OT: How to charge battery on the trailer?

I have a trailer that has a battery powered hydraulic system on it. I want to hook it to the truck charging system to keep it charged. My first
thought is to run a small wire(8 gauge?) from the truck battery to the trailer battery. The problem I see with this is that when the trailer battery is being used and starts losing power, the hydraulic system would begin pulling power from the truck battery. This drain would go thru the 8 gauge wire and burn it up. Do I need to run a large battery cable all the way to the trailer? What size? Am told welding leads are good for this. Also told that 4/0 copper entrance cable would be better, but very stiff to work with. What kind of disconnect plug would be good? Would appreciate any suggestions. Thanks, Chief
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A local club has a golf cart with a solar cell on the roof. Seems like it hardly ever needs to be plugged in.
Steve.

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Around here solar trickle chargers are real popular for this kind of application. You can get them at RV supply stores.
Of course the one thing we've got lots of around here is sunshine.
--RC

You can tell a really good idea by the enemies it makes
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What I would do is put a relay on the trailer between the 2 batteries. The batteries would be connected through normally-closed contacts and the relay would be actuated when the hydraulic system is operated. I.e., the batteries are connected only when the hydraulics are *not* operating. Thus the connection is only used to charge the trailer battery.
I'd say that you'd only need a size of cable and relay necessary for the charge current, but I don't know how you would limit it. Maybe somebody has an idea for this?
Bob
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Hey Bob,
A decent diode will fix one problem you foresee, but won't fix the opposite situation. Also, in general, it's not a good idea to have two batteries in parallel and expect them to charge equally, or even properly. You have to install a special "dual battery charge" device.
What you can do for under 50 bucks, is install a trickle charger to the trailer battery, run off an inverter from the vehicle cigar-lighter, or as they are called now, the "power outlet". On my Chrysler van, the rear "power outlet" as sent from the factory, is controlled by the ignition switch, so the inverter would only work while your vehicle is running.
Take care.
Brian Lawson, Bothwell, Ontario. XXXXXXXXXXXXXXX
wrote:

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You need to emulate what they do on an RV where you have a "chassis battery" (the truck battery) and a "coach battery" (the trailer battery).
The first thing you need is a battery isolater. This allows the truck alternator to charge both batteries when the engine is running, but when the engine is off the trailer battery won't suck current from the truck battery. Battery isolators are available both in relay form and diode form but in either case they work automatically, you don't have to switch them. Search around the web a little on Battery Isolater and you'll find lots of info on them that will help you pick one out.
Put an automatically resetting circuit breaker in the line from the battery isolator to the trailer battery. This is something you can get from NAPA for a reasonable price. Size the circuit breaker depending on your wire size. If you search the web on "12 volt systems" you'll find plenty of info on current carrying capacity for different wire sizes.
Run a ground wire from the truck to the trailer also, don't depend on the hitch for that. Use the same size ground wire as you use for the charging wire.
Good luck-
Paul T.
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wrote:

Do a web search for "battery isolator". You can get them at most auto parts stores. This one is typical http://www.pplmotorhomes.com/parts/rv-converters/rv-battery-isolator.htm
I've done what you intend, used wire on hand, probably about #8. Used a large (10 pin IIRC) trailer connector. I may have used more than one pin for the charging circuit.
Wayne
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I forgot to add a few more things- use a standard trailer connector for both the tailights and charging wire to the trailer. The "7 pin" trailer connector is the smallest size you would want to consider using, you can get a set pretty cheap at any RV or trailer store. Don't buy one of the cheapo ones that has plated contacts as they will corrode, only get the ones with solid brass contacts. The 7 pin connectors have one large connector pair that you can use for the charging wire.
Since the only current going through the charging wire will be charging current from the alternator (the battery isolator will keep you from pulling big current from the truck battery), it doesn't have to be super heavy since it probably won't see more than 30 amps or so. 10 gauge is probably ok, I'd use a 30 amp circuit breaker with that.
Good luck-
Paul T.
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Go to the nearest RV dealer and buy a trailer battery isolator. Your problem was solved long ago by the RV people. The isolator will allow the truck alternator to charge the trailer battery, but will not allow the trailer electrical load to discharge the truck battery. The conductor from the truck to the trailer battery will only need to be big enough to accommodate the maximum charging current, probably #10 will be enough.
Randy

want
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spake the words:

A relay inline with a 10ga wire from a switched hot lead from the alternator would protect your wiring from that kind of misuse.
Alternatively, put an inline circuit breaker on the trailer where you can see and get to it. And tag the equipment switch on the trailer to "Disconnect the trailer cable from the truck before using!" just in case "somebody" forgets. ;)
Schematics here: http://www.etrailer.com/faq/wiring.asp

Use a standard 6-wire trailer connector for the thing. You're charging batteries, not running several horsepower worth of equipment through it.
When you run the hot wire back, make sure you avoid little things like the plastic fuel line and brake lines, just in case.
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wrote:

On my old tent trailer I had a relay on the trailer AND one on the car. The one on the car was activated by the ignition switch, and was protected by a circuit breaker. I had a switch to disable it so the trailer plug was not always alive - this reduced the corrosion problem.
The relay on the trailer was to keep the cable from being live when sitting - again helping control corrosion (stored in a damp area). The relay was controlled by the tail light circuit - as I always drove with head-lights on (before the coming of DRLs)
On a previous vehicle I used the diode isolator box - and had no end of grief. The battery charging voltage was always too low (sense was on the alternator side of the isolator) and I could never get more than a season out of an isolator without loosing either the car battery, trailer battery, or both from the circuit.
Been using constant duty solenoids, a-la Ford trailer towing special, ever since with no problems.
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Chief
I would expect the battery for the hydraulics to be adequet for doing whatever it 'hydraulicing' is needed without needing any help from the other batteries in the system. If thats not true in your application, ignore this following.
If the hydraulics are operated only when the vehicle is "engine off", it would be easy to install a relay between the vehicle battery and the hydraulic battery. That relay will make contact only when the vehicle engine is charging. (after engine crank). The size of the wire that connects the two batteries needs to be large enough to conduct the maximum charging current to the Hydraulic battery, which can be controlled by the appropriate series resistance.
Jerry
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Chief McGee wrote:

How much current does the hydraulic unit pull?
Are you prepared to risk killing the truck battery if you use the hydraulics for some time with the truck engine off?
I would suggest buying a diode isolator (RV shop). It feeds from your alternator to two batteries but won't allow current flow between the batteries. A #10 wire should be big enough to go from isolator to trailer.
Ted
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vaguely proposed a theory ......and in reply I say!:
remove ns from my header address to reply via email
How much does the hydraulic system's electric motor draw?
Batteries can charge at up to 30A, and maybe more. It may well be that your battery, if low and charging fast, will draw more than the motor.
It may also be that as long as you keep the engine running, you can simply either have only one battery, or keep both charging on a parallel circuit. Use similar batteries and you should have no problems.
Many alternators run up to 50 Amps. Some to 70 A. A few to 100 A. If you siomply stop at a site, and use the trailer with the engine still running, your main battery should not be drawing much if in reasonable condition.
Again all this depends on the power that the motor uses.
The only problem with using an isolator is that if you lower the trailer battery really badly, and your main battery is well-charged, you can get problems charging both at once. This gets messy.
I actually thought the idea of the charger running from the lighter socket was not bad. You get charging while running from job to job, and a separate regulation of charge for the secondary battery. It would be a costly way to go, depending on the required charge current. thast in turn would depend on the current draw of the motor, and how often it was sued vs not used in a day's run.

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With my trailer, it uses the battery to power the brakes in the event of a breakaway, and it also powers the electric winch for rigging. The trailer manufacturer sells a part that acts as a diode.
http://www.centrevilletrailer.com/trailerParts/trailerParts.htm
I wired it in to an unused pin on the trailer plug
Tony

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Chief McGee wrote:

Big wire and diode isolator is the way to do it right. But... If you're trailer battery is up to the trailer task and you only need to keep it charged, you can run a small wire between batteries and use a self-resetting circuit breaker that limits the current to less than the fusing current of the wire. The resistance of the small wire limits the current. If it gets too high, the circuit breaker opens. If you don't let the trailer battery get too low while disconnected from the truck, the breaker won't ever open. The weak link is the connector. If you have low battery you'll get some connector damage every time you plug it in hot. How serious depends on how big the connector is. mike
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wrote:

You can also cheat and put a bulb in the line to limit current; a headlight limits current to about 4 amps and you can get more by wiring the Hi and Lo beams together. At low current there will be little voltage drop.
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I'd suggest using a relay that is activated by the truck ignition - that way when the ignition is off, the trailer is not connected to truck's battery, when you start the truck it will connect the two batteries in parallel and charge both. A circuit breaker or fuse is a good idea in case the wires short. Locate the relay in the engine compartment where it's easy to hook up.

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go to an RV store ( or marina) and get a battery isolator. ( ~$50 for a 120 Amp version)
Pat

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