750w Inverter. How much Draw?

[Danno]

One danger is keeping things too tight, one individual in this forum had an experience of waking up gasping for air. I used computer case fans in the vent.

I was just looking at some at Radio Shack the other day, because they seem to be rated good enough CFM-wise for a TD.

[eamarquardt]

My BTU calculators say that I should be able to heat the joint with two bodies and 200watts as long as It doesn't get too much below freezing.

According to the calculator at: http://www.bdbatteries.com/acdcrv.php it will take about 8 batteries weighing 56# each or over 448 pounds of batteries to run your 200 watt heater for about eight hours. Then you will have to put a maximum of about 60 amps over a minimum of about 4 hours back into your 448 pounds of batteries to recharge them. Where do you plan on getting that much electricity?

In addition the battery ratings are at 25 degrees C, not the 0 degrees C that you expect to encounter so eight batteries probably won’t be sufficient (cold batteries don’t put out as much juice).

The numbers above don’t include any lighting or running any of the other items you discuss. I’m beginning to think you’ll need 1000# of batteries to meet your demands. On a positive note, however, if it’s that cold where you plan to camp, your refrigerator won’t be running that much and will save you a bit of juice.

Food for thought.

Cheers,

Gus

[Danno]

That calculator came up with slightly different results for me on that one. Wonder what I'm doing wrong.

I appreciate the help, but everyone here must think I'm a complete idiot, because they are missing that I am saying (over and over) that I WON'T BE USING 12v to 120V INVERTER AS A DEDICATED POWER SOURCE. I just want to be able to do this stuff in case of an emergency, or for very short periods of time should the need arise.

If I'm gonna rarely use something like a heater, then I think that it might be ok to (just that one time) draw the battery below 50%. I'm pretty sure the world won't end if I do. Is it good to constantly do that? NO. I get that.

I was beyond the the scope of the original post at this point, which was just to determine that, indeed, the inverter does draw at a variable rate that is dependent on the load presented to it by 120v devices. I guess the smart thing would have been to start another thread (or find an existing one) to discuss the heat loss and ability of a 200w heater to work in that TD. Then I should have found another thread for the AC and again another thread for the 12v options of computer fans. I just though that the thread could have a nice flow to it like a conversation, where you move on to the next thing.

[eamarquardt]

I used 120 volt load, 2 amp load for your heater, 80% efficiency inverter and the "answer" was 4 of their batteries +/- 1 battery for 5 hours use. So, if you go with their worst case 5 batteries for 5 hours it would take 8 batteries for 8 hours. Given that at low temperatures the batteries would put out even less energy, the numbers could be "worser". This number is just for your heater not allowing for your other "demands".

Cheers,

Gus,

[eamarquardt]

........but everyone here must think I'm a complete idiot,

Nah, I'm sure some pieces are missing (just like all of us are "missing some pieces").

I just think using batteries and an inverter, even in an emergency, for heating and other sizeable loads (hair dryer, microwave, toaster oven) is optimistic and ill advised. Better a generator. I've picked up a 1000 watt Honda for $100 at a garage sale and a 1500 watt Coleman for $100 (also at a garage sale) and they both work fine.

Some folks requiring “serious heatâ€

[Danno]

Is this a good or a bad time to mention that I have 3 generators? I have two 1200w ones and one 2000watt one. those are max ratings, All are cheap Black Friday deals. I got the two little 2stroke ones at menards last year for 80 bucks each, and the bigger 4stroke one this year for 150 bucks.

I think the running watts on them is 1000, and 1500, respectively.

[eamarquardt]

Is this a good or a bad time to mention ...........

No time like the present!

I think the generators are a better option than the battery/inverter option but still think running a 120 volt reefer w/o shore power is optimistic at best.

Cheers,

Gus

[Shadow Catcher]

What I used were Antec three speed case fans and we used them on low or medium.
Version 2 will be wired in direct, here is a picture of version 1

[eamarquardt]

What I used were Antec three speed case fans and we used them on low or medium.
Version 2 will be wired in direct, here is a picture of version 1

Except for the exposed wiring (which you're planning on concealing) it looks verrrrrrry nice, cost effective, and energy efficient!!

Cheers,

Gus

[Danno]

...still think running a 120 volt reefer w/o shore power is optimistic at best.

Cheers,

Gus

I keep trying the math on the fridges and I think that they really don't consume much because they cycle on and off. I think that people see the running numbers and calculate like they are on all the time. When I do the math on the little fridge in my Home office, it's running .23amps at 120Volts (on average). enter that into the calculator and I can run the thing all day long on two batteries and only draw them down to 50%. This, of course, doesn't include the extra amps the inverter would be pulling even when the fridge is off. Even in that case I think I can make an 8hr trip on one battery if it's only powering the fridge. I'm probably being a little optimistic, because the temp in my office is 70, and the temp outside could be 100+ on the road inside a TD. the fridge is gonna have to work harder, of course, in that situation. Then you have the coils on the back of the thing heating up and reducing efficiency.

At any rate, I know there are a lot of variables, but not enough to make it seem unreasonable.

[eamarquardt]

......... enter that into the calculator and I can run the thing all day long on two batteries and only draw them down to 50%. This, of course, doesn't include the extra amps the inverter would be pulling even when the fridge is off. Even in that case I think I can make an 8hr trip on one battery if it's only powering the fridge. I'm probably being a little optimistic, because the temp in my office is 70, and the temp outside could be 100+ on the road inside a TD. the fridge is gonna have to work harder, of course, in that situation. Then you have the coils on the back of the thing heating up and reducing efficiency.

At any rate, I know there are a lot of variables, but not enough to make it seem unreasonable.

Couldn't find many specifics on wattage of small a/c reefers but did find this:

http://www.diychatroom.com/f18/mini-refrigerator-wattage-9999/

Unfortunately, GE, and most other manufacturers, don't list this information. They do show their energy guide however, which states that this product consumes 345 kWh a year.

The figure above was with the reefer probably in a 75f room temp and an internal temp of 35f for a 40f difference and will use 1000 watt/hour/day. If you're ambient temp is higher your draw/useage will increase proportionately to the increase in temperature. Double the internal/external temp difference and you draw/useage will double (or worse).

Your estimate that you can get by with two 100 amp hour batteries may be attainable under "good" conditions and a very efficient inverter but you will be pushing your batteries to the limit each and every day. The BIGGEST issue to me is how are you going to recharge the batteries which will (at the max charge rate recommended) take 20 amps for 5 hours? Solar? Generator? Shore power? You've never shared your plan to recharge your batteries.

Solar doable but expensive and you need sunshine. Running a genny 5 hours a day will be a PITA and your neigbors "may not not love you for it" as they probabaly won't appreciate your cold beer as much as you do. Running your car engine for 5 hours a day will also be a PITA. Shore power will make it a breeze but only if you have shore power. If you have SP, why not use it to begin with and just use your inverter while driving and you have your alternator turning to power the reefer.

I believe your approach is not used in marine or rv applications for a reason. It's simply not practical day in/day out.

Cheers,

Gus, out

[Engineer Guy]

Inverters of different sizes from various Manufacturers have their 'standby' or 'quiescent' current draw, and it tends to increase with Inverter Output capacity. Other 'phantom loads', like TT Propane Detectors or lil Power Supplies and Chargers, are also becoming more widely understood as loads not to be ignored when calculating power requirements.

A 1,500 Watt Wally World Heater indicated 5,100 BTU output. So, your ~200 Watt Heater would output 680 BTUs at 100% efficiency [theoretical]. Only an empirical test will tell if that's enough and if it's practical, energy use-wise.

I'm a big fan of Insulation, so I'd do what you can in that regard. Then, shift from theoretical discussion - which will only get you in the ballpark anyway - to a practical test setup. Hook up your lil Heater, and borrowed other stuff via a temp Power Strip, and see how it does in Real World conditions in your Driveway with variables like Bedding or a BTU- producing Dog in place. Outside the TD, hook up a few Spotlights, or some other load, the same total wattage as the 115 VAC Fridge of interest. Flick them on once, then off, for ~1/2 hour for each on/off condition to simulate the ~50% duty cycle of a chilled-down Fridge. I strongly suspect you'll rediscover what's already known: 115 VAC Fridges, Hairdryers and Heaters really suck down the Inverter input Amps, even over the short time periods calculated accurately above. I was surprised and educated the other day to learn that 3-way Fridges use ~Amps to operate their internal cutover electronics that auto-switch optionally between energy types. On Big Rigs trying to Boondock efficiently on Solar, it turns out the Leveling Jack Sensors are always on and consuming ~Amps. Not a direct TD concern, but insightful. Lil 'gotchas' abound.

As mentioned above, Inverter losses and phantom loads likely will be known only when you grab that 'Kill-A-Watt' and Voltmeter and check Battery condition for yourself in drawdown scenarios. Then, you can tweak your setup to achieve the desired performance. As I read this Thread, part of the meandering is in trying to supply well-intentioned numeric answers based, in part, on non-numeric inputs like 'run very rarely' or 'reasonable'. G.I.G.O..

One fellow I just read was a big fan of pricier AGM Batteries, due to the ability to slam more Amps into them during the charge cycle. His 'vision', and experience, was to use a High Output Alternator in his Expedition Rig, or TV, to achieve short charge cycles [~1/2 hour] that Solar Panels or a Genny couldn't achieve; not even close. He got me to re-thinking my future design... I've also bookmarked a lil Project where a Lawn Mower Engine [not designed for long duty cycles] directly runs a typical 37 Amp Delco Alternator w/built-in Regulator for charging. The trick is add a better Muffler to achieve Honda Generator-type low dB levels [<60 dBs].

One Big Rig Owner cited his direct experience to claim he got ~4 years life from paralleled +12 VDC Deep Cycle Batteries. If he discharged them below 50% 'too often', the life dropped to 2 years. This is not inconsistent with other data I've read. Going to +6 VDC Golf Cart Batteries in Series/Parallel, he got ~6 years life. So, he was a devotee of using +6 VDC Golf Cart Batteries. As the Masthead of a lil CO Mountain Town Newspaper says: 'You hear what you want to hear. You see what you want to see'. So true.

These folks linked below have some good observations. Also, Off Grid Solar/Alt. Energy Geeks have reasons to run the numbers exhaustively. So, along with the Mr. Solar Website above, check those Alt. Energy Resources on line. They will have answers for things like Fridge start-up draw, and long term effects of exceeded 50% drawdown, or if Heaters/Hair Dryers are practical. My own bias is to skip most conversion inefficiencies, like Inverters and 115 VAC -> DC Adapters, and run things directly. My Cellphone optionally runs/charges off a +5 VDC USB Port. There are AA Battery Chargers, and all sorts of neato things, now running off of USB efficiently [1 Amp max. per the latest USB Spec]. Not to mention integral USB outputs now on higher-end Charge Controllers...

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