What size inverter do I need?

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What size inverter do I need?

Postby earl84 » Sun Feb 04, 2018 9:42 am

I have a small heater that works well for my insulated Grumman 2ish build when on shore power. I want to be able to run it to keep things warm while boondocking. I bought an inverter that I thought was powerful enough, but it keeps tripping off after about 2 seconds. I only run it a little bit and have a Group 24 battery, can’t remember to ah, but it was way overkill for my little trailer.
What size inverter do I need?

Specs on the heater:
Lasso Model 100
120V, 60hz, 200 Watts

Specs on inverter that wasn’t strong enough:
Bester Model MRI3011BU
Input 12v DC, 28.5A
Output 115vAC, 2.6A, 300W
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Re: What size inverter do I need?

Postby Socal Tom » Sun Feb 04, 2018 11:13 am

That inverter should be big enough, it may be tripping due to low voltage if the wires feeding it are too small. How is it hooked to the battery?
That heater will use 16 Ah per hour of use. That battery probably has less than 100 Ah capacity. Using the 50% rule , it shouldn’t run more than about 4 hours. Watch your battery voltage, if it gets below 12.2 then you are at about 50% .


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Re: What size inverter do I need?

Postby troubleScottie » Sun Feb 04, 2018 12:29 pm

Really need all the information:
heater information: how many amps draw; assume the highest draw if range;
your current inverter information: max wattage, max/surge amperage;
the actual number of AH in the battery as Group 24 batteries have a range;
and as above, size of the wires connecting it.

There is a nice summary and a simple calculator at the bottom: http://www.donrowe.com/power-inverter-faq-a/258.htm
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Re: What size inverter do I need?

Postby earl84 » Sun Feb 04, 2018 5:27 pm

That’s all the info I have for the inverter and heater, came from the data plate. Guess I’ll have to look up the specs online. So have to look up the Ah capacity of the battery full answer to that, but that’s more for how long it can run. The wiring from battery to PD4045 is 10 gauge, then 12 gauge from there to outlet. Run from battery to PD4045 is 12 feet, PD4045 to AC outlet about 10 feet.
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Re: What size inverter do I need?

Postby Trebor English » Sun Feb 04, 2018 8:13 pm

Measure the 12 volts at the inverter. The drop when you turn on the heater will be due to the battery size and wire resistance. Try turning on the inverter with the heater off then turning on the heater as well as turning on the inverter with the heater already on.

Just for a test can you supply shore power to the PD4045 and use both the battery and the converter?

My group size 24 battery is 75 amp hours. The 20 hour rate is only 3.75 amps. You are trying to get 20 amps from it. The voltage will drop. It can only run a couple of hours at 20 amps.
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Re: What size inverter do I need?

Postby Graniterich » Sun Feb 04, 2018 11:38 pm

Trebor English wrote:Measure the 12 volts at the inverter. The drop when you turn on the heater will be due to the battery size and wire resistance. Try turning on the inverter with the heater off then turning on the heater as well as turning on the inverter with the heater already on.

Just for a test can you supply shore power to the PD4045 and use both the battery and the converter?

My group size 24 battery is 75 amp hours. The 20 hour rate is only 3.75 amps. You are trying to get 20 amps from it. The voltage will drop. It can only run a couple of hours at 20 amps.
I disagree that it's big enough, I would expect at least double the wattage draw And good large short wire to battery. Ratings are best case

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Re: What size inverter do I need?

Postby Socal Tom » Mon Feb 05, 2018 10:07 am

earl84 wrote:That’s all the info I have for the inverter and heater, came from the data plate. Guess I’ll have to look up the specs online. So have to look up the Ah capacity of the battery full answer to that, but that’s more for how long it can run. The wiring from battery to PD4045 is 10 gauge, then 12 gauge from there to outlet. Run from battery to PD4045 is 12 feet, PD4045 to AC outlet about 10 feet.

I think that is your problem. Essentially you have 44 feet of 12 guage ( you have to account for both legs). If you are using the cigarette lighter plugs, they are very hit and miss as far as contact. The voltage drop over all that distance is just too much. The charts I'm seeing on line would recommend 6 guage for a run that long. Your best bet is to install the inverter near the battery and run an extension cord to the sleeping area. Failing that, then some sort of dedicated 12V plug with a shorter length and thicker wire. I would probably get rid of the cigarette lighter. The stuff I see on line says they are rated for about 10 amps, and 200 watts would be 16 amps, the full 300 would be 24 amps ( @12.5 volts).

To confirm you could test it connected directly to the battery. If it works like that, then you are on your way.
Tom

I do echo what the others have said. Even with best case 100aH battery, you only get a few hours, and most likely you don't have that much capacity. You may want to rethink the whole thing before you get too much into it. On the other hand, if you just want to warm the cabin in the am before getting dressed, then this could work, but you need to have a reliable method to charge the battery every day.
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Re: What size inverter do I need?

Postby absolutsnwbrdr » Mon Feb 05, 2018 10:58 am

Trying to run any size electric heater from an inverter while boondocking is not really a reasonable expectation. Converting electricity to heat takes a significant amount of energy, and you're compounding the electrical losses by first converting from 12VDC to 120VAC. 100W @ 12VDC = 8.33A, so in a 100aH battery, assuming you only take it to 50% discharge, you're only going to see maybe 6 hours of heater usage. And that's not taking into account the conversion losses through the inverter, or a smaller battery.

Without a massive power bank, you're going to be very disappointed, even with the "little" 100W heater.

Your best bet would be to get a 12V electric blanket for when you're off-grid. I have this and it works well....https://www.amazon.com/Roadpro-12-Volt- ... 0009Y2CFS/

If you're set on having a warm camper while boondocking (and not just a warm bed), you'll want to run the heater from an generator, or invest in a catalytic propane heater like an Olympian Wave-3.
Zach

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Re: What size inverter do I need?

Postby Aguyfromohio » Mon Feb 05, 2018 1:02 pm

absolutsnwbrdr wrote:Trying to run any size electric heater from an inverter while boondocking is not really a reasonable expectation...Without a massive power bank, you're going to be very disappointed, even with the "little" 100W heater...


Exactly. People use the words "power" and "energy" interchangeably but they are very different ideas to an engineer or scientist.
Energy is an amount. Power is a rate.
Batteries are great for power. Whatever energy is in them can get out fast, at a very high rate. But there really is not much energy in them, so you can end up with a dead battery quickly if you try get out a large amount of energy.

Let's use a Duracell marine deep discharge battery, model SLI31MDC for this example.
(105) amp hours x (12) volt = 1,260 watt hour

We can convert watt hours of energy to BTU directly, because energy is energy in all units. BTUs are handy for comparison, because chemical fuels like propane and gasoline are usually measured in BTUs. One watt hour equals 3.4 BTU. Let's convert the battery energy to BTU:

(1260) watt hour x (3.4) BTU per watt hour = 4,284 BTU. So we have 4,284 BTU in that battery.

A standard gas grill propane bottle is 20 pounds, and propane has a lower heating value of 20,990 BTU per pound.
(20) pounds x (20,990) BTU per pound = 419,800 BTU. So we have about 419,800 BTU in a bottle of propane.

This allows us to directly compare the energy in batteries with the energy in a bottle of propane.
4,284 BTU in a marine battery
419,800 BTU in a bottle of propane.

It would take about 100 marine batteries to equal the energy in a 20 pound bottle of propane. It's actually worse than that, because you lose energy in the inverter and in the charge/discharge of the battery. But you get the point.

A nice toasty warm heater is a heater that delivers a lot of energy. If you want a toasty warm heater, use propane or some other chemical fuel.
I think we are not aware of the difference because our portable electrical devices use only tiny amounts of power and energy, and our heavy electrical devices are connected to the grid. Things like charging a phone, or running a stereo or a laptop or a few LED lights are all small energy and small power. But space heating or heating water or running an air conditioner is heavy lifting, and you want to use either a chemical fuel like propane or be connected to the grid where some machine burning natural gas or coal somewhere is sending you gobs of energy.
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Re: What size inverter do I need?

Postby earl84 » Sun Feb 11, 2018 9:15 am

I do have a 12v blanket that I use to warm the bed. I think the plan will be to give the cabin a blast of warm heat from a propane heater (with adequate ventilation) before retiring for the night, then the 12v blanket for warming the mattress, then body heat. It stays pretty cozy with just body heat and blankets, coupled with insulated walls and a small space. Save the heater for shore power.
Thanks, everyone.
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