Charging batteries AND running the DC fridge on the road: a DC-input battery charger

[ShrimpBurrito]

Like several other folks here on the forum, I could not run my fridge on DC while traveling without consuming battery power on the TM -- in my case, there was a 3 amp draw from the TM battery. As has been discussed in numerous other threads, this is due to inadequate wire size and numerous connections in both the TV and TM, both of which are factors which cause resistance, thereby reducing current. There have been numerous solutions to this problem, including:

1. increasing the wire size and possibly reducing the number of connections and length of wire
2. running the fridge on propane while on the road
3. load some ice (like a frozen 1-gal milk jug) in the fridge to provide cooling, and then do not run it at all on the road
4. running an inverter in the TV and running an AC extension cord back to the TM to power the converter
5. running an inverter in the TM, powered by the charge lead from the TV, and using the AC power to run the converter
6. running a DC-input battery charger in the TM (which I think is essentially the same thing as solution #5, but it's all contained in a small box)

Additionally, not only do I want to run the fridge on DC while traveling, but I also want to charge the TM batteries at the same time. Of course, there are various ways of charging batteries, including solar panels, generators, additional batteries, etc. The only solution among these that I considered was solar, but ultimately decided against it because we typically like to camp in shaded areas and in the winter, both times when sun isn't exactly at optimal levels. I don't have the weight capacity for another battery, and I am not at all excited about carrying a generator either. It's yet another thing to set up, and it makes noise, both things I try to avoid while camping.

Since increasing the wire size in both the TM and TV would solve the problem, and after consulting with Bill here on the forum, I initially decided to implement that change. Since I also wanted to charge my batteries vs. simply prevent the fridge from draining the TM batteries, this required a little more thought and design. This was my thought process:

The most commonly recommended max charging rate that I found for lead acid batteries is C/8 to C/10, meaning the max rate in amps should be no more than 1/8 to 1/10 the total capacity (at the 20-hour rate) of the batteries in Ah. I have 2 Trojan T-105's that have a combined capacity of 225 Ah, so my max charge rate is about 23-28 amps. The fridge and fridge fan consume about 10 amps (mine is actually about 9.2 amps, but I'll use 10A for this calculation), so I need to deliver 33-38 amps back to the trailer. For this amount of current, I calculated that I needed either #4 or #6 wire, so I chose #4. I initially thought about using the TV frame, and perhaps the TM frame as well, as a ground, but I was concerned that I would not be able to keep the connections clean and uncorroded. Since dirty or clean connections cause resistance, it felt silly to spend a bunch of money and time implementing a solution that could cause more resistance. So I decided to run both positive and negative leads. I needed a solid 50-feet of wire to run from the TV battery to the TM battery, so that's a 100-feet total.

Unfortunately, most wire you come across is not designed to withstand the heat of an engine compartment. Guidelines set by the Society of Automotive Engineers (SAE) require that the wire have heat resistance of at least 125 degrees C, and also be self extinguishing. Being that I was going to run 40-amps and the power source was unlimited (the alternator), I felt a short to some place like the frame could easily start a fire, and thus, I felt SAE's guidelines are appropriate.Such wire is very difficult to find, especially in small quanities, but I eventually found SGX wire on eBay meeting these requirements. It was only 20% or so more expensive than lower temp wire. SGR cable also meets these guidelines, and is actually resistant up to 180 degrees C. Incidentally, the factory battery cable in my TV is SGX.

Because these wires would be regularly connected and disconnected, they need to be flexible. If it is not flexible enough, it is not only difficult to work with, but it is also more suceptible to individual strands breaking. The more strands you have, the more flexible the wire. I felt a bunch of different wires with varying stand quantities, and after consulting with a wire manufacturer, determined I needed at least 400 strands in 4 AWG wire. Most #4 wire, including that sold at the big box stores, don't have nearly that many, so you need to source specialized online vendors.

Running 40-amps through a single conductor requires high capacity connectors. I thought about dividing strands of the wire and connecting sets of them to multiple pins, but that didn't seem very elegant. NAPA carries big weather resistant connectors for tractor trailers rated for 200A, and they aren't very expensive, so I decided on those.

I did not want the TM fridge or battery to run down my TV battery when I stopped somewhere, so I designed in a solenoid. As it turns out, most solenoids are NOT designed to withstand engine compartment temperatures. I contacted a few manufacturers of those sold in typical automotive stores, and they specifically recommended against an installation there. I really wanted to install it there because I was going to use a fuse tap to control it from an ignition-controlled fuse socket under the hood. I did eventually find one, and it wasn't much more expensive, but as it turns out, I later found a good alternative location in the rear of the vehicle near an ignition-controlled cigarette socket with a well protected passageway to the exterior of the vehicle for the 4 AWG power leads.

This all quickly became much more expensive that I anticipated. From a quick glance:

$150 - wire
$20 - circuit breakers
$40 - solenoid
$5 - fuse tap
$10 - wire lugs and crimping
$50 - tractor trailer connectors, mounting brackets, weather seals

$275 = TOTAL

That seemed like alot of money for a solution that was ultimately going to require alot of work on my part. Furthermore, the current transmitted back to the TM would not be regulated very well. I found myself running calculations to use wire size to regulate current, as I did not want to actually exceed the recommended charging rates of the battery. Excessive charge current results in heat, which can boil the battery and even crack the case in extreme cases. But using wire size to regulate current is obviously trickly when the voltage fluctuates at both ends. However, I agree these charge periods would not be for very long, and thus possibly not very detrimental, but since the price point was now the same as a DC-input battery charger, I decided to go that route.

....CONTINUED BELOW.....

[ShrimpBurrito]

....CONTINUED FROM ABOVE.....


Why? Several reasons:

1) it's plug-and-play. You mount it to the wall, connect the wires, and you are done. No rewiring the TV or TM
2) It's a 3-stage charger, and thus a more elegant charging solution. It seems silly to spend $400 or more on batteries only to abuse them by ramming power into them.
3) It's basically the same price
4) If I switch TV's, it remains in place. No need to rewire the new TV.
5) If I purchase a new TM, I simply disconnect the wires, unmount it from the wall, and reinstall it in the new TM. No need to rewire the new TM.
6) No extra connector to connect when hooking up the trailer

The only disadvantage I could think of is that the charging rate would be significantly less than 23-28 max rate I was using in the above calculations. A big disadvantage, but I decided it was worth the tradeoff.

DC-powered 12v battery chargers that supply any decent current (> 10 amps) are hard to find. I eventually found one made by a company in Australia. It had a max output of 20A, which meaned the max rate I could expect to charge the batteries at was 10A with the fridge on. One of their techs also said many of these units can also put out up to 22A, so my charging rate could perhaps be as high as 12A. It also had a fused input at 30A, which was good because that the charging circuit in my TV also happens to be fused at 30A, and that meant I did not need to modify the TV wiring at all.

Before I installed it, I inspected the wiring running from the fridge to the DC distribution panel -- as suspected, it was only 12 AWG. With the batteries fully charged, I was getting a 1.2v drop at the fridge. To maximize current flow to the fridge, I ran 8 AWG wire (positive AND negative) parallel to the factory wiring, which runs under the kitchen sink and bathroom floor. That reduced the voltage drop to 0.5v, which I think is a significant improvement. Interestingly, it still draws the same amount of current, so I presume that means it is consuming more power, and thus will get warmer and make the fridge cooler. Time will tell. I have a Dometic fridge, but for those with newer TMs who may have the Norcold fridge, take note: the Norcold service manual provided by forum member Rickst29 and posted here:

http://www.trailmanorowners.com/foru...ad.php?p=67910

...specifically states on page 5 that the minimum input voltage is 13.5v and that "Operation out of these limits may damage the refridgerator's electrical circuit parts and will void the warranty."

I have no idea how they can reasonably expect that kind of voltage in a trailer, even when connected to a vehicle with a charging circuit. I am not aware of a 12v battery that has a voltage of 13.5v, especially under a 10A load.

I haven't used the charger much in our typical camping routines yet, but from my initial testing, it works ok, but not quite as well as I had hoped. With the fridge off, the max charger output I have measured is 17A. So with the fridge on, I should be getting 7-8 amps to the battery when the charger is in bulk charging mode, but I need to do more testing. I'm not positive why it isn't at the full 20A output, as the batteries were at ~50% state of charge....unfortunately, I didn't measure the input current at this point, but my guess is I was close to the input max. I had taken a reading at one point where the output was 13A and the input to the charger was 19A -- that's roughly 70% efficient overall taking wire resistance into account, so a 17A output may mean it was drawing 24A (@ 70% overall efficiency). With a 30A fused input, the input max is probably real close to 24A, so wire resistance is again probably the ultimate limit here, as the unit's efficiency is supposedly ~95%. Once I have more time to work on it, I'm going to consult with their tech, who is responsive to both emails and phone calls, and make sure there isn't something amiss with the unit's sense wiring or anything else.

More info on the charger is available on their website:
http://www.redarc.com.au/products/pr...rt-start-bcdc/

The AUD$495 price listed on their website is for Australia customers. They charged me AUD$300 + AUD$80 shipping, and delivery only took 3-4 days. With the 3% currency conversion fee on my credit card ($10), the total cost was ~USD$340. The unit is small -- about the size of a regular pack of hot dogs (the ones pictured are bun length -- I hate short dogs), and weighs about a pound. I have it mounted to the side wall in the rear compartment directly on the aluminum, as it does get rather warm during operation.

It also appears weather resistant. Being a gadget guy, I took it apart as soon as I got it and immediately noticed that the entire circuit board was dipped in some sort of soft squishy plastic. There are no exposed electrical components. It is in fact designed to be installed in an engine compartment, so it should be fine installed on the exterior of a TM for those folks with batteries on the tongue.

Dave

[wbmiller3]

"...specifically states on page 5 that the minimum input voltage is 13.5v and that "Operation out of these limits may damage the refridgerator's electrical circuit parts and will void the warranty."

Think it might be a typo for "maximum" in the manual? I can't make this make sense any other way.

Great writeup.

[Bill]

Bill is right about there being a typo, although he has surmised the wrong typo. My Norcold manual is a bit different from the one posted by Rickst, but in the Installation section, it says "DC Operation: 14 VDC (11.5 V min, 15.4 Volts max.)"

In the Operating section of my manual, Page 5 doesn't say anything about electric operation. On page 6, I find the statement "While the vehicle engine is running, have a qualified service technician make sure the voltage of the DC power supply leads at the refrigerator is more than 11.5 VDC."

Bill

[ThePair]

So...as to this charger. Is it in parallel with the TM system? Series? How would it be installed?

For efficiency it seems like it should be in parallel with the TM charger, although you'd need separate wiring to the TV to do so. If there weren't inefficiencies (or if they're not big enough) it almost seems like you could do something in series, connecting this at the bargeman on the TV and then into the TM bargeman as well (an in-line current boost?) although, again, it would then be running the current through two chargers, which seems like a less than ideal situation, although the installation would be simplicity itself.

I really don't know enough about this stuff...

[ShrimpBurrito]

This charger operates totally independent from the TM charger, as the TM charger is not operating while running down the road.

The charger output is simply connected directly to the battery terminals. It's no difference from having an auxiliary charger at home. For the charger inputs, the negative lead is connected to the TM ground, and the positive lead is connected to the charging circuit coming from the TV through the Bargman. This positive lead from the charging circuit can be found connected to the DC distribution panel right above the converter, so access is easy. There are a few other wires coming from the charger, but no additional wiring has to be run. It's as close to plug-n-play as you can get.

When installed, it operates behind the scene. You simply connect the Bargman as you have always done, and that turns it on. That's it.

Dave

[ThePair]

Ahh, I think I see. Under normal conditions, the TV sends DC to the TM charger, and thus to the battery. In your installation, you're removing the hot lead from the TV to the TM's charger, and connecting that wire to this thing instead, so the TM charger does nothing through the Bargeman, but will still charge the battery fine through 120V AC. This device will directly charge the battery independent of the TM converter/charger, but only turns on when the bargeman is connected to the TV. And, the upshot is higher Amps for charging the battery from the TV.

If that's it, it's quite a clever solution, and for a single battery setup like mine, could indeed make a non-electric site a possibility, as the recharge time at idle wouldn't be too outrageously long (I'd hope!).

Something to think about, for sure...

[ShrimpBurrito]

Originally Posted by ThePair

Ahh, I think I see. Under normal conditions, the TV sends DC to the TM charger, and thus to the battery.

No. The TM charger is actually a function of the converter, which only operates on AC power. If you do not have AC power, the TM charger does not function. What provides DC power to the TM while on the road is simply a wire that runs from the TV battery, alternator, or charging circuit directly to the TM battery. It actually is connected to the TM battery via the TM's DC distribution panel, which is simply a screw. From that same screw, there is another wire that goes to the TM battery. There is no fancy circuitry involved. In fact, it's the same circuitry as if you were to "jump" the TM's batteries with a pair of jumper cables coming from the TV.

Originally Posted by ThePair

In your installation, you're removing the hot lead from the TV to the TM's charger, and connecting that wire to this thing instead, so the TM charger does nothing through the Bargeman, but will still charge the battery fine through 120V AC. This device will directly charge the battery independent of the TM converter/charger, but only turns on when the bargeman is connected to the TV. And, the upshot is higher Amps for charging the battery from the TV.

Yes, all of this is correct.

Originally Posted by ThePair

If that's it, it's quite a clever solution, and for a single battery setup like mine, could indeed make a non-electric site a possibility, as the recharge time at idle wouldn't be too outrageously long (I'd hope!).

If you find yourself wanting to do more dry camping, a battery monitor like the LinkLite or Trimeric is invaluable. That is the only real way to get a sense of how much power you are using and budget your energy accordingly. During summer time, we might use 10-15 Ah/day, but it's around 30 Ah/day in the winter due to longer light and furnace use. If this DC-input battery charger were to output 17A to the batteries, maybe 15A is actually being absorbed by the batteries (the rest is output as heat). So you'd have to idle the car for 1 hour to recharge 1 day's worth of juice during the summer, and 2 hours to recharge 2 day's worth during the winter.

Idling a car at a campsite is not ideal, but I'm not sure it's much worse than running a generator, especially for short periods.

Dave

Originally Posted by ShrimpBurrito

Idling a car at a campsite is not ideal, but I'm not sure it's much worse than running a generator, especially for short periods.

Dave

fwiw, my Honda 2000 generator will run for 45 minutes on a pint of regular gas and is very quiet. I typically run it for 45 minutes every other day. I have never measure how many amps the converter can put into the batteries.

I have never measured how much gas my GMC 2500HD would use in an hour while idling. I do know that on a good day I can get 13 mpg. 11 when towing.

[Wavery]

Originally Posted by wbmiller3

"...specifically states on page 5 that the minimum input voltage is 13.5v and that "Operation out of these limits may damage the refridgerator's electrical circuit parts and will void the warranty."

Think it might be a typo for "maximum" in the manual? I can't make this make sense any other way.

Great writeup.

I don't think that it's a typo.....I think it's more of a "Dummo". If this fridge was a compressor type fridge (like the larger fridges) it would require 13.5V to start the compressor while cycling (just like your AC unit requires 110V).