200W Power inverter

vitaminh, Dec 3, 1:25am
Hi. How fast would a 200w power inverter drain a car battery! ie how often would I have to start the car if it was say running a small fridge/TV! Cheers.

intrade, Dec 3, 1:30am
it be drained way to fast. volt times amp = watt so you usually loose 25% of power on converting , for fridge i would use a gas model instead and for tv you want a led preferably one whom runs 12 volt by default

intrade, Dec 3, 1:35am
lets say you run the 200 watt inverter at its max capacity right thats 200 watt and volt times amps = watt so 240 volt is the output of mainsso its 1.2 amps it draws the problem is its the same on 12 volt battery so lets sayyour 12 volt battery has got 100 amp houres thats its maximum capacity and you cant drain it to 0 you only can drain about 10% of that befor the car wont start so you only got 10 amps and volt times amp= watt 10 times 12 volt = 120 watt so within less the 30 minutes the car wont start no more when you drain 200 watt via a inverter on a 100 amp battery. my figures are not fully correct but the calculation factors are correct.

intrade, Dec 3, 1:40am
you would have to start the car every 5 minutes and the alternator makes most times 40 amps that is 480 watts only that goes back in the battery plus the engine needs power also to operate so it would probably just power the 200 watt inverter if you constantly run the engine . unlessyou got a huge mother deep cycle 1000 amp hour and a big mother alternator pumping out plenty amps when the engine is operating.

mm12345, Dec 3, 1:44am
You don't lose 25% on inverting - they are >90% efficient.
But even very small compressor fridges draw about 4 Amps though, the 3-way fridges absorption fridges draw more (when on 12/24 or 230v), and 12v (peltier device) perhaps even more.
The small 230v compressor fridge might also draw much more than 200 watts starting, so you'd need a big inverter.
From my experience with people on boats, it's a real struggle to keep up with a small fridge with 80 watts of solar panel in summer.Without charging, even a large battery won't last a day.
Gas is the way to go.

mm12345, Dec 3, 1:44am
You don't lose 25% on inverting - they are >90% efficient.
But even very small compressor fridges (ie waeco) draw about 4 Amps, small 3-way absorption fridges draw more (when on 12/24 or 230v - a small electrolux chest fridge draws 6A / 12v continuous), and 12v (peltier device) fridge perhaps even more.
The small 230v compressor fridge might also draw much more than 200 watts starting, so you'd need a big inverter.
From my experience with people on boats, it's a real struggle to keep up with a small fridge with 80 watts of solar panel in summer.Without charging, even a large battery won't last a day.
Gas is the way to go.3 way Electrolux (about 40 litre!) chest fridge use ~ 200g of gas per day when turned up full (less when turned down) - a 5kg gas bottle will last weeks.There are (or were) other brands based on the Electrolux design going cheap at the Warehouse etc - they seemed to be perfectly okay.

ralphdog1, Dec 3, 5:23am
Short and simple answer is what you are proposing is not practical.
A 3 way gas fridge is.

meathead_timaru, Dec 3, 5:48am
What he said.

chebry, Dec 3, 5:59am
Mate o mine ran a whole house off a 400 watt inverter and a $50 injected Volvo he used to run the generator every morning to charge his battery no problems at all

thewomble1, Dec 3, 6:09am
But the fridge would not be running 24/7. Off on so as to keep it cold.

bitsy_boffin, Dec 3, 6:21am
Let us assume, 200w continuous, zero losses, both of which are silly assumptions, but for the sake of argument.

200w out, is 200w in (no losses remember).At a nominal 12v, that's 16.67 Amp.

Now find how many Amp-Hours your battery is rated for, divide, and there you go for a ball-park.

I believe that a typical car battery is in the region of 40 amp/hour (before you get down to 10v) that gives you 2 and a bit hours supply if you are sucking up 200 watt.But of course, if you want to start the car, you're going to need a good bit of juice left.

And remember, this is in the very much theoretical ideal condition of no losses.That said, you might not be using 200 watt the whole time, and probably aren't.Your fridge only needs to run when it needs to cool.

But of course, that's not the whole story, because car batteries just are not designed for this, and they are not generally rated in Amp-Hours because of it.Car batteries are designed to provide a very large current for a very short time (which is why they are rated in CCA, a measure of the current they can throw at the starter in a cold condition).Even if you could get the theoretical 40 A/hr out of the battery, it's going to be bad for the battery and have a really short life.Car batteries are not designed to deep-cycle.

meathead_timaru, Dec 3, 6:39am
But it's not 200W that a small fridge draws. It's more like 50W and the duty cycle is lucky to be 20%. So say 4A for 12 minutes total each hour, say 3 minutes at a time.

But the real problem is as you've stated - it's the wrong type of battery (starting vs deep cycle) to run low draw/long duration appliances. A lead-acid starting battery will be killed in short order.

mm12345, Dec 3, 7:00am
These:
http://www.my-caravanning.com/en/products/cooling/compressor-coolers/WAECO-CoolFreeze-CF-35/24024
Are about as good as it gets for something of useful size.Claimed "average" power use <30 A/h per 24 hour period.
But this is what an 80 watt solar panel charging a deep cycle battery will struggle with.With panels, battery, and charge controller, the kit will cost north of ~$2,000 (and you'll probably have to p1ss around to keep the panels orientated to the sun, or get more than 80w - which will really hurt the wallet.
An absorption fridge of similar size can be bought for $500, connect to a gas bottle - a 9kg bottle will keep it running for about 2 months.

scoobeey, Dec 3, 10:43am
Solar panels are cheap as chips now thanks to china:)