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.

[DavidP]

jkwilson wrote:

Rustofer wrote:
Another thing that could be happening is that your alternator is pushing out current based on the needs of your starting battery, which gets replenished quickly. It's frustrating that today's trucks come with high-capacity alternators that basically do almost nothing because the sophisticated controls tell it the starting battery is fully charged. I've wondered if a way around this would be to wire the trailer connection directly to the starting battery with an isolator that opens when the truck is not running. Hmmm . . .

That’s not really how an electrical system works. The trailer charge line will draw current from the vehicle electrical system and that draw will cause the voltage to drop and the charging system will increase output. The trailer batteries are in parallel with the starting battery, so they are all one electrical system.

Not exactly. The remote/trailer battery will receive the full voltage the tow vehicle provides but the tow vehicle in many applications and in all modern (factory install) cases uses a battery isolator to protect from discharge of heavy loads and rapid discharge of the remote/trailer. True they are installed in parallel but it is one sided. No matter the state of charge/discharge of the trailer battery the tow vehicle battery is isolated from loads/discharge originating from the remote/trailer battery

[SoundGuy]

landyacht318 wrote:
There are also DC to DC converters which can step up vehicle voltage, and can speed the charging of distant batteries.

The best solution for those wanting to charge a deep cycle battery while driving.

EXAMPLE

[brulaz]

We have the solar that usually charges up the batts when travelling, but on cloudy rainy days last summer it wasn't doing it.

So installed a left-over 1000W inverter in the truck (which has a 220A alternator). Good for other things too (like charging tool batteries), but primarily use it to run a 650W battery charger in the trailer. Added an extension cord with water tight couplings between truck and trailer and we're good.

[jkwilson]

Rustofer wrote:
Another thing that could be happening is that your alternator is pushing out current based on the needs of your starting battery, which gets replenished quickly. It's frustrating that today's trucks come with high-capacity alternators that basically do almost nothing because the sophisticated controls tell it the starting battery is fully charged. I've wondered if a way around this would be to wire the trailer connection directly to the starting battery with an isolator that opens when the truck is not running. Hmmm . . .

That’s not really how an electrical system works. The trailer charge line will draw current from the vehicle electrical system and that draw will cause the voltage to drop and the charging system will increase output. The trailer batteries are in parallel with the starting battery, so they are all one electrical system.

[landyacht318]

mattherrington wrote:
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.

I can spin a potentiometer on my Dash and watch both an Ammeter, and a voltmeter whose voltage sense leads are on the battery terminals themselves, respond to me spinning that voltage dial.

In general if I have it set for 13.6v, and give it a few minutes of driving, then ask for 14.7v, about 2/3 more amps flow into my battery. This is true whether the battery is at 50% or at 95%.

So I choose 14.7v until I know the batteries are full, then 13.6v.

Rather than beefing up the wiring, some will power an inverter off enginne battery, powering a 25+ amp charger hooked to trailers house batteries.

One can also run thicker copper from alternator and ground through a hefty connector to the trailer batteries, and bypass the 4 or 7 pin cable and get much more voltage at the trailer batts.

Hefty

But when the vehicle chooses 13.6v and says Good enough, the inverter powering a higher amp 'smart' charger will likely deliver more juice to depleted trailer batteries.

There are also DC to DC converters which can step up vehicle voltage, and can speed the charging of distant batteries.

[Rustofer]

Another thing that could be happening is that your alternator is pushing out current based on the needs of your starting battery, which gets replenished quickly. It's frustrating that today's trucks come with high-capacity alternators that basically do almost nothing because the sophisticated controls tell it the starting battery is fully charged. I've wondered if a way around this would be to wire the trailer connection directly to the starting battery with an isolator that opens when the truck is not running. Hmmm . . .

[RoyB]

My 2010 F150 Truck had an unmarked envelope in the glove box with a fuse and relay in it... Turns out this had to be installed in the truck main fuse panel and then would send back 12VDC to the trailer when the ignition key was turned on. Was labeled TOW something in the truck manual...

Of course this only sent back 12VDC at a low amperage rate but getting a good trickle charge while going down the roadways for hours on end works wonders for the trailer installed battery bank...

Might check your truck for this...

Roy Ken

[profdant139]

For someone who says he's electricity dumb, you sound like you are doing quite well! Just FYI, I have a similar trailer (even smaller), and we get along great with a 120 watt portable solar -- plug and play, easy to use, store, and deploy. There are pluses and minuses to rooftop systems (or should I say positives and negatives?).

Also, replace those incandescent bulbs with warm white LEDs asap -- big savings on juice!

And welcome to the forums! What kind of a trailer do you have, and where are you going next?

[mordecai81]

I have a 155 amp USBattery 12 volt battery. If your 6 volts are like my battery they take quite a few discharge/charge cycles to settle down and charge and measure accurately. Before that point mine was difficult to charge fully and each cell's specific gravity to measure consistently. Early on the cells would just bubble like crazy and not charge fully. Every chance you get hit the batts with some higher voltage charging and eventually you may see some better results from your alternator charging but it will never be enough to really get them close to full.

[time2roll]

You need to boost voltage. I recommend 200 to 400+ watts solar on the roof. Might be in the shade periodically when parked but should work well driving on a sunny day.

The DC/DC charging link above is also a good solution.

The wire from truck to trailer s/b #10 minimum.

[mattherrington]

Rookie expectations. Twas my first real outing with this big battery bank Oldwizard's gadget is cool though. Who knew I was searching for the Holy Grail?

Worked fine when I was using the simple battery gauge and just charging that Chinese marine battery on the tongue!

Appreciate the help everyone.

[jkwilson]

You could potentially lose 1.5V from the truck to the trailer with 14AWG, which would be enough to make it hard to do any charging.

[PopBeavers]

In many trucks if you engage tow/haul mode then the alternator will put out a little more voltage to compensate for the long distance to the trailer batteries.

If you do not have tow/haul then you can turn on the headlights to get the same voltage increase.

Read you truck's owner's manual.

[SoundGuy]

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.

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

Read the OP's post again - he did replace the original 4-pin with a 7-pin and added a charge line. :R

[SoundGuy]

mattherrington wrote:
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.

To my surprise though I didn't get the charge I thought I would when we were out on the road.

You're expecting far too much from a vehicle charging system designed to maintain that vehicle's starting battery. If you're really serious about this then the solution is to invest in a DC > DC charger ... you'd do well to read the discussion linked to by theoldwizard1.