Charging trailer with tow vehicle

[J.Shea]

I like to boondock in my trailer. I will use up about 100 amp hours of battery in 3 or 4 days. This brings my batteries to about 50%, time to recharge. I tow with a 2011 f150 which will charge my battery while on the road. The problem is after the battery voltage is raised by the alternator, the charge rate drops to nil. In a 4 hour drive it only charged about 30 amp hours. I know this is probably do to a small gauge charging wire. Would a DC to DC converter work which would input higher voltage into the battery than the charging line itself? Any thoughts on the best solution for this problem?

[larry2c]

Here's my 2¢ - the system on your truck isn't really designed to do much more than trickle charge your trailer battery. There are folks who have done things to improve the amount of current flow but for the money and effort, you'd be better off looking at solar or a small inverter generator (like a Honda EU2000). Then you aren't dependent on how long your drive is or trying to charge the battery at your camp site with your truck running.

[HangDiver]

I boondock almost exclusively (except while enroute). I added two 100-watt solar panels that generate more than enough amps for what I use day/night.

[Theb2b]

Same question poised by myself concerning device to charge trailer batteries while towing, going to end up using solar on the roof given the total cost.


http://www.forestriverforums.com/for...er-183238.html


But if your intent on using your TV this is a solution.

[TitanMike]

Originally Posted by Theb2b

Same question poised by myself concerning device to charge trailer batteries while towing, going to end up using solar on the roof given the total cost.


http://www.forestriverforums.com/for...er-183238.html


But if your intent on using your TV this is a solution.

Can't say anything bad about the Redarc unit other than it's more expensive than it needs to be.

I just installed a Renogy 20 amp Battery to Battery charger designed specifically for charging from an alternator in tow vehicle or MH

https://www.renogy.com/renogy-12v-dc...ttery-charger/

Only $127 and will be able to use existing wires in Tow vehicle.

I've tested mine and even with a nearly full (99%) Lithium battery I was able to read 10 amp charging current. With a more discharged battery I have no fear I'll see the full 20 amp charge current and it will remain that high until the charge controller switches to a lower voltage for the absorbtion phase.

With a Redarc selling for just under $400 and a Sterling for $300, the Renogy unit is very affordable. Everything about it looks well made and best of all, it works without having to upgrade vehicle wiring.

These units are a good constant voltage supply and are programmable for different types of batteries so you can adjust the voltage where it switches from Bulk Rate to Absorbtion to Float, depending on battery. Even if the supply voltage drops as low as 8 volts it will still deliver the full charging voltage and current.

I used to be a skeptic but when I installed mine I had my mind changed by the evidence.

I'd still recommend a secondary means of battery charging for the times you don't drive 4-5 hours between sites or if you stay longer. You really don't need a huge generator to run your Converter/charger. Unless it's larger than a 60 amp unit you can get by with a 2 kw inverter generator (that will support 1600 watts when running) that will be more than enough for battery charging.

Champion has one for $429 on Amazon.

https://www.renogy.com/renogy-12v-dc...ttery-charger/

The OP could use some of the money saved by buying the Renogy unit to boost your tow vehicle charge rate

[Boomerweps]

Or you could always turn your TV around and use a set of jumper cables from TV to TT, IDLING FOR 15-20 minutes. That Renogy System sounds nice.

[babock]

Originally Posted by Boomerweps

Or you could always turn your TV around and use a set of jumper cables from TV to TT, IDLING FOR 15-20 minutes. That Renogy System sounds nice.

15-20 minutes won't do much. You need hours to charge a battery. Plus, at idle, the alternator isn't putting out much.

[TitanMike]

Originally Posted by babock

15-20 minutes won't do much. You need hours to charge a battery. Plus, at idle, the alternator isn't putting out much.

Surprisingly, alternators put out quite a lot at idle speed. That is unless you have a lot of high current accessories like A/C, heat, electric window defog, heated mirrors, seats, etc turned on high.

Automotive Alternators generally reach max output around 6,000 rpm and have a 1:3 engine to alternator drive ratio. At idle an alternator is capable of producing around 1/3 of it's rated output. If the only loads are basic engine operating systems and battery charging it's possible to get pretty close to max charge rate for the usual lead/acid battery. Naturally the larger the alternator, the more output, even at idle speed.

The issue(s) with charging using an idling truck engine and jumper cables are:

Huge fuel consumption for energy produced

Modern alternators are controlled by starter battery SOC with sensors are placed at starter battery to monitor current flow as well as charge voltage. Connection MUST be made at starter battery terminals and be electrically sound enough to trick system into thinking the starter battery is discharged.

Idling a tow vehicle engine for long periods has never been ideal.

In some jurisdictions letting a vehicle sit and idle is illegal. Part of clean air regs and usually only enforced at places like airports, etc, but nonetheless, illegal.

Considering fuel costs and the wear/tear factor I'd just buy a small inverter generator.

[babock]

Originally Posted by TitanMike

Automotive Alternators generally reach max output around 6,000 rpm and have a 1:3 engine to alternator drive ratio. At idle an alternator is capable of producing around 1/3 of it's rated output. If the only loads are basic engine operating systems and battery charging it's possible to get pretty close to max charge rate for the usual lead/acid battery. Naturally the larger the alternator, the more output, even at idle speed.

You would be very surprised how much power fuel injection, fuel pump, ignition, and ECUs draw. I used to be an EE for the electronics division of GM and it was close to 20A on some vehicles.

[TitanMike]

Originally Posted by babock

You would be very surprised how much power fuel injection, fuel pump, ignition, and ECUs draw. I used to be an EE for the electronics division of GM and it was close to 20A on some vehicles.

No argument on how much electrical power it takes just to make an engine run today. That said we've come a long way since the first Delcotron 35 amp Alternator was introduced by GM.

My truck has a "modest" 130 amp alternator so there's more than enough to keep the engine running as well as all the other things we've become accustomed to ------ Including charging auxiliary batteries.

FWIW, I started my career in the automotive related business in 1961. One of the first training courses I took was on the Delcotron, from diagnosing problems to complete overhaul. Hurray for low cost silicon diodes. Otherwise I'd still be replacing bushings, turning commutators, replacing brushes, and bending spring anchors to adjust voltage, current, and "cutout relays".

Thinking back though, if we still had to rely on "thirty something amp" alternators we'd probably not have to put up with "BOOM-THUMPA, BOOM-THUMPA stereo's in the car next to us at stop lights. If they tried to put one of today's stereo's in a car with that small an alternator the engine would probably die at the first "BOOM" considering how much power the "Amps" draw.

[Iwannacamp]

Originally Posted by babock

15-20 minutes won't do much. You need hours to charge a battery. Plus, at idle, the alternator isn't putting out much.

Mmhmmm. Surface charge.

[Iwannacamp]

Originally Posted by TitanMike

Surprisingly, alternators put out quite a lot at idle speed. That is unless you have a lot of high current accessories like A/C, heat, electric window defog, heated mirrors, seats, etc turned on high.

Automotive Alternators generally reach max output around 6,000 rpm and have a 1:3 engine to alternator drive ratio. At idle an alternator is capable of producing around 1/3 of it's rated output. If the only loads are basic engine operating systems and battery charging it's possible to get pretty close to max charge rate for the usual lead/acid battery. Naturally the larger the alternator, the more output, even at idle speed.

The issue(s) with charging using an idling truck engine and jumper cables are:

Huge fuel consumption for energy produced

Modern alternators are controlled by starter battery SOC with sensors are placed at starter battery to monitor current flow as well as charge voltage. Connection MUST be made at starter battery terminals and be electrically sound enough to trick system into thinking the starter battery is discharged.

Idling a tow vehicle engine for long periods has never been ideal.

In some jurisdictions letting a vehicle sit and idle is illegal. Part of clean air regs and usually only enforced at places like airports, etc, but nonetheless, illegal.

Considering fuel costs and the wear/tear factor I'd just buy a small inverter generator.

I hate smart people....well the ones that make me feel stupid! [emoji23][emoji23][emoji23]

[pgandw]

Which brings up the other point - time to charge a lead-acid battery. The maximum charge rate of a modern LA battery is about 20% of its AH rating. If you have a bank of 220AH 6V batteries, the maximum charge is about 45 amps. Doesn't matter if you have a 130amp alternator or a 55 amp converter - the batteries won't accept more than 45 amps - and then only for about 30 minutes before the charge rate starts to taper off. Meaning it takes about 3 hours of bulk charging to go from 50% SOC to 90% SOC. The 3 hours assumes your charging source maintains 14.4V bulk charge to 90% SOC (WFCO converters and vehicle alternators do not).

It then takes another 3-6 hours to finish charging the batteries to 100% SOC.

Which is why most dry camping and solar installs run the batteries in the 50-90% SOC range. Which also implies that 40% of your battery bank is sufficient to meet your needs until you are ready to recharge again.

Fred W
2019 Flagstaff T21TBHW with 2 Costco/Interstate GC-2 batteries for 4 nights dry camping, no generator, no solar.