Charging While Towing

[mattherrington]

Newbie to the forum here. Looked around a bit and didn't see anything specific so worth a fresh post.

We just took our first long trip in my 16ft TT after I installed a new (4)6-volt 460AH system in lieu of the single group 27 marine battery. Installed a Victron 700-BVM which was/is the coolest gadget. Love the AH consumed/charged function as that and voltage were my real measurements of state of charge. Didn't quite like the percentage used so that'll take a little programming change (hopefully).

Overall on the six night trip we used 170AH with the kids watching DVDs every night, ending voltage when we got home was around 12.25V. Pretty respectable. Just the smoke detector, carbon monoxide, propane detector, propane fridge controls, and a USB that stays hot draws about .85A an hour and if we were out for the day I'd just shut it down.

To my surprise though I didn't get the charge I thought I would when we were out on the road. We relocated twice 2-3 hours each and drove home 8 hours yesterday and the change in AH and voltage was seemingly negligible each time.

I've got a 30A lead wired from a empty fuse slot to the 7-pin connector. My big battery system install is right to the original connection points on the TT to keep things simple. When I start the truck I can see the Amps inbound to the trailer peak at about 5A but then really quickly it dropped down to 1/0/-.85. The AH meter moves backwards and forwards so I recorded a little dent in AH used right at startup but that was it.

My simple math said even at 5A inbound I'd see a real dent in total AH consumed on our crazy long drive home and it was at 175AH when we left and 171AH when we got home. Every time it basically felt like it was a wash and I thought I'd see a 30-40AH pickup after a whole day on the road. Truck voltage the whole ride was considerably above (runs around 13.4 to 13.6 on the move) the trailer batteries in the mid/low 12's.

Only thing I can come up with is that I run full lights on (lights on for safety!) all the time and the running/marker/brake lights on the trailer are still incandescent and that's consuming most of that trailer feed?

Seems like I'm missing something. Any insight from you experienced folk would be appreciated.

[GordonThree]

mattherrington wrote:

Tow vehicle had a 4-pin so I had to wire in 7-pin for the trailer brake controller and to get power back there. Looking at a 7-pin diagram again you're right those lights come through what were those original four pins.

This explains it all, no charge line in a 4 pin connector.

[mattherrington]

I get it now.

I can be at 13.4V all day in the truck and it's not going to dent my 4 GC2 US Battery bank. Bulk and absorption charge for these batteries is spec'd at 14.7V to final 97% of capacity and then drops to 13.0 for the float. I'm really sending a trickle back there to batteries down 30-40% hence net zero on the road.

That explains the amp drop too. I am sending amps back there but not with enough voltage to overcome battery resistance. So it pushes in 5A for but only for one minute(ish) and then normalizes because of the lack of voltage.

I'm a off-roader so I'm brakes, suspension, load, tranny temp, gear oil (reasonably) smart but electricity dumb. Bottom line today is that I'll have to A. keep hauling the generator just in case or B.) mate the setup with a 3-4 stage solar charger that gets to 15V.

Thanks all.

[mattherrington]

Wire size? Pretty light (maybe 14g). Seems super light in retrospect but that was the same gauge as the make-a-tap fuse connector thingy.

Tow vehicle had a 4-pin so I had to wire in 7-pin for the trailer brake controller and to get power back there. Looking at a 7-pin diagram again you're right those lights come through what were those original four pins.

It's the amp drop thing that gets me and I can't be back there to see it. I just presumed watching at startup it's at 2000 RPM I get the alternator spinning and at idle I get back down to the minimal feed. Still means I'd be great running in 4th gear all day. Maybe I get the bluetooth dongle for the BVM so I can see it on the road.

[DutchmenSport]

Sorry, you lost me after the sentence, "We just took our first long trip..." After that ... um ... total ignorance on my part. Glad your trip went well. Sounds like you had a great time.

Welcome to the forums, hope you get "it" figured out ... well ... whatever "it" is! But most important, have fun doing it! That's what RV ownership is ALL about!

Welcome!

[DavidP]

Trailer/running lights are not drawing from the trailer battery and are normally powered from the tow vehicle. If you were seeing 13.5-13.7 (normal range) on the vehicle but < 12.6 on the trailer you either have a bad connection somewhere in the battery charge line or upstream from the 7 pin. Did you meter the charge line at the 7 pin? It should mirror the voltage at the battery of the vehicle. That same voltage should be present at the battery bank on the trailer. If the voltage is the same at all points you had batteries that were more severely discharged then you suspected. That is a pretty big reserve you are trying to replenish basically on a float voltage level. You would need hours of bulk charging at plus 14.6-14.8 voltage to bring batteries reading 12.2 back to 90-100%. Your truck is not going to typically do that.

[theoldwizard1]

Read this regarding charging a house battery from a vehicle alternator.

DC-DC battery charging

[GordonThree]

The so called "charge line" in the Bargman connector is really only intended to recharge a small lead-acid battery that controls the emergency brakes. It doesn't supply much power.

The charge line on my Ram 2500 supplies 10 amps, which is just enough to cover the idle load of my RV systems, so net gain is 0 amps.

[jkwilson]

mattherrington wrote:


Only thing I can come up with is that I run full lights on (lights on for safety!) all the time and the running/marker/brake lights on the trailer are still incandescent and that's consuming most of that trailer feed?

The power to the lights comes in on their own lines, so your 12V charging feed isn't affected.

Curious why you connected your own line when the tow vehicle likely already had one? What size is your wire?

[BarneyS]

Welcome to the forums! Glad to see you here and hope you get all you questions answered.

I think you will have better response to your question in the Tech Issues forum. I will move this thread there. Thanks for your post and I'm sure some help will be coming forth quickly.
Barney