> How much current / how big a wire do I need for the power wire to the
> trailer? I was hoping that by putting a battery or two on the trailer
> that current for the winch would be supplied by that, and everything else
> would be low current. Is that silly?
The wire doesn't _have_ to be big, it's just that using a small wire will
greatly slow the charging of the trailer battery. Perhaps that's not an
issue for you.
Lets try some off-the-cuff numbers. Just for example, lets say the charging
rate is limited to 20 amps, and your trailer has a 100 ah battery that's
about 80% discharged. That means it's going to take 5 hours of running your
truck to recharge your trailer battery (assuming the battery has an 80%
charge efficiency). If the alternator output is at 14.4 volts, and the
camping battery is at 13.0; 20 amps & 1.4 volts means .07 ohms. If it's 25
feet (as the wire lies) between the alternator & trailer battery, you need
14 AWG wire (.003 ohms/ft) to get 20 amps.
If you want it to charge in 2 hours, that means 50 amps, which would require
8 AWG (.00074 ohms/ft). Of course, I'm ignoring things like the contact
resistance in the trailer connector and the resistance through the ground
path (and assuming your connector can handle 50 amps).
> I like the idea of a battery isolator though... From what I can see, the
> way they work is to essentially have the altenator charge each battery
> independently. Is this essentially only usefull if the trailer battery
> goes dead, draining the truck battery along the way?
Pretty much. One other advantage is that the wire to the trailer socket
isn't hot when the truck isn't running, but that's minor.
> Someone else mentioned wiring in a 110VAC trickle charger that would keep
> the 12V battery topped up when the trailer was plugged into 110VAC...
> Anything weird here? I'd like to just have this wired up and automatic as
> soon as I plug in the trailer to 110. Is that a realistic thing?
Yes, but it's not a good idea to leave an ordinary trickle charger connected
for a long time. If you plan on leaving the trailer plugged in for more
than a day, I'd look for a smart battery charger instead. Again, check with
the camping/RV people. These are extremely common problems, and there are
lots of solutions out there.
Randall
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