How much battery charging from alternator?

ewarnerusa wrote:
Having spent a few years here I often read about the concept that while a typical trailer wiring connection from the vehicle to the camper will supply 12V, it is of low amperage and as such the camper batteries are just being trickle charged. The reasoning being very long wire length of inadequate diameter between the alternator and trailer wiring harness. However, this wiring can be upgraded which results in better charging capacity.

I've recently begun reading a solar panel forum and sticking to the RV section. I've noticed that every time a discussion comes up, one of the frequent posters immediately chimes in to first install a battery isolator to allow for alternator charging and that the alternator will provide more power in an hour than solar panels can in days. No mention of wiring upgrades, just this battery isolator concept. This poster clearly knows his solar but I don't get the impression of actual RV knowledge beyond carry over solar concepts. This poster clearly knows solar and electricity? Somehow I think they do not understand that a electronic battery isolator has a voltage drop across it, up to 1.5 volts during high amperage flow.



I recognize that alternators are rated to deliver a healthy supply of amps, but this concept from the solar panel forum doesn't mesh well with what seems to be the common thought here on rv.net. So can anyone enlighten me on how many amps one can expect for battery charging from an OEM trailer wiring arrangement and from an upgraded wiring arrangement, and what this battery isolator may do? Thank you.

A typical RV plug to the truck is going to have 7 wires. They are typically either #12 or #14, rated at 20 and 15 amps, with some wires being smaller, say #16 for back up lights, or turn signals, rated at 10 amps. The "Ground" wire or return path is only #12 wire. So potentially you could have 3 amps going to a turn signal, 12 amps going to the running lights, and some to the battery. The return path will be congested with all that amperage going back on one small wire. Yes more amps can return via the hitch steel connection, but it will offer some resistance.

Upgrading to #8 wire rated at 40 amps will offer a low resistance way to charge the RV battery. Having a relay type isolator in the truck will allow it to stay plugged in at night, without a problem of draining the truck battery while using the furnace in the RV. This will allow up to about 30 amps per hour to come from the truck alternator while driving, and fully charge the battery in a 3-4 hour or longer drive. Once the battery is about 90% full, it normally takes a really long time to reach 100% full, no matter the power source.

On the solar side - I can fully charge my house battery bank in one day, even if down 100 AH overnight. I have a very accurate E-Meter to measure the amps going into and out of my battery bank. I have a 400 watt solar system, and it can put out 22 amps for the peak 3 hours of sunshine, and about 100 - 120 AH daily to the battery. Sure when I start my engine, it's 130 amp alternator and short #2 wire to the battery can provide more power, but normal charging is via solar power.

With a trailer, the truck will not provide much of the battery charging if you have a solar panel or two. This is because of the distance and small wire size. Upgrading the wiring, and normally you will not see a huge increase from the truck, unless you don't have solar, and start each drive with really depleted batteries.

So for me, I would spend the money to install a couple of solar panels, and then not bother with upgrading the truck to RV wiring. Solar panels are cheap now. SunElec.com had a 140 watt panel with aluminum frame (to attach to your roof brackets) for about $230 recently.

You can look up my prior posts, and search 'solar' to find a older one with how I made brackets, and even see pictures I posted of my solar panels, and mounts.

Good luck,

Fred.