Pure Sine Wave Versus Modified Sine Wave For Electric Space Heater?

Last weekend I went out to my favorite camping area in Maryland - Green Ridge State Forest. The temperatures got down to the mid 20s over night. With a nice winter sleeping bag and a hot water bottle in the bag I was toasty warm sleeping all night however getting up and out of the sleeping bag was chilly!

I have small propane heater that can take the chill off quickly but I don't like running it inside the trailer - it's a little finicky and if it does fall over while hot bad things will happen. This got me to thinking about trying to use my electric space heater when boondocking.

I've used this model from West Marine (see pic below) when connected to shore power and found that it works well. The lowest power setting pulls 600 watts and will heat the space up in minutes. I think that I only really need to use it for about 10 minutes before going to bed and 10 minutes when I get up in the morning so it won't have a huge draw.

Now for the more technical part, I have 2 lithium (LFP) 100 AH batteries that I use for my trolling motor. I plan to take these along and and power a 1000 W inverter. I had originally thought I'd put the inverter and battery inside with me but now I am thinking I can set this up in the galley and plug the teardrop shore power to the inverter and use the 120 v circuit in the trailer.

My question is whether it will make a difference using a modified sine wave (cheaper) versus pure sine wave inverter for a space heater as well as the charger for the house battery which will also be powered during this set up? Thanks all. Ben H

 

An electric heater will be just fine with the modified sine wave, especially if it doesn't have a fan. If it has a fan, it depends on what type of motor it has if it will be happy.

However, if you plug the shore power plug into the inverter it will power up anything else in the trailer not switched off, most notably the battery charger which will not be happy with the modified sine.

 

I’d certainly check with Cary to make sure that process will work. If it does and if you’re only looking for an inverter large enough to run your heater I’d only use a pure sine. The cost difference wouldn’t be worth it to me.

Depending upon your other potential future uses you might consider a portable solar generator which would have a built in inverter and the convenience of being able to keep and run it inside. Definitely more costly but definitely more convenient.

 

I do have a portable solar generator (ie battery with built in inverter) but it only has around 450 watt hours capacity, between the 2 12 volt 100AH lithium batteries I have roughly 2400 watt hours which would be a heavy and expensive solar generator to match this capacity. For now I think the inverter is the way to go for this use. Sounds like I'm better off to spend a bit more for the pure sine wave here.

 

Yeah, cost is definitely a downside to the sogens. But they are getting cheaper. I just can’t imagine carrying around two 100Ah LFP batteries and an inverter.

 

I hear ya. They each weigh about 25 pounds. I will only use this set up for the really cold weather camping.

 

Another benefit of a pure sine wave inverter is that they are the most efficient. Ultimately that’s good for a power hungry heater, whatever battery capacity you have will be maximized.

200ah of lithium does seem like a lot. Do bear in mind the lower temp limit for your specific batts! There should be a minimum charging temp, and a minimum operating temp.

I too would be concerned about using the shore power input to get power to your heater. Energizing the CI battery charger seems like a bad idea!

 

Great point about the battery operating temp. According to their website (Renogy) I am good down to 5 degrees F with one of the batteries and -4 F with the other.

Probably a question for Craig, but I wonder if it's possible to shut off the main battery switch and bypass the charger and house battery but still keep the 120 volt circuit hot when plugged into the inverter?

 

If you dip into the electrical diagrams in the manual for your model year, I think you’ll find that the battery disconnect switch is on the load side only, meaning that the AC/DC charger is always hooked up.

For those wondering where their manual is, it can be found in the What’s New section at tinycamper.com - make sure to grab the one for your model year.

I just double-checked my 2012; the manual shows that all charging sources and the optional electric brakes emergency switch are always connected to the battery, the disconnect is between the Batt + post and the fuse block.

 

Seth, would your caution apply to running a dometic, recharging a small sogen from the in camper 120v plugs, as well as running a small AC powered hearer off those outlers? Why?
Is the older battery charger somehow affected by having high loads flowing thru the shore power plug to appliances also plugged into the CI 120v outlets?

 

Let’s talk power inputs and outputs!

Shore power —> CI shore power input —> CI AC socket/output —> Solar Generator (sogen) charging input. Seems fine! Basically, you’re using shore power supplied from a campground power post or AC socket in your garage to charge your sogen. And maybe also simultaneously running a fridge, air conditioner, ceramic heater, charging the CI battery, etc. Should be no problem until you pull so much power the circuit breaker protecting the shore power source trips - the breaker on the campground power post, or in your house/garage. But really, the biggest consumers here are the air conditioner and heater, and you’re unlikely to be running both at the same time! Grasshopper, is this what you’re asking about?

In Ben’s situation, we have a couple possibilities:
12v lithium battery or battery bank (12vdc) —> 1000w inverter (12v DC to 110v AC) —> (110v AC) CI shore power input —> Heater, CI battery charger, fridge.
The sensitivity here is charging one battery (CI) from another (12v lithium). This seems unintended, but would result from the hookup detailed above. Then, you’re taking maybe half the capacity of the lithium battery and just moving it into a depleted CI battery, while trying to heat the cabin with a power-hungry method. It *could* be fine, but without battery management and monitoring it’s hard to know what’s actually going on with both batteries.

Instead:
12v lithium battery or battery bank (12vdc) —> 1000w inverter (12v DC to 110v AC) —> (110v AC) Heater.
Here it’s clear - the only loads drawing down the 12v lithium battery are the heater and whatever load is from the inefficiency of the inverter.

Hmmm… that’s a lot of words, I hope this helps! Electricity can be fun! Reading electrical schematic diagrams is always a headscratcher for me…

 

Hi Seth, That is a very helpful and apt description. Thank you!

I don't use a fridge so that won't be an issue. I normally use very little of the house battery - occasionally the lights for brief periods but prefer other lighting solutions and the fan only when it's really warm which obviously won't be the case when I am wanting to run the heater.

I am not sure how many watts the CI charger tends to pull just to maintain the battery when it's not really being used. I'll be curious to experiment a bit with this. I should plug in the killawatt and see what it typically takes.

I agree that it would be more efficient to go with the 12v lithium battery or battery bank (12vdc) —> 1000w inverter (12v DC to 110v AC) —> (110v AC) Heater. The reason I am contemplating the more circuitous route is an attempt to declutter the cabin and to get the noise of the inverter out to the galley. I wish there were a pass through from the galley into the cabin but I'm not willing to cut my own.

 

Thanks Seth, and Ben for clarifying.

 

I know this is quite subjective, but from the department of FWIW, I have used this cheap space heater a few times when I had shore power. It has two power levels, 375W and 750W. My experience was that 750 was way too hot inside the camper but 375 was just fine. I only used it before bed while reading. Through the night I did not run it.

It doesn't solve how to get power to it while boondocking, but if you can get by on fewer watts it could makes solving the power problem a bit easier.

https://www.amazon.com/Vornado-VH202-Personal-Space-Heater/dp/B00VXEJ6PC/

 

I think that’s a 1 LB. bottle that fits in the regular Mr. Buddy. The standard tank on a CI is 11 LB., and the big tank (which also fits the holddown on the tongue) is 20 LB. And I think you can feed it from the port in back with the whip from CI.

Such a heater isn’t safe for inside a sealed space, but people use them in tents. I think the CI cabin is sealed too tight for a Mr. Buddy. I’ve thought about getting one for our ARB Awning Room if we extend into cold weather.

That’s real useful data, but not very encouraging. 500w heater burned through 50ah of capacity in one hour. I think that math is roughly accurate.

If Ben has 200ah of batt
Less 40ah, should maybe leave a 20% charge on them batts,
Less 16ah, the inverter might be 90% efficient,
Equals 144ah available to a heater,
Divided by 50, grasshopper’s heater’s performance at 500w,
Equals 2:50 of heating time delivered across the two 100ah batteries IF their performance isn’t degraded by age, abuse, or cold temps.
Or about 3:40 of a 350w heater.

This math looks better than it is, because it’s based on some rough numbers and assumptions. But it’s probably good for ballpark.

 

P.S. I just pulled a rando 12v electric blanket off Amazon, 50w! Yes, that should go about 30 hours with Ben’s batts and no inverter.

 

Through some poor planning I've put myself in an interesting position. I totally screwed up while trying to book Yosemite this morning. Got very lucky and this afternoon I found a vacancy pop back up that fits my window exactly and for 6 consecutive nights with no moving. However, it is definitely back in the jungle where little light gets through all the trees.

There's no way that fridge won't drain the battery if I can't get some juice to one or the other. Kinda like boondocking in the middle of Yosemite Valley with no sun. LoL! So, it looks like I may be spending more time either charging the battery with an external panel (assuming I can reach some sun) or possibly running the fridge off one of my sogens and then finding a way to recharge the sogen via sun or 110. (Fortunately free 110 is available in the village. But of course you have to sit there and babysit it.) I'm definitely going to be spending a lot more time messing with this than I ever expected, but it's probably a good experience, right?

 

I hadn't thought of a portable ice maker! That's a great idea while out boon docking. I thought of getting one for the house rather than repairing our fridge while we wait for a new fridge. Any idea how many watts one of these draws? That could be a reason to consider an even larger inverter if needed. If one could run the ice maker for an hour or so and have enough ice for a day or 2 this could be super helpful in the summer.

 

I haven't used a fridge (yet) with the teardrop so I may be missing some of the good aspects of it however I have been pretty happy with my Rtic 50 quart. To those unfamiliar with this brand - I'd say it is a knock off of Yeti for about half the price. I usually use a couple of tall rectangular plastic containers the type you can pick up at the grocery store and meant for hold cereal and the like to store the ice. I put one on either end and my food in the middle. This keeps all the food from getting soggy but more importantly, as the ice melts I put it right into my insulated water jug. It's easy enough to buy ice in most camp grounds (though now I'm thinking about an ice maker for boon docking!). This might save you some battery capacity anxiety. Let us know how the trip goes.

 

Thanks. There's definitely trade offs to coolers vs fridges. I recently transitioned to a fridge shortly after picking up my new 560 in June 2021. I have to say I don't miss digging through ice and cold water to find food. I was reminded of that on my most recent trip when my fridge broke before leaving and I had to revert back to the cooler. It's also annoying (to me) having to seek out ice every couple of days. The only real downside to the fridge is that it is a power hog so you really need a constant supply of power to run it. I always knew that was a downside but it hasn't been an issue previously. I've always had enough sun to run it. I have 3 sogens of varying sizes and I always anticipated they might need to run the fridge at some point in time. This could be the time.

I should add that I will be very interested to see how the rooftop solar does in a very dappled light condition. I've had it in partial shade before but there was always a good window of full coverage during the day. I don't think that will be the case this time. I'm actually looking forward to the challenge. It should be fun.