Boondocking capable tiny trailer battery backup system with air conditioning

[lisalizajohn]

I have finally started working on a long postponed project: building a small, lightweight enclosed trailer to be able to camp in Florida with a fridge and an AC. While I expect to spend most of my time in locations with access to at least an extension cord, I'd like a battery system to get me through the semi-frequent power outages I experience in campgrounds, as well as be at least theoretically capable of boondocking. Ideally I'd be able to use my Chevy Bolt as an additional power source while boondocking, but I'm open to using a gas generator.

I have an inverter/no-surge portable air conditioning unit (ASIN B09L15X5MD ) that maxes out below 1500w, and easily cools a large poorly sealed section of my workshop at less than 400w. I have a small refrigerator (ASIN B01DZQICZK ) incoming that is rated for .8A@110v, with an estimated 26w average - I won't know until I test it how accurate that is or demanding the surge will be. I also have an induction burner (ASIN B08YJ6ZPFJ ) with 600/900/1300 watt settings - I have not tested it but reviews indicate that it does properly limit the power consumption. I am comfortable with turning off the air conditioner and/or fridge to use the induction burner, but I am hopeful that will not actually be necessary.

The initial plan in my head is to purchase eight 280ah EVE cells and a 2200w Samlex (EVO-2224) inverter/charger. I should be able to limit the current of the AC input to be able to use the Bolt's output (albeit through another inverter, so some hefty additive losses there) or an extension cord from a friend's back porch. My area for solar panels is pretty limited - I would likely not even bother with solar initially, though I can theoretically mount up to ~750w of flexible solar panels.

Am I on the right track? Is there a better way to accomplish my goal? Your experience and knowledge are very much appreciated.

 

[Zwy]

You might be but those are some large loads over extended time frames. You can supplement charging with occasional shore power or the generator.

My suggestion would be to purchase a Kill O Watt meter and monitor the Kwh used by each appliance over the course of a few days while actually camping using shore power, even in your backyard. This would give a baseline of how many watt hours you will need to generate and store.

Forget the flexible panels. Pretty high failure rates.

 

[lisalizajohn]

You might be but those are some large loads over extended time frames. You can supplement charging with occasional shore power or the generator.

My suggestion would be to purchase a Kill O Watt meter and monitor the Kwh used by each appliance over the course of a few days while actually camping using shore power, even in your backyard. This would give a baseline of how many watt hours you will need to generate and store.

Forget the flexible panels. Pretty high failure rates.

Unfortunately, my trailer is as-yet just a pile of plastic lumber and foam panels - I can't test the AC in it for quite some time. The inverter AC is currently cooling an 8'x12'x8' portion of my workshop, and the killawatt meter says that even with my incredibly shabby sealing, the AC never pulls more than 400w after the initial cooldown, which takes less than thirty minutes. That's 768cf, the trailer will be less than 200cf with much better insulation and sealing. I'm not certain my meter does a good job of detecting surges, but I'll be able to get a good over-time estimate for the fridge as soon as it arrives. I'm less worried about the induction burner, I'm content to only use it in campgrounds or with a generator.

I expect to have shore power at least 80% of the time we use the trailer. I'm definitely ok with supplementing, that's actually the plan - use shore power when available, have battery backup in case of outages, and use my EV (65kwh battery, ~1000w continuous available, as-yet unknown losses) and/or a generator to keep the battery topped off when I'm off-grid. As long as the battery can provide adequate power during an outage, it can provide power long enough for me to drive the EV to a charger for a refill.

Solar is (I think) totally unnecessary to this system - if I do it, it'll just be to gain familiarity for a future system. I'll almost definitely be holding off on the solar bits until later, if I do it at all. It's possible that I could come up with a way to use traditional panels, but the weight and the setup are both unattractive things to have to deal with. It's definitely not feasible to mount or even haul enough panels to cover my consumption, and it's hard to prioritize it when I drive around a large battery that can feed me 24kw a day without breaking a sweat.

 

[Zwy]

Unfortunately, my trailer is as-yet just a pile of plastic lumber and foam panels - I can't test the AC in it for quite some time. The inverter AC is currently cooling an 8'x12'x8' portion of my workshop, and the killawatt meter says that even with my incredibly shabby sealing, the AC never pulls more than 400w after the initial cooldown, which takes less than thirty minutes.

400W x 10 hours = 4Kwh

That's 768cf, the trailer will be less than 200cf with much better insulation and sealing.

But it will sit in the sun.

I'm not certain my meter does a good job of detecting surges, but I'll be able to get a good over-time estimate for the fridge as soon as it arrives. I'm less worried about the induction burner, I'm content to only use it in campgrounds or with a generator.

I expect to have shore power at least 80% of the time we use the trailer. I'm definitely ok with supplementing, that's actually the plan - use shore power when available, have battery backup in case of outages, and use my EV (65kwh battery, ~1000w continuous available, as-yet unknown losses) and/or a generator to keep the battery topped off when I'm off-grid. As long as the battery can provide adequate power during an outage, it can provide power long enough for me to drive the EV to a charger for a refill.

Solar is (I think) totally unnecessary to this system - if I do it, it'll just be to gain familiarity for a future system. I'll almost definitely be holding off on the solar bits until later, if I do it at all. It's possible that I could come up with a way to use traditional panels, but the weight and the setup are both unattractive things to have to deal with. It's definitely not feasible to mount or even haul enough panels to cover my consumption, and it's hard to prioritize it when I drive around a large battery that can feed me 24kw a day without breaking a sweat.

280Ah x 24V= 6.72 Kwh of which 90% is usable leaves 6Kwh of power. The AC will take 4 Kwh over 10 hours, that leaves you 2 Kwh for other loads.

I would not rely on the EV battery except in a possible emergency, how do you drive an EV if you drained the battery in it? Have you tested range pulling a load?

 

[lisalizajohn]

400W x 10 hours = 4Kwh



But it will sit in the sun.



280Ah x 24V= 6.72 Kwh of which 90% is usable leaves 6Kwh of power. The AC will take 4 Kwh over 10 hours, that leaves you 2 Kwh for other loads.

I would not rely on the EV battery except in a possible emergency, how do you drive an EV if you drained the battery in it? Have you tested range pulling a load?

I had imagined that a large leaky space inside a 90f warehouse would make for a fair estimate of power required, but you're right, I don't have a handle on how direct sun would play in. I could perhaps rent a U-Haul for a couple of days to try to get a more accurate estimate. I definitely need at least four hours of usable backup, ideally triple that. Doubling the battery pack would mean an extra $1200 and 100lb, but it wouldn't break the bank in dollars or pounds.


I'm not clear on why I shouldn't use the EV battery as a power source - it is a built in function on many EVs now, though for mine I do have to pull off the 12v and can't exceed the ~1600w rate (less vehicle system overhead) at which the DC-DC converter can fill the 12v. Having a mobile battery capable of charging 60kwh in 90min and then generating 1000w for a couple days was one of the selling points of buying an EV.

I would not be letting the EV battery get anywhere close to drained, though I'd have the generator if I ever grossly miscalculated - I would typically go recharge at whatever interval made sense based on my distance from a DCFC charger. Any eventual solar would extend this interval somewhat. I wouldn't be pulling the load when I go back and forth to the charger, only to get the trailer to and from the campsite. I am willing to run the generator in situations when it would be inconvenient to leave camp to recharge - two gallons of gas over six hours would produce ~10kwh, so six hours of AC/fridge and ~7kwh to the vehicle.

Perhaps I didn't do a very good job of explaining my goals. I want to:
A) weather power outages at campgrounds without losing my AC or refrigerator
B) use the 1000w the Bolt can produce continuously (or that I might draw safely from a friend's backyard extension cord) to maintain a battery system with an inverter capable of producing more than that 1000w when needed
C) maintain AC and refrigeration in the trailer when I leave the site with my EV

For boondocking, I fully intend for the EV to be a working component of the system. Also, for what it's worth, I don't anticipate boondocking for more than a handful of days at a time - my use cases are more like a comfortable place to sleep at a music festival or a dry campsite on the beach rather than an extended trip into the wilderness. I would rely heavily on the generator if I ever did find myself wanting to spend more than a weekend far from an EV fast charger.


I appreciate you letting me talk this through with you! I'll definitely have to get better estimates for actual consumption. I've also used an even more efficient inverter style window unit before - it's busy cooling my mom's Florida room - and while window units aren't welcome at most campgrounds, it might make sense to design the trailer to allow me to swap one of those in when I know I won't have shore power. I do welcome any additional thoughts you or others might have.

 

[Zwy]

I'd rent the UHaul and do some range tests plus what it takes to cool the interior. Might be a cheap investment to get an idea of how much power you need and how many Ah's you can pull off the EV yet still be able to drive home.

 

[Bluedog225]

Fun project. I wonder if there are big panels that could form the roof of a teardrop type trailer?

 

[lisalizajohn]

Fun project. I wonder if there are big panels that could form the roof of a teardrop type trailer?

That hadn't occurred to me - traditional panels are so heavy that I thought I'd have to use practically disposable flexible panels or skip out on solar altogether. If I were replacing the roof weight, though, rather than adding, then maybe? A single 'seam' should be doable, I think - perhaps one large panel fixed and one on a hinge to increase interior height and add some good tilt?

I'm still solidly in the plotting stages, I definitely welcome any ideas like this!

I'd rent the UHaul and do some range tests plus what it takes to cool the interior. Might be a cheap investment to get an idea of how much power you need and how many Ah's you can pull off the EV yet still be able to drive home.

I don't have to make it home under load - I just have to make it to the nearest DCFC station. For recharging mid-trip (or right before I head home) I won't be under load at all. When I stay at the Suwannee music park, in one of the least EV friendly stretches in Florida, I have six DCFC chargers within fifty miles. I'd have my generator in a real pinch, though even in the week after Ian I didn't have to use it once in over 3000mi.

Some of your concerns sound similar to when people tried to warn me when I decided to drive my 80,000mi a year in an EV - but in my experience, if you're good with maps and good with math, you won't have any trouble in Florida. My generator has so far gone unused in the nine months since I took the leap. There are areas I make sure I head into full, just in case I run into a power outage at an inopportune part of my route, but to me that's not much different than wearing a jacket when the weatherman says it's going to be cold. I keep an umbrella in the back beside the generator, too.