hennesseystealth]

As others said, taillight as a battery charging circuit could work, sort of...
One thing could be a problem. Without manual or automatic isolation, when the car is switched off, the trailer would put power back into the lighting circuit of the car. Depending on wiring configuration of the car, could illuminate all or a portion of the cars lighting as well as the trailer lights.

Possibly create additional problems to the car as well.
(I say "possibly" as I am old-school in regards to vehicles. Cars I have good expertise of are 20 years plus...)
Some cars nowdays have electronic supervision of taillights and such, Back feeding power into a lighting circuit might be bad for the cars electronics. (Rumours too of CAN BUS type controls of some lighting systems ??)
Lastly, Many new cars are LED taillamps, At the least, I expect their wiring would be sized only to accommodate the LED.

There is something to said for keep it simple & old-school regards to trailer charging...

If I tried this approach, I would definitely include a battery isolator and relay. Less than $20 part and fixes the drain issue.

[MtnDon]

Am I wrong in thinking that connecting a trailer battery to a TV the battery in the trailer will only charge at the rate that the TV's battery requires. So unless your TV's battery is very low the trailer battery will be charged at a low (amperage) rate.

First I should say I have never stopped to measure the charge currents on either the TV or the trailer/coach batteries. I've always used a continuous duty relay to act as the isolator; it gets activated when the TV engine runs. All I can say is that in over 40 years of connecting one or two batteries in a trailer or a self contained RV (coach battery) to the charge circuit in the TV I have never had a problem with the depleted batteries not being charged even though the TV battery was quite full. I have looked and can not say that the TV battery ever used more water than expected. Ditto the trailer/coach batteries. My guess is that if the coach/trailer batteries were depleted more than the TV the current flowed mainly into the depleted batteries. Just a guess.

Anyone else have similar experiences or have a battery problem when doing something similar?

In theory I would expect the battery that was already full to use more water or that the lower charge batteries would not receive a good charge. In an off grid system with charging coming from a solar charge controller the usual thing to happen is that the CC "sees"the higher voltage reading on the charged battery and cuts back the charge, leaving the lower battery only partly charged. Not sure how an automotive system reacts. But again, I have not had battery issues with the RV batteries over many years. At least no issues since I stopped using the so-called RV/Marine batteries.

[Dale M.]

Am I wrong in thinking that connecting a trailer battery to a TV the battery in the trailer will only charge at the rate that the TV's battery requires. So unless your TV's battery is very low the trailer battery will be charged at a low (amperage) rate.

Yes you are sort wrong in the thinking ... Almost all modern auto charging system put out a pretty specific voltage (usually 14 volts (+/-) any battery in the system will try to charge to that voltage when hooked in parallel... IF the voltage in charging system (alternator) is correct all batteries connected in parallel will charge to its maximum capacity unless there is physical problem in a specific battery and battery has the time to reach full charge. The condition of the individual battery charge is dependent on the specific battery IF charge voltage/current is correct....

A lot of information about batteries and charging here....

http://www.batteryfaq.org/

Dale