Charge Batteries by Idling Tow Vehicle?

Hi Mex,

Ok hairpin alternator--but is there a brand name? Or one that is easily available for a Ford v-10 in an E-450 chassis?

I don't take the risk with jumping or charging with the engine if I don't have to. I've blown alternators, way back when I had a Chevy and was helping out with other people's cars. If it can trash an alternator, I wouldn't want to know what it would do with the automotive electronics.

Instead, normally I keep my generator in the bed of my pickup, and have a rapid-start battery charger that can pump a couple hundred amps into a battery. 200 amp-hours do a good job starting most vehicles, no matter what the battery's condition. It takes a lot less gas running the generator than the battery, and I don't have to worry about car electronics frying.

Mex, alternator with gauge 4 wires is "a fast way to charge", as long as the goal is not to charge fully. Because full battery rejects the charge, yes. When mostly full, it will mostly reject it :).

To do the last 15-20% you need a lot of time and not too many amps. This is a job for solar. For a light user that cycles from, say, 70% to 100%, solar alone can handle this range.

This is the output curve of a 270-amp hairpin alternator.

Remember, the RPM is of the alternator rotor, NOT the engine. The pulley diameter differential allows the alternator to spin three-times faster than the engine. Therefore when you read 3,000 RPM on the chart it means the engine is at 1,000 RPM

THIS IS A FACTORY ALTERNATOR NOT AN AFTERMARKET!

Take 2 batteries. One full the other optimally hungry. The full battery will reject a charge more intensely than a ten-year-old will reject a double helping of broccoli. It's automatic.

Keeping on topic.

Show me a cheaper and easier way to recharge to fast recharge (mostly) 200 amp hours worth of batteries than connecting a set of 4 gauge jumper cables between a hairpin alternator and the batteries. For a short bout of boondocking...

Generator and charger? Solar panels? Whistling The Star Spangled Banner Upside Down? Shirley You Jest...

j-d wrote:
charging the battery in a fiver through the Seven-Wire from a V10 pickup takes all day, even at road speed.

Yes. Because 7-pin wire is thin.

Won't the truck's alternator bring the truck's battery up to charge quickly and probably first? If so, won't it then throttle down and offer decreased voltage to the Seven-Wire therefore the trailer's battery?

With a HUGE empty battery of trailer connected in parallel to SMALL starter battery, the total bank voltage will average between the two and becomes close to the trailer voltage. So alternator will keep on pumping 14.5V until trailer battery is near full.

Motorhomes are a different breed, charging on the road makes more sense (though mid-size solar is still a plus). Wires in MH are thicker than in 7-pin plug and (I guess) there are devices limiting the charging of one bank when another one is already full.

The factory wiring to a 7 pin connector is designed for maintaining a charge on the trailer batteries, not for a full recharge. Voltage drop over that long of a run with any significant load will be significant.

j-d wrote:
I've been trying to follow all this. What I can actually add is that charging the battery in a fiver through the Seven-Wire from a V10 pickup takes all day, even at road speed.

What I wonder is this: Won't the truck's alternator bring the truck's battery up to charge quickly and probably first? If so, won't it then throttle down and offer decreased voltage to the Seven-Wire therefore the trailer's battery? I guess there are devices that would create a system that could recognize the truck was charged, trailer wasn't, and adjust alternator output to the trailer accordingly...?

The problem is the tiny wire that travels 20 or 30 feet to the trailer. It doesn't carry any amps. Big fat wire will. That's my take anyway.

I've been trying to follow all this. What I can actually add is that charging the battery in a fiver through the Seven-Wire from a V10 pickup takes all day, even at road speed.

What I wonder is this: Won't the truck's alternator bring the truck's battery up to charge quickly and probably first? If so, won't it then throttle down and offer decreased voltage to the Seven-Wire therefore the trailer's battery? I guess there are devices that would create a system that could recognize the truck was charged, trailer wasn't, and adjust alternator output to the trailer accordingly...?

John & Angela wrote:
I wouldn't recommend it for a trailer battery charging through the 7 wire harness but for the class C motorhome bunch you can kind a make it work. But you need to put in some decent size wiring. We made a direct run of 2 or 4 gauge (can't remember) from the alternator on the V10 to the 2 AGM batteries in the coach. We also put a meter in line. When they are run down we will see charge rates north of 40 amps at idle, sometimes quite a bit north. A v10 is quieter than any generator I have ever heard, ANYWHERE. Fuel usage is around 1/2 gal an hour. Wear and tear, couldn't give a care. I bought it to use it. It already has over 150,000 K on it and a couple hundred hours idling probably won't hurt it. Beats hauling around a generator and fuel, neither which I have room for.

Having said that we have around 400 watts of solar on the roof, which works great until you have endless days of rain.

Too each his own.

Well stated ... and right on target!

For staying put in one drycamping spot with enough average sun day after day, solar is the way to go. But for hit and run type of drycamping with some driving miles every day, or every few days - we've just stayed with the alternator/generator approach so that we're ready for any kind of sky to keep the battery bank charged when drycamped in between driving spurts. I've considered portable solar, but decided that doesn't make as much sense as rooftop solar if we should ever add solar to our mix. Our rig came with an excellent Onan built-in and the little very quiet 30 year old Honda was given to us years ago and it's still in pristine shape - so why not bring it along and use it after boost charging with the V10's alternator.

From the curves on page 6 of this .pdf, our Ford 130 amp alternator is good for up to around 80 amps (temperature dependent) when the V10 is idling. Note that the alternator is spun 2.72 times faster than the engine's crankshaft RPM:

https://www.fleet.ford.com/truckbbas/non-html/2005/vs_pdf/05elect.pdf