As posted above, the tow truck battery would eventually run down with an extended race outing. You have two easy solutions:

*First, unplug the tow truck connector during the race day [or days]. Even charge the trailer battery from your generator if needed.

*Second, a battery isolator can be installed on the tow truck that will not allow the tow truck battery to be drained. It will charge the trailer battery while driving but protect the truck battery when parked. It is a simple install and common with RV setups.

Some very good info you guys are handing out. Thanks. I have been doing a little research on batteries themselves, and looking at their reserve capacities. I assume that would be one of the more important considerations. I also assume that cranking amperage and cold cranking amperage are important, but likely not as important as the reserve capacity. Have found one rated as 205 minutes reserve, cranking amp @ 875 and cold cranking amps @ 675. it's a group size 29. I assume that's physically much larger than a typical one.

I had not thought of running the trailer brake away off of this battery, but will likely do this now. Also had not considered wiring into the truck charging system. I may do that as well. Thanks again.

quote:

Does the tow vehicle charge the trailer battery when connected?

My motor home is wired to charge the battery while towing. While I'm at home and the track I keep the battery tender connected and charging.

Bob

All three of my batteries are stand alone while parked with ignition off. I have the trailer battery wired with 10 gauge into the seven way connector into a relay excited by engine ignition and hooked to the positive side of the coach battery. The motor home engine battery is isolated from the coach battery while engine is off as I think all motor homes are. When engine is running all three batteries are charging. If you are towing with a pickup, the trailer can be wired the same way using just the battery on the tow vehicle thru a relay. Standard 40 amp relay or even a maintained contact switching solenoid will work.

Lots of good info has been presented here.

The idea of using a constant duty solenoid to isolate and charge the trailer battery from the rig is a very good one.

The battery isolators that lot's of folks use for RV use works, but the one thing nobody ever seems to mention or understand is that they drop the output voltage to both of the batteries about .7 volts. This is due to most of them using high current diodes to accomplish the isolation and the forward voltage drop on a silicon diode is around .7 volts. Using a good quality relay or solenoid will typically only drop the voltage to the trailer battery less than .1 volts.

I have a deep cycle battery in my trailer that gets charged anytime there's daylight by the solar panel on the roof. It works out really well and I have a panel to charge the race car too.

The panels are 30 or 35 watts, picked them up on eBay for around $40. They run through a battery tender solar charge controller to the batteries.

I always had battery charger running off trailer generator, coupled with solar tender. Never experienced any trouble.

I've been using two optima for years now, a red top and a yellow top that are about 8 years old. They still work, but won't take a full charge anymore. One of the updates I'm doing to my trailer over the winter besides the new jack (same one Ed is using), is brighter 12v interior lighting and a new battery, mounted under the trailer, along with a new junction box and breakaway battery. The new batter I'm getting is this one:

https://www.batteriesplus.com/...ycling)/bci-group-8d

Bit overkill maybe, but it has more than enough capacity that I won't need to worry about charging it all the time when I'm not home, and will power the inverter I'm also getting for my laptop. I'm trying to minimize how much I run my generator and this setup should do it. Yes, I'll have a battery tender on it all the time at home.