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:
- increasing the wire size and possibly reducing the number of connections and length of wire
- running the fridge on propane while on the road
- 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
- running an inverter in the TV and running an AC extension cord back to the TM to power the converter
- running an inverter in the TM, powered by the charge lead from the TV, and using the AC power to run the converter
- 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.....
Charging batteries AND running the DC fridge on the road: a DC-input battery charger
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