Charging your battery from your tow vehicle - R-pod Owners Forum

Author

Share Topic

Topic Search

Topic Options

offgrid

Senior Member

Joined: 23 Jul 2018
Online Status: Offline
Posts: 5290

Topic: Charging your battery from your tow vehicle
Posted: 03 Sep 2018 at 6:32pm

David raises a good point. Battery isolators are usually high current diodes, essentially the electrical equivalent of one way valves. They are used on boats and RV's to keep from discharging the start battery into the house batteries. They are wired between the alt and the TV and RV batts.

The problem with the diode ones is that diodes have a significant voltage drop, right when you are trying to keep voltage drops to a bare minimum. There are some FET based ones now that have much lover voltage drops than the diode ones, but they're a bit more spendy. There are also automatic charging relays that only allow dual batts to be connected when the relay senses that the engine is on and the alt is charging. This gear is common practice in marine applications.

If I was going to leave the campsite for more than a few minutes or go to bed with my TV engine idling charging the trailer battery then I would want an isolator or an automatic charging relay. But I don't really see leaving my TV running unattended anyway or my genny either for that matter.

If you are there monitoring things then you can simply manually disconnect the trailer from the TV when done charging, no need for an isolator. You can do this by pulling the connector between the TV and the trailer or get a marine style battery selector switch with 1, 2, both, and off positions.

1994 Chinook Concourse
1995 RV6A Experimental Aircraft
2015 Rpod 179 - sold

offgrid

Senior Member

Joined: 23 Jul 2018
Online Status: Offline
Posts: 5290

Posted: 03 Sep 2018 at 6:51pm

Yep, its a bit of work to get the TV charge current to a level that makes you feel more or less Ok about running a big engine charging a little rPod, and then you still can't run your AC appliances unless you also buy a 3kw or so sine wave inverter.

My inverter generator runs on propane too. I hate messing with stinky gasoline. That approach has a lot going for it, its simple and versatile, clean burning with no residual nasty smells. But swapping 20lb propane bottles is a very expensive way to get electricity. We ran our air conditioner 12 hours straight once on the genny and went through a whole propane bottle in the process. $25 to stay cool for 12 hours. Ouch.

Noisy too, the inverter gennys aren't really any quieter than the cheap ones when running fully loaded. My Highlander is a LOT quieter, I can hardly hear it idling 10 feet away. Right now I'm leaning toward giving the TV charging option a try. If I do I'll report back how it went.

1994 Chinook Concourse
1995 RV6A Experimental Aircraft
2015 Rpod 179 - sold

CharlieM

Senior Member

Joined: 23 Nov 2012
Location: N. Colorado
Online Status: Offline
Posts: 1797

Posted: 03 Sep 2018 at 9:00pm

I'll jump in here with my two cents worth, pun intended Smile

. I'm a believer in charging from the TV via heavy copper jumper cables. However I also have another option: my Tacoma comes equipped with a 400W inverter. Not sine; probably corrupted square/sine. I ran some tests to see how various methods might work. These tests were with two 6V golfers and Precision Dynamics converter but results are probably generic. From my original post:

CAMPER BATTERY CHARGING TEST
(12/28/2014)

1) Discharged two 6V GC2 batteries to 12.1V unloaded (approx. 50% SOC). Residual camper current 0.150 Amps.

2) Connected to Tacoma battery, engine running, with 20 ft. #4AWG copper jumper cables. Measured 55+ Amps charging. Disconnected.

3) Tacoma gas consumption while idling was 0.5 GPH.

4) Plugged camper shore power cord into 400 Watt Tacoma inverter. Measured 20 Amps charging. Disconnected.

5) Plugged camper shore power cord into normal house 30A circuit. Measured 39 Amps charging.

I didn't pursue the truck inverter/camper converter method to find out why it was so low. I suspect the converter didn't like the voltage or waveform from the truck inverter. Hooking up with extension cords and adapters was no easier than the jumper cables so I dumped that approach.

Overall the heavy jumper cables appear the best. Agreed the truck is an expensive battery charger but I have it, it works, and it is the biggest source of stored energy available at the campsite.

Charlie
Northern Colorado
OLD: 2013 RP-172, 2010 Honda Pilot 3.5L 4WD
PRESENT: 2014 Camplite 21RBS, 2013 Supercharged Tacoma 4L V6 4WD

Guests

Guest Group

Posted: 04 Sep 2018 at 7:11am

Thanks for the "real world" numbers, Charlie. They confirm what I have always suspected. However, you are making me be brutally honest about the situation.

Truth be told, if the generator were to go away today, it is very doubtful that I would replace it. For me, it's more trouble than it is worth - hauling it around and maintaining it.

If I were approaching the situation today, I think I would go with the second battery under the hood, along with a good isolator. However, instead of swapping batteries (truck/camper), I think I would make up a good "extension cord" with the nice Anderson connectors, that would reach from the truck to the camper. When done with the truck for the day, connect the extension cord from the truck (second battery) to the camper. The batteries would equalize overnight. I'm thinking this would cover me 99% of the time. If needed, I could always idle the truck for an hour or so, for an extra boost.

As best I can tell, this would be the least expensive, least work solution for me.

offgrid

Senior Member

Joined: 23 Jul 2018
Online Status: Offline
Posts: 5290

Posted: 04 Sep 2018 at 7:36am

Great info Charlie. And very useful numbers.

I suspect that the issue with using your Taco's 400 watt inverter is that the WFCO charger in the rPod needs more like 700 watts when running at 40-50A. So its voltage and/or waveform probably got really wacky and it would eventually either shut off on thermal overload or cook itself or the WFCO charger or both.

But recharging via the TV as a generator by installing a large inverter in it is an interesting idea. For me it has a lot of merits, because the main reason I'm using may Honda 2000 genny now is to also have AC power for the microwave and air conditioner.

Installing say a 3kw inverter in the TV has several benefits for me. I could run the rPod AC appliances without the genny and leave it at home, saving weight and fuel storage. I could use the standard AC cord to connect the two vehicles, no need for heavy gauge 12Vdc conductors. As you say it also gives access to the largest energy source available at the campsite.

I would also have use of the TV to run other high power electrical loads when not camping. My TV would become an additional backup power source for my house when we get one of our long power outages here on the OBX after a hurricane.

The disadvantage is of course that the TV charging process becomes even less efficient because both the inverter and the WFCO efficiency losses are now added to the alternator losses and the already high pumping losses of the large idling TV engine. Of course charging from the Honda inverter/generator has these inverter and charge losses too.

Turns out there is a huge unused space in the center console of the Highlander I could locate the inverter in, but I'd have to figure out how to run 4/0 cable to the start battery through the firewall...

1994 Chinook Concourse
1995 RV6A Experimental Aircraft
2015 Rpod 179 - sold

offgrid

Senior Member

Joined: 23 Jul 2018
Online Status: Offline
Posts: 5290

Posted: 04 Sep 2018 at 8:10am

David, that's an interesting idea.

Its also interest to see folks on this thread gravitate in different directions. You obviously don't use your microwave or air conditioner when boondocking so having an AC power source is not important. To me it is. Others here who camp in sunny places don't see why anyone needs more than just a 100w solar module. Different strokes.

The issue I see with your idea of using a second battery on the TV to recharge the trailer batt is that the trailer battery state of charge (SOC) will steadily go down over time unless you run your TV to recharge.

Say you have a 100 amphour battery on the trailer and your house load is 50AH a day. So you start your recharging with a 100AH trailer battery that is at 50% SOC. Say you connect that to a equivalent battery on your TV that is at 100%. Both batteries will end up at 75% SOC in the morning.

The next day say you recharge the TV second battery back to 100% driving around. The trailer battery provides the 50AH so it winds up at 25% SOC at the end of the day. You connect them together the next night and both will end up at 63% the next morning. And so on...

So if you do what you are thinking about, and are camping for more than a day or two, I'd suggest you plan to also run the TV for an hour or so every evening after you've connected the two batteries. If Charlie's numbers work for your vehicle that would bring both back up near full charge and create a stable situation for an extended stay.

Other than that tweak I think your plan is a good one and should work well for your situation. You might want to add a marine style rotary selector switch to your circuit. These have batt 1, batt 2, both, and off positions which could be helpful in directing charging and load to the two batteries as needed.

1994 Chinook Concourse
1995 RV6A Experimental Aircraft
2015 Rpod 179 - sold

CharlieM

Senior Member

Joined: 23 Nov 2012
Location: N. Colorado
Online Status: Offline
Posts: 1797

Posted: 04 Sep 2018 at 10:26am

DAVID: That's an interesting concept. I'd use a relay for the isolator to avoid the voltage drop of a diode. However, the overnight charging of the TT battery might be minimal unless you use 1" copper buss bars as your connection cable. A fully charged battery, not under charge, will only be 12.7V or less when discharging. That won't do much for the TT battery. That said, your scheme would basically be the same as swapping out the extra battery to the TT each night without having to physically move the heavy beast. As OFFGRID suggests, a little time at idle every other day or so with the TT connected would work well. That basically implements my jumper cable method without the hassle of the jumper cables. Sounds like a plan.

OFFGRID: You're probably right about the Tacoma inverter. I couldn't look at the waveform because my ancient vacuum tube Tektronix scope died and I'm too lazy and old to fix it. I should send it to the Smithsonian and look at the newer SS digital scopes. The TV mounted HP inverter would work as long as the engine is running and your alternator can produce the current. You might also look at the 1" Cu buss bar I suggested to David Wink

Charlie
Northern Colorado
OLD: 2013 RP-172, 2010 Honda Pilot 3.5L 4WD
PRESENT: 2014 Camplite 21RBS, 2013 Supercharged Tacoma 4L V6 4WD

Kevinscamps View Drop Down

Newbie

Joined: 14 Jul 2018
Location: Fort collins, c
Online Status: Offline
Posts: 37

Posted: 04 Sep 2018 at 10:48am

Check out this product. This is pretty much doing what you want. Just think of the Rpod as the towed vehicle.

https://rvibrake.com/products/towed-battery-charger?variant=8804388741&_vsrefdom=adwords&gclid=CjwKCAjw2rjcBRBuEiwAheKeL-DxgajOUoVKwzNUIxt8VwLAcywG_2EF77va7azPDFy1EcVxq_TKghoC_fkQAvD_BwE#shopify-product-reviews

CharlieM

Senior Member

Joined: 23 Nov 2012
Location: N. Colorado
Online Status: Offline
Posts: 1797

Posted: 04 Sep 2018 at 10:57am

Originally posted by Kevinscamps

Check out this product. This is pretty much doing what you want. Just think of the Rpod as the towed vehicle.

Not so good. Ten Amps is way to little and 16AWG is way too small. To quickly charge the TT battery you need bunches of Amps which requires bunches of copper.

Charlie
Northern Colorado
OLD: 2013 RP-172, 2010 Honda Pilot 3.5L 4WD
PRESENT: 2014 Camplite 21RBS, 2013 Supercharged Tacoma 4L V6 4WD

Guests

Guest Group

Posted: 04 Sep 2018 at 11:35am

As mentioned above, folks gravitate in different directions based on their needs and desires.

I run two 12 volt "RV/Marine" type group 24s. Frankly, I don't know how long they would last in fair weather. I've never stayed in one place long enough to find out. In cooler weather, it becomes more of an issue, as temperature and battery capacity drops.

Yes, Offgrid, you are correct that I don't use my A/C or microwave when dry camping. My personal practice/philosophy is that if it is hot enough to need the A/C, I'll stay somewhere (developed) and pay extra for the pedestal. I don't think I would enjoy running a generator (at high RPMs) for hours/all day. I don't think my neighbors would like it either.

I think we all agree that under the proposed scheme, I would fall "behind the curve" as days progressed, regarding SOC of the camper battery. However, if I could stretch out (a day or two) the times that running the TV is needed, that should do it for me. Occasionally, I'll stay a week or more somewhere but, most of my stays are of 3-5 days in length. The camper batteries will last for the shorter stays, with no help. During the longer stays and/or cooler weather, a little help is needed.

For an isolator, I was thinking something like this: http://www.westmountainradio.com/product_info.php?products_id=iso_pwr

If I ever decide to proceed however, I may shop around to see if there is anything that will handle more than 40 amps.