Re: [A/S] Charge rates from tow vehicle

["Jim Dunmyer" <jdunmyer@xxxxxxxxxxx>]

Rik,
 My relay has 2 small connectors, one for each end of the coil. You ground
one of them and apply power to the other to energize the relay. Some of
those relays have only 1 small connector; the other is usually connected to
one of the large terminals. In that case, +12VDC is applied to that large
terminal and the small one is grounded to energize the coil. This is common
for a relay that is used for the horn. Other relays could have one end of
the coil grounded internally so you would just apply power to the one small
terminal to energize it.

You want the one with 2 small terminals or the one with the coil grounded
internally.

Again, I got my relays from NAPA by just describing what I wanted to the
counterman.

If you use a relay, you want one like this. It's very heavy-duty and will
last a long time. A common problem with a lighter-duty relay is contact
welding. The scenario goes like this: you start the vehicle and get hitched
up to your trailer with low batteries, then shut it off to finish loading
and hit the road. You attempt to start the engine, only to find that the
battery is low and she won't go. The problem? The relay contact welded,
causing the low trailer batteries to draw down the vehicle battery. That
shouldn't happen with the relay that I use.

I prefer a relay over a diode isolator for a couple of reasons: there's no
need to cut into the vehicle's charging system and there's no voltage drop.
The diode system causes voltage drop in the vehicle's system, possibly
affecting the regulator's control of the alternator and causing a lower
charge rate to the trailer. Besides, it's cheaper and simpler to install.

You cannot use the trailer batteries to assist the starting battery because
the wiring between them won't handle the current. A motorhome is generally
wired with heavy cables between the House and Chassis batteries, so it's
practical to have a hookup like you describe.

If your vehicle's battery was low and if you could get the umbilical
connected, you could install a jumper around the big terminals on the relay
so the trailer could charge the vehicle's battery. Or, you could disconnect
the relay wire that goes to the ACC line, then jumper that terminal to the
battery to energize the relay. You would NOT want either connection in place
when you attempt to start the engine because of the excessive load on the
circuitry. If you've installed a fuse or circuit breaker in the system, it
would prevent the use of the trailer batteries for starting anyway. (we're
assuming that the trailer battery is being charged somehow)

                                                                  <<Jim>>