Charging 2nd Battery from Tow Lights

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I tow a small camping trailer on which I have a 12v spare car battery for charging batteries and for emergencies. I'm thinking I can keep it charged just by running two leads from the trailer lights. I know it will only get current when the lights are on and that I must disconnect when they are not to avoid drain. Is this an OK thing to do.

1997 Land Cruiser 130,00 miles
Cheers,

Mike Stevens
 
I just ran a wire from my main battery to the trailer battery to charge it. It's charging anytime the trailer plug is connected. I used 10ga (or 12 ga? forgot) to act as a current limiter.
 
Even if the voltage were high enough to charge a battery, those wires aren't big enough to support any real current (you'll likely blow fuses).

Not recommended.
 
I just ran a wire from my main battery to the trailer battery to charge it. It's charging anytime the trailer plug is connected. I used 10ga (or 12 ga? forgot) to act as a current limiter.

That's a bold statement. In case there is a short circuit in the wire between the cruiser battery and the trailer battery, there is a great fire risk.:bang:

This wire has not the same properties as the OEM fuse link.

You must protect this circuit by hooking the wire to the vehicle battery through a 40 Amp breaker (which will trip right away if the trailer battery is completely depleted). If the breaker is the type that autoreset, then it will cycle on and off until the battery reduces it's charging current demand.

By the way I don't recomend using the standard trailer connector because it will melt the positive carrier pin in the socket. You must use a separate heavy duty plug for the battery charging circuit.

If you don't mind to charge the battery on a slower pace and want to keep your standard trailer socket, a old (and cheap) trick to regulate safely the charging current is to get a single filament turn signal bulb (and it's holding socket) and hook it in series with the wire that goes to the positive trailer battery (at the trailer).

When the battery is fully depleted the bulb will illuminate brighter and once the battery reachs it's full capacity the bulb will be dimmed or off at all so it works also as a charged battery indicator. Of course keep the bulb in a waterproof enclosure which can stand the heat created at first by the bulb.
 
Thanks:

Will method described above by robmir require that I do NOT park with the trailer still connected to the cruiser with the lights off else the trailer battery will illuminate the trailer lights and drain? IE when parking I must disconnect trailer electrics?

Mike
 
That's a bold statement. In case there is a short circuit in the wire between the cruiser battery and the trailer battery, there is a great fire risk.:bang:

This wire has not the same properties as the OEM fuse link.

You must protect this circuit by hooking the wire to the vehicle battery through a 40 Amp breaker (which will trip right away if the trailer battery is completely depleted). If the breaker is the type that autoreset, then it will cycle on and off until the battery reduces it's charging current demand.

By the way I don't recomend using the standard trailer connector because it will melt the positive carrier pin in the socket. You must use a separate heavy duty plug for the battery charging circuit.

If you don't mind to charge the battery on a slower pace and want to keep your standard trailer socket, a old (and cheap) trick to regulate safely the charging current is to get a single filament turn signal bulb (and it's holding socket) and hook it in series with the wire that goes to the positive trailer battery (at the trailer).

When the battery is fully depleted the bulb will illuminate brighter and once the battery reachs it's full capacity the bulb will be dimmed or off at all so it works also as a charged battery indicator. Of course keep the bulb in a waterproof enclosure which can stand the heat created at first by the bulb.


dude, I meant current limiter as in increased resistance, not fusible link... And I put fuses inline too at the battery...

and how do you think all these 7 pin plugs for trailers work...?
 
I'll combine your idears.

I'll use and extention cord from my battery to a plug on the wires from the trailer's battery. I will put a 40amp inline fuse at the battery and wire a cheap 12v bulb into the +ve at the battery end for safty and to monitor chargheing. Souns OK?
 
dude, I meant current limiter as in increased resistance, not fusible link... And I put fuses inline too at the battery...

and how do you think all these 7 pin plugs for trailers work...?

And you believe a #10 or #12 gauge wire, will limit the current because it will have a increased resistence ?

I am sorry dude, but a #10 wire can carry up to 40 amp which will be enough to fry the plug no matter if it's the 4 pin or the 7 pin plug.:grinpimp:

You HAVE to limit the amount of current relayed to the trailer aux battery because a fully depleted battery can draw more than 50 amps.
 
Mike: I use for all my medium power requirements, extensions made with the small quick connector sold by Warn and #8 wire. This connectors are capable to handle more than 40 amps without any risk.

My Engel 60 qts refrigerator is powered through this type of connectors. When I camp with my roof tent I also use one of this extensions hooked to a small metal box on which I installed 4 standard female lighter sockets to plug on them any 12 volt accesory I carry upstairs.
 
And you believe a #10 or #12 gauge wire, will limit the current because it will have a increased resistence ?

I am sorry dude, but a #10 wire can carry up to 40 amp which will be enough to fry the plug no matter if it's the 4 pin or the 7 pin plug.:grinpimp:

You HAVE to limit the amount of current relayed to the trailer aux battery because a fully depleted battery can draw more than 50 amps.


it is true I would not put a fully discharged battery back there. I always charge the trailer battery before connecting it to the truck to avoid high currents. And, I've put a fuse inline that is rated for less A than the wiring. Never blew to date.

So, what kind of current limiter do all these trucks with 7 pin plugs out there have then if you think the pin will so readily melt? What's the battery equivalent inner resistance?

(There are also these little thermal protectors / breakers one can put in line with the charging circuit. I have one on my trailer battery as well.)


Mike, be sure that the wire you use is compatible size wise with the fuse... And yes, the bulb approach is a good one, much higher resistance. Lessee, say a 12 W bulb (e.g. rear light), so 1 A so 12 ohms when fully lit. Say your trailer battery is really low, you would have 2V across, so 0.2A, easily handled by just about any wire. (Actually the resistance would be lower at lower temperature so higher current, but then again there is also the battery resistance.) Problem with that, of course is that at 0.2A, you'll need to leave the engine running for a week to charge your battery.... :)
 
In a perfect world, I would attach an alternator to the trailer's axle and charge a battery (or 2) that way.
 
OK guys and thanks again.

I used, from the cruiser main battery to the trailer 16 gauge, it was in fact a cheap 15' electrical extention lead. For safety I put a auto-reset 30 amp breaker at the front end, at the cruiser battery, and the single filament indicator light in series at the battery end.

This seems to include the safety issues you have raised.
 
OK guys and thanks again.

I used, from the cruiser main battery to the trailer 16 gauge, it was in fact a cheap 15' electrical extention lead. For safety I put a auto-reset 30 amp breaker at the front end, at the cruiser battery, and the single filament indicator light in series at the battery end.

This seems to include the safety issues you have raised.


well, did you check the current limit for 16ga? Varies depending on insulation, stranding etc, of course, but IIRC 16 ga is usually talked about for less than 20A current for 12V apps. If that is true -not sure- your breakers are too big. If it's an extension cord, you could run both leads in parallel to double the rating (and then get ground in the rear, although that's not great).

I'd be curious to find out how long it takes to charge a battery with the bulb in series. Care to do an experiment?
 
In a perfect world, I would attach an alternator to the trailer's axle and charge a battery (or 2) that way.

and then you can have fun towing the trailer around the campground to top off the battery for the night lights... well assuming your axle is spinning... :D
 
e9999 you are right I think regarding breaker. My wire is rated at 13 amp so I need change the 30 amp breaker for a 10. I'm now really thinking the second battery should be under the hood and not on the trailer. You got me scared of fire risk if my 16 gauge wire overheats. It is a tree wire lead so I could double-up on the hot side.
 
e9999 you are right I think regarding breaker. My wire is rated at 13 amp so I need change the 30 amp breaker for a 10. I'm now really thinking the second battery should be under the hood and not on the trailer. You got me scared of fire risk if my 16 gauge wire overheats. It is a tree wire lead so I could double-up on the hot side.

dammit, why is everybody so worried about fires...? :)
if you have a bulb in series, the resistance will be huge. No way that wire will get 13A unless you short the bulb somehow. (Also if the 13A is for 120V AC, would be different for DC.) But even then, if you have the breaker or fuse matched to the wire you've got extra protection so should be fine.
 
oh lord the things I could say but shall refrain.......................
 
Here is the final setup. Starting with the +ve terminal of the Cruiser battery is a 15amp auto-reset breaker. Then down a 16 gauge 20' extension cord run under the vehicle ending with the female plug at the tail of the Cruiser. On the trailer a 2' length of extension cord starting with the male plug to connect to the Cruiser goes into a single filament side running light on the front of the battery compartment and then on to the +ve terminal of a heavy-duty deep cycle battery. The -ve terminal feeds back to the -ve terminal of the Cruiser.

Didn't try it yet.
 
What about a set of Anderson plugs. I have one on the back bar of the Cruiser and one on the draw bar on the caravan. Hook them together and charge the batteries in the van. If the batteries in the van get a bit low it's just a matter hooking up with the motor running and putting some power back into the batteries. Usually do it when running the engine to heat the water through the glind for showers.
 

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