Inverter Questions

spongerx

spongerx

Member
I just installed a Cobra 400 watt inverter right off the battery. With the key off, I'm running a small heater (200 watts) but can't run it without the inverter giving me, I assume, the low battery beep. The battery is about three years old. Should I expect this?

The reason I added the inverter is because I'll be living out of my 4runner for a month this winter and wanted to be able to run this heater while sleeping (windows cracked or course). Maybe I need a dual battery set up... Any advice is much appreciated.
 
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spongerx said:
I just installed a Cobra 400 watt inverter right off the battery. With the key off, I'm running a small heater (200 watts) but can't run it without the inverter giving me, I assume, the low battery beep. The battery is about three years old. Should I expect this?

The reason I added the inverter is because I'll be living out of my 4runner for a month this winter and wanted to be able to run this heater while sleeping (windows cracked or course). Maybe I need a dual battery set up... Any advice is much appreciated.
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If it alarms from the second you turn the load on, the first suspect is the DC-side wiring. You're probably pulling 20 amps on the 12V side. The Cobra alarms at <10.5V. So check your battery and your inverter terminals with a voltmeter under load. If over 10.5 V at the battery and under at the inverter, time to redo your DC wiring (I'd say minimum 10 ga., 8 ga. would be better.) If under 10.5 at the battery, you need more battery or that battery is on its way out.
 
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I hope your plan is to warm up the interior and then rely on a sleeping bag, no way are you going to be able to run that through the night on your primary battery, or even on a second battery, without severely discharging/damaging it.
 
Definitely need a dual battery for that. Even then, you may run out of juice before the night is over. My stepfather uses a heat blanket off an inverter when cold camping. ..has a group 34 battery he uses just for the blanket. First night is ok, second night runs out of juice. If you have a chance to fully recharge a deep cycle battery daily (a loose battery, plug in a trickle charger to a wall plug during the day) then maybe there is a chance. Just driving a bit daily won't be enough to fully recharge.
 
Not even close to a full night. 20 amp draw, if continuous, think two hours max in cold weather if you want car to start (just a wild guess, depends on battery, temperature, charge needed to start). If it has a thermostat, and runs only 20 minutes an hour, maybe you get a six hour sleep. 200 watt resistive heat is equivalent to having two extra people in the truck, (1 body = 100 watts heat) so it's not a big deal. It's gonna have to run if it's cold, maybe flat out. Definitely dual battery situation. A separate deep cycle would be best. You will kill a starter battery going to minimum restart charge very often.

Good points above about the recharge. Stick to one-night stands.
 
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TheDurk said:
Not even close to a full night. 20 amp draw, if continuous, think two hours max in cold weather if you want car to start (just a wild guess, depends on battery, temperature, charge needed to start). If it has a thermostat, and runs only 20 minutes an hour, maybe you get a six hour sleep. 200 watt resistive heat is equivalent to having two extra people in the truck, (1 body = 100 watts heat) so it's not a big deal. It's gonna have to run if it's cold, maybe flat out. Definitely dual battery situation. A separate deep cycle would be best. You will kill a starter battery going to minimum restart charge very often.

Good points above about the recharge. Stick to one-night stands.
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Ooh boy I love this stuff. Bottom line you all are mostly right, not going to split hairs about it actually being under 18amps draw. But if you get a good deep cycle battery and use this calculator you can figure out EXACTLY how long you can run it.

BatteryStuff Tools | Determine Run Time for Specific Load Calculator

:pop2:
 
Just a side observation not relevant to the questions:

Windows cracked just lets your heat out. I can't think of a reason to crack them other than reducing condensation. That electric heater isn't going to be giving off carbon monoxide.
 
Moisture build up from your breath condensates very quickly inside a closed vehicle like that. Ultimately raising the humidity levels and making it feel much much colder inside the truck.
 
IBallEngineer said:
Ooh boy I love this stuff. Bottom line you all are mostly right, not going to split hairs about it actually being under 18amps draw. But if you get a good deep cycle battery and use this calculator you can figure out EXACTLY how long you can run it.

BatteryStuff Tools | Determine Run Time for Specific Load Calculator

:pop2:
Click to expand...

Huh? Under 18? While 20 was a guess, I fine calculate it to 200watts/12Volts=16.6amps, then a 15% bump for inverter losses, giving me 19.52 lovely 12V DC amps from the battery, rounds to 20. Of course, you could calculate the average delivered battery voltage from full charge voltage of maybe 13 in cold weather to low alarm of 10.5 volts, to get an average draw for the full cycle, and with the shape of the voltage curve being pretty steep at the top, the delivered voltage probably averages even less than 12V, bumping the inverter draw even more. I'd say 20 is conservative. 20% could be argued for inverter loss as well.

Actually, I enjoy splitting hairs.

....if you want to split hairs.:gun5:

It's on!

P.S. The site with the calculator you used also has a draw calculator that ignores inverter loss, as is discussed in the comments. This seems a basic flaw to me. The runtime calculator you posted works off 12V load, so the loss factor does not apply.
 
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hour said:
I hope your plan is to warm up the interior and then rely on a sleeping bag, no way are you going to be able to run that through the night on your primary battery, or even on a second battery, without severely discharging/damaging it.
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Yeah, I got a couple of good sleeping bags - one inside the other. I was hoping to be able to run the heater before bed and then in the morning so I don't have to have the truck running all the time. But I was also hoping to be able to run a set of boot dryers while sleeping. Wet snowboard boots will ruin your day.
 
I've done this a couple of time and the best thing that I have found to heat my Rig while sleeping inside safely is survival candles and I have used a alcohol burning heater i made out of a paint can which will run u out of your runner and its cheap to fuel. 3 bottles alcohol is only a couple bucks verses $100 bucks a battery
 
TheDurk said:
Huh? Under 18? While 20 was a guess, I fine calculate it to 200watts/12Volts=16.6amps, then a 15% bump for inverter losses, giving me 19.52 lovely 12V DC amps from the battery, rounds to 20. Of course, you could calculate the average delivered battery voltage from full charge voltage of maybe 13 in cold weather to low alarm of 10.5 volts, to get an average draw for the full cycle, and with the shape of the voltage curve being pretty steep at the top, the delivered voltage probably averages even less than 12V, bumping the inverter draw even more. I'd say 20 is conservative. 20% could be argued for inverter loss as well.

Actually, I enjoy splitting hairs.

....if you want to split hairs.:gun5:

It's on!

P.S. The site with the calculator you used also has a draw calculator that ignores inverter loss, as is discussed in the comments. This seems a basic flaw to me. The runtime calculator you posted works off 12V load, so the loss factor does not apply.
Click to expand...


Yeah it was designed to determine battery amp hour needs, not truly to give you only run time. And yes inverter loss is sadly ignored, unless you look at the RV sites, but then we open up all kinds of cans of worms.

:yield:
 
200 watts at 120vac is 1.6a. Inverters to 12vdc is 10:1 so 1.6a*10 is 16a on the 12vdc side.

I used a Johnny hand warmer when it was cold outside. It used lighter fluid. The other burner sounds good too.

Use the RC of the battery to figure out how long it will last. I think most RC's are at 20a.
 
Get a good winter sleeping bag and put a 12v heated blanket inside it. Don't bother with the inverter. Your battery will be stone dead before you can warm up the runner


You need a bigger inverter to run that heater. Your probably hearing a overload buzzer
 
spongerx said:
I could always go with something like the Mr. Heater Little Buddy. That would help on the really cold nights.
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That's what I'm getting for inside my camper shell on truck, maybe the next size up so I can use it in a tent too should I set that up in the winter.

Shouldn't kill you since you have to crack a window anyway, and you can get battery operated alarms for cheap on amazon - a couple of em even of various brands for redundancy.

Even still, I was just going to warm up the interior before hopping in. No suggestions on the boot warmers, but your general plan is going to be rough no matter how you do it. I've slept multiple nights in a row in the back of my 4runner, and I know others on here have done the same, but I've always been rather drunk and had nothing physically demanding to do the next day(s). Snowboarding, sleeping in a confined space in the cold, and doing it all over again the following day for a month? Ooooof
 
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