Battery-to-Battery Charger

[charliem]

Hey Guys,

I'm back! Still alive and anticipating upcoming camping season. Hopefully free of Covid.
I'm thinking of getting a battery-to-battery charger to charge the camper batts from the TV battery. My method of heavy jumper cables isn't working as well with the voltage limited charging systems in the newer vehicles. I'm thinking a B-to-B charger with good cabling would extend my time at no hookup sites. I'm looking at the Renogy 40A unit:

https://www.amazon.com/Renogy-Battery-Batteries-Multi-stage-Charging/dp/B07Q4SVX3M/ref=sr_1_3?keywords=battery%2Bto%2Bbattery%2Bcharger%2B12v&qid=1646518442&sr=8-3

Anybody got any thoughts, experience or recommendations?
Charlie

[DavidM]

That is a good plan, but I have a couple of concerns:

The OEM power supply to pin #4 of the 7 pin connector is typically 10 gauge with a 30 amp fuse near the TV's battery. You need to upgrade it to #8 or even #6 to carry the 40A of current and protect it with a 50A fuse near the TV's battery.

40 amps probably won't put too much stress on your TV's alternator, but recognize that it will be 40A until the batteries are charged, unlike the OEM setup where the current is limited by the voltage drop in the #10 wire. Check your alternator's case temp with an IR gun. You should stay below 200 F to keep from overheating the alternator.

David

[charliem]

Hi David,

Good to hear from you. I was hoping you would respond. The problem I was having with the heavy jumper cables directly between the batteries was the charging current would start off high but quickly drop to 5-8A as the TT battery voltage came up.  The truck battery voltage would be held at 13.5V by the "smart" truck electrical system so TT recharge would take years. Not good, hence my interest in the BtoB.    My plan would be to mount the BtoB permanently under the hood of the truck with short heavy wire to the starter battery. The Renogy BtoB has a connection to the ignition 12V so it should be shut off if the engine is not running, but I might add some sort of disconnect or removable fuse in line just to be sure. The connection to the trailer battery would be entirely external, maybe with heavy wire and clips like a jumper cable. Maybe a short cable with Anderson power pole connectors on each end. Not planning to use the 7 pin connector at all. This setup would only be for recharge each morning at the campsite. For recharge while in motion I would still rely on the existing internal 7 pin connection and the BtoB would just be along for the ride.

My alternator is part of a towing package upgrade and is rated at 130A. Should easily handle to 40-50A of the BtoB.

What's the take on Renogy these days? The recent Amazon reviews indicate their customer service might be nonexistent. Any reports?

Charlie

[charliem]

After thinking a bit it might be better to mount the BtoB on the trailer and connect to the TV battery with jumper cables. Line voltage drop might be better tolerated on the BtoB input rather than the output to the TT battery.

[DavidM]

My gut tells me to mount the B2B near the truck alternator, although I don't see why near the trailer batteries wouldn't work. Renogy has come a long way and I think they are a good supplier. Redarc and Victron also make B2Bs.

David

[charliem]

My thinking is any voltage drop between BtoB and TT battery will subtract from charging capability since the BtoB output is regulated and controlled at the BtoB output terminals. It's much like a solar charger where the input drop can be partially compensated by the controller. The specified input range is 8-16V with an 8V low shutoff. Any slight voltage drop on the input just makes the BtoB boost inverter work harder. Also the cable from truck to BtoB on the trailer would be dead until I made the final connection at the truck battery positive. Maybe safer.

BTW, how are you doing converting from massive Class A motor coach to tiny micro trailer?

Charlie

[DavidM]

OK, that makes sense.

With regard to the tiny trailer, it is a project still in the works. I am trying to convince my wife to do a long, 2-3 month trip out west and the MH is perfect for that. Then I might downsize to the tiny trailer. The Aliner Ascape is the only trailer of its size and weight that is decently comfortable for two. I went to the Northeast RV Show last week and saw lots of small trailers but all but the Ascape were too cramped. What makes that one work is its squared off back. All the others are more tear drop shaped- front and back which probably makes for better gas milage at the expense of interior comfort.

David

[charliem]

I'm glad it's you and not me looking for a new trailer. We've made several 2-3 month trips n the 21RBS and really enjoy the space. I'd go crazy in a tiny trailer for only a weekend.

Be careful assuming the teardrop is more aerodynamic. May be more marketing than engineering. Years ago we had a Forest River RPOD. Nicely rounded front and back so everyone assumed it had been designed with wind drag in mind. Then some engineer ran a wind tunnel simulation and determined it was like towing a parachute tied to a brick wall.

Good luck on your trip and trailer search and keep us informed.

Charlie

[Merlin]

Hey Charlie, Here are some ideas for you. I've got some enforced time inside right now as I recover from ankle surgery, so I figured I'd look up some things.

It's common for the off-road/overlanding folks to charge a second battery either in the vehicle or trailer. They are often used for winching or running a portable refrigerator, or various outdoor fun things. DC-DC charging in those installations isolates the starting battery, but sometimes has the capability to connect that one and the accessory battery for situations needing huge amp supply (winching). In cases where their is no trailer, the converter is mounted right in the engine compartment. To me, that's a hot, rough place to have aftermarket electronics, so I'd vote for have the converter in the camper battery box.

The most common brands used are those DavidM mentioned, Redarc and Victron. I do use a Renogy flexible solar panel, because I think they do well with their panels. I have no experience with other Renogy stuff. For all electronics, including DC-DC conversion, solar charging, etc, I use Victon equipment because most have Bluetooth capability and excellent control, monitoring, and customization through their app.

The one I might suggest for DC-DC camper battery charging is the Victron Energy Orion-Tr Smart 12/12-30A. If there is room in your battery box, that would be a convenient place to mount it. Running 8AWG wire to-from the Tacoma battery using Andersen connectors would provide the full 30A and charge things up fairly quickly without straining anything. Installing some new double terminals on your Tacoma battery with a short pigtail using Andersen connectors on it (and a fuse!) would make it easy to plug in a long wire from the camper battery box for charging.

As far as equipment support is concerned, individual companies are hit-or-miss now. You best bet for product support is to buy from a reputable supply company like either Northern Arizona Wind and Sun or the altE Store. Both have excellent customer support and can provide good advice pre-sale too.

https://www.solar-electric.com/victron-energy-orion-tr-smart-12-12-30a-dc-dc-charger.html

https://www.altestore.com/store/voltage-converters/dc-to-dc-voltage-converters/victron-energy-orion-dc-dc-chargers-p41653/#VICORI121240110

If you don't like that Victron, peruse their stores for alternatives, or give them a call and ask.

[charliem]

Roger,

Glad you popped in and glad you are still kicking. Temporarily disabled but I assume you will recover fully. I'm familiar with such disablements such as both knees replaced but you do heal.

I'm looking at the Victron models too. Of course Victron is the gold standard brand but more $$$. At first I discounted the Bluetooth stuff but more study reveals it can be pretty handy. Being able to tune the profiles and check voltages remotely can be useful. I wish it would display current so I didn't have to get out the clamp on meter but maybe I just haven't found it yet. I'm also considering non isolated vs isolated. Not sure why I need isolation since everything is grounded to the frames but I've also been bitten by the under buying bug before so I'm evaluating. I'll probably apply my time tested motto of "Buy it right and buy it once" to this project. The Renogy is cheaper and 40A vs 30A but I'm still a bit leery of Renogy. I would prefer 40A but my battery bank is 200AH so 30A is still 1.5x the recommended C/10 charge rate. Just takes longer each day.

On a related subject, and following our last discussion, I did go with five new Goodyear Endurance tires. No road time on them yet but it was time to replace the originals. What a shame to toss those factory GYs with perfect tread but they were getting old. Don't need a blowout on the road, even with two axles.

Spring/Summer/Fall is coming and Covid is becoming controlled so we're anticipating some camping. My daughter (40+) and family bought a camper last year and are already planning joint outings for us. Hence I need to get the CL ready. Always a fun time when I discover which water line I failed to blow out two years ago.

Take care, stay safe, and recover fast,
Charlie

[Merlin]

I can kick only with my right foot for a while, but I guess that counts 

I can confirm the Victron App does not report current in or out on the charger, so if that's important and you don't want to get out the clamp, there are probably better choices. 

I like Bluetooth and the VictronConnect so much I dedicated an older iPad as a permanent monitor. My wife and I use it daily to track battery SOC and current in/out on the solar system that charges our electric car. Other suppliers may also have good apps, but for now I'm sold (literally and figuratively) on Victron.

I love the simplicity of using the Tacoma as a generator. In general, it seems like using tow vehicles to charge house batteries is way underrated as a solution for RVs compared to running a standalone generator. And way quieter!

Not to make your life complicated, but did you consider a portable solar setup for battery charging? 

This month is tire month for me too. In my 13 inch wheel size I'll be getting either the Maxxis or Carlisle. In the larger wheel size I would have bought Endurance.

Sounds like fun times coming up with family camping. I hope all your water lines hold water and all your trips are downhill with a tailwind.

[charliem]

That sounds like a ringing endorsement for Victron. I'll probably go that way and mount the charger on the trailer near the battery. It looks like Victron even suggests this in their manual. Funny you should mention the EV solar charging. My wife is after me to provide solar backup for the house. If I had panels and a modest battery bank I would be set up. Since we heat with a gas fired furnace and cook with gas we just need electricity for the central fan and refrigeration. You have solved it all with the solar and the car battery. Now if I just had the Tesla.

On the Tacoma as a charger, it seems you, I and DavidM agree hauling the generator and all that gas is a lot easier if it's all built into the truck. That plus solar panels is the ideal solution. I have also thought of the solar panels for the camper but one thing at a time. The truck is always there; the sun may not be.

As for the down hill and tailwind thing, you obviously haven't driven across my neighboring state of Kansas. Perfectly flat and always a headwind no matter which way you're going.

Camp on,
Charlie

[DavidM]

FWIW, it takes a pretty large generator to start a home furnace fan. When we lived in Atlanta an ice storm took out our power for a couple of days. I hooked up a portable 2,500 watt generator to the home's gas fired furnace and air handler. It would not start the fan. Maybe a cheap Supco starting boost capacitor would do it or a more expensive soft start system would.

David

[Pinstriper]

Our new TT has one of the big refrigerators with a separate freezer. To my surprise, these are not available as a 3-way, only Gas/AC. No 12v. Which means rolling down the road you either run it on propane, or leave it turned off and hope to get reconnected to shore power before the temp rises too much.

So that's something that no longer pulls from our battery (we wouldn't run 12v at destination anyway). Other than air conditioning, which we rarely use, our battery needs are pretty modest - most of our camping is near the ocean where we don't need air conditioning anyway.

When I do use a generator, I have an 800w unit what weighs less than 20#, and runs on 1# propane cylinders (7 hours at a time; you can get an external regulator to run off your big bottles). So that's putting out ~7 amps, roughly what you could plan on with a 100w solar briefcase assuming less than 100% best conditions for the whole time. It makes a quiet purring sound.

Charging from the tow vehicle would certainly fill our needs to keep charged in a pinch.

[charliem]

David,

You party pooper. Now I've got to go fire up my 3500W gen and see if it works. Never tried it, just assumed it would work. Well, you know what they say about assumed 

Thanks for the heads up,
Charlie