Updated: Jul 10, 2019
NOTE: There is an update to this post at the end.
Powering everything inside an RV requires one of the following: being plugged in to "shore power" with an electrical cord, starting up the noisy generator, running the vehicle engine, or using the coach ("house") batteries. If we're at a campground without electricity, it is taboo to be running the generator for long. If we're out in the middle of nowhere boondocking (dry camping), we can run the generator but it's going to use up some propane which is also used for cooking and by the furnace. The ideal dry camping setup is to have lots of battery power and to have lots of solar panels for recharging those batteries.
We just want to occasionally stay one or two nights without shore power and not run the noisy generator. I also want to avoid starting and idling the engine for a long time, which I've heard can be hazardous to the health of the Mercedes diesel emissions system. Now that we're out of warranty, visiting a Mercedes dealership could require the sale of a kidney.
Caution: Techno-babble lies ahead. Time to pour a glass of wine.
Our Airstream Interstate came with 160Ah of AGM deep cycle batteries which I upgraded to slightly larger 220Ah batteries. In order to preserve their life, it is recommended to only let them discharge 50%. This means I really only have 110Ah of usable capacity. I was on the verge of adding another pair of batteries but that step alone was going to cost $1500 for the batteries with the extra battery holders to be installed under the coach, including modifications to those holders for the bigger battery dimensions. The extra batteries would add another 110Ah for a total of 220Ah of usable energy. That's an improvement but is still a limiting factor. Plus, that's an extra 150 lbs of weight in total.
To do it right, I would need to add more solar panels and upgrade the solar charge controller costing around $4k-$5k if installed professionally. Then I'm still hobbled by the standard Magnum 1000W inverter because it can't run the microwave (which needs almost 2000W) so that would need upgrading too. The total cost is getting close to $8k just so we can have the flexibility to not be hooked up to electricity at a campground for a couple of nights here and there. Might as well spend that money on some nice hotels. But that's not why we bought this studio apartment on wheels. And we haven't even addressed powering the air conditioner. That still requires running the generator if there is no electricity to plug into.
To really do it right, people go with lithium-iron batteries. They have about 3x the energy density per pound and per cubic foot of space. They also cost about 3x as much as AGM batteries. If you want to run the air conditioner, you need a LOT of them. Now you're spending $10,000 to $20,000 and for us that's just not going to happen.
After much research I decided to go down an unconventional path and buy a Goal Zero Lithium Yeti 3000 power station. It functions as an alternative power source, a silent generator. You can charge it by plugging into 110V AC power or you can use solar panels mounted on the RV roof or placed out on the ground. It's a nifty little box with Lithium-ion batteries inside that can supply 110V as well as power 12V items and recharge your portable electronic devices directly.
The Yeti has a capacity of 3084 watt hours. That's 250Ah at 12v (3084Wh / 12v = 257Ah) . What's great about lithium batteries is they don't mind being discharged a lot. An 80% depth of discharge works just fine, so that works out to 205Ah (2465Wh) of usable energy. That's in addition to the 110Ah of usable juice from my current AGM batteries.
The Yeti is rated for 1500W of continuous output and it will permit a 3000W surge for 30 seconds before resetting itself.
To get power from the Goal Zero to the RV, I would normally have to plug in to the input power receptacle on the outside of the coach. The Yeti 3000 costs $3000, so I definitely don't want to set it outside for thieves to see. I could run a power cord out through the side awning window and into the side of the coach, but that's not desirable.
Building A Better Mousetrap
I wanted to keep the Goal Zero inside and integrate the unit into the coach electronics. I explained this to Kelli, the electronics wiz at Portland's Airstream Adventures NW, and she came up with a nice solution for me by installing a second transfer switch. Power cables from the Yeti and my noisy generator feed into the new transfer switch which then hooks up to generator side of the original transfer switch. Did you catch all that? If I turn on the Yeti (or the generator) this takes over and becomes the power source for the coach, bypassing the batteries. Only $400 for this conversion and it works like a charm ! The two transfer switches are hidden in the black box you see in this photo.
Are there any drawbacks? Well there are some significant power losses going from 12v to alternating current then into the coach which converts back to 12v for some items like the interior lights. So I really won't be getting the full 205Ah; more like 120Ah.
Also, it should be said that the Goal Zero Lithium products use Lithium-ion and not Lithium-Iron, which limits the number of rated charge/discharge cycles to 500 vs 2500 cycles for the Lithium-Iron. So is this a big deal? Well, for us, being on the road six months per year and dry camping two nights per week = 52 times per year that I need to run down the Yeti battery. This equals nine+ years of usefulness before it gets to 80% of rated capacity. You can still use it, it just won't store as much energy as it did when it was new.
Considering that kind of light usage and long life span for the Yeti, I plan to exercise this device a lot. As a test, I ran an extension cord direct to the microwave to bypass my anemic onboard inverter and it works great! The Airstream microwave on full power pulls 1875W. I ran it for one minute without tripping the Yeti protection circuit. Our Instant Pot only draws about 700W, coffeemaker about 1000W. The more I can power things like this directly from the Goal Zero Yeti, the longer my house batteries will last.
For another test, I was powering the entire coach like I would at a campground. I had all the lights turned on plus the refrigerator and the separate freezer running. The display on the Yeti revealed it was delivering 330W. I checked the monitor panel inside the Airstream and found it showing 12A going toward recharging the coach batteries at 14.6V on an Absorb charge (12A x 14.6v = 175W). The Yeti performed admirably, powering the coach and recharging the house batteries a the same time. I need to see how things go in the real world because each of these loads will go on and off as they need to. More testing to come.
Recharging the Goal Zero Yeti 3000
Using 110V to recharge the Yeti is dog slow even when using the maximum of four of the special chargers for a total of 288W of input. If I discharge the device by 80% I'll need to put 2400Wh back in to get it up to full charge. So when we get back to a campground with electricity, it will recharge from shore power in around eight or nine hours. It can also be partially recharged when I'm driving, while the engine alternator is charging the house batteries and the Yeti at the same time (the Yeti is plugged into a 110V outlet, which is energized by the onboard inverter). In fact, the engine alternator feeds the house batteries at a higher amperage level so is really a great way to get things juiced back up.
In May 2018, I added two more 100W solar panels on the roof but they only feed into the house batteries. I could add some wiring to feed energy from these into the Yeti's special input ports but that's not in the near future.
In summary, using the Goal Zero Yeti 3000 in a dry camping situation allows me to run the microwave, coffeemaker, Instant Pot and hair dryer without using the normal house batteries. I can also use it to power the coach and recharge the house batteries as needed. With this solution there are no high expenses for wiring, installation, charge controllers or an upgraded inverter.
1) If/when I sell this coach, I can keep the Yeti. 2) I can pull the Yeti out and store it in my garage and use it at home or at a remote site. 3) Saves weight (for me, about 80 lbs). 4) Saves thou$ands. 5) Provides more usable Ah than the extra AGM batteries I was about to install. 6) Cheap to add portable solar panels to further extend the dry camping time. No additional charge controller needed. 7) As an option, Goal Zero sells a manual transfer switch for your home that permits you to use the Yeti to power up to four circuit breakers when there's a power outage.
1) Not a perfectly elegant solution; requires once in a while manual cord hookup to power the microwave directly. 2) There is about a 40% power conversion penalty transferring from 12v to 110v and back to 12v. 3) It uses up a little over one cubic foot of my precious "garage" space behind the sofa. 4) My wife will spend some of the $avings on a better kitchen faucet in the galley, and maybe a new sink if we can fit one in. That's in another blog post.
UPDATE Summer 2019:
I have discovered two methods of managing energy that permit longer dry camping periods.
#1 - When the Goal Zero is connected to the coach electrical system, it drains the GZ battery quickly because it is working to recharge the house batteries while it also powers the refrigerator and freezer. Due to the approximate 40% power loss converting to 110V and back to 12V, this can drain the Goal Zero faster and farther than I want. I accidentally discovered that if I turn off the coach 12V system and have the Goal Zero connected to power the coach the only power being drawn from the Goal Zero is the 100-110W needed to run the refrigerator and freezer. This is fantastic! It only uses about 30-35% of the GZ 3000 battery capacity to power these appliances overnight. During this time there is no 12V power going to the lights or water pump but you can dry flush or turn on the 12V system power for a few seconds. And we have a desk lamp with an LED bulb being power directly by the Goal Zero.
#2 - I had upgraded my battery isolation module to a Blue Sea ML-ACR which automatically connects the house batteries with the chassis battery under certain conditions. I can manually override this to force the two battery banks to stay connected. My Victron BMV-712 battery monitor reveals that when these are combined, the consumption from my house batteries drops about 33% since the chassis battery is helping out by sharing the load. This effectively expands my total house battery bank capacity by about 50% so I can power the coach 50% longer before engaging the Goal Zero battery. I am being careful not to discharge the combined batteries below 12.0 volts so I can still start the engine.
By using these two methods of battery management, I can now dry camp for three nights if there is some good solar energy being utilized during the day. I have three 100 watt solar panels and am considering adding another 100 watts.