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one for the electrickery boffins


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Watts/Volts=Amps, 150/12=12.5


So if you divide your battery capacity by the amps we've just determined you will reveal the time you can run the appliance. Assuming your batteries are both 90Ahr




But I wouldn't suggest you use the full time calculated, if you don't want to damage your battery. Personally, I'd stop at 7hrs and that's, also, assuming you're starting from fully charged.





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Would I be correct in thinking you want to heat your van with a 150w ceramic heater? If so I would say don't bother trying, I use one of these in my Phaeton, it can can just about manage to keep clear a square foot of windscreen it provides no perceptable heat in the vehicle.
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Pete-B - 2012-11-03 8:26 PM


Theory question.


how long would it be safe to run a 150 watt 12 volt heater off 2 90 amp connected leisure batteries?


From personal experience, these 12v 'heaters' are an absolute waste of time as they give off less usable heat than a 100 watt light bulb!


Two x 90 ah = theoretical 180 ah when new and fully charged.

Possibly about 80% of that in normal use due to the inadequacies of the on board charging system and battery deterioration over time at a rough guestimate?

I wouldn't want to let my batteries get much below 40% capacity to avoid the risk of damage to them which gives you about 40% of 180 ah which is about 6 hours worth - always assuming the 'heater' is rated for continuous use?

Personally I would stick to using the on board gas heating or mains if on EHU.

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Mike B. - 2012-11-04 7:54 PM


peter - 2012-11-04 6:33 PM


Light a candle..................it will give out more heat. :D


What about if you had fork'andles? :-D


You would get a lot more heat from four candles and probably as much heat by rubbing two fork 'andles together as one of those 12 volt gizmos?

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Pete-B - 2012-11-03 9:24 PM


Sorry I should have added the batteries are fed from a 100 watt solar panel so would it be fair to say I could safely run the heater for say one hour per day?


It really depends on what you mean by "safely".


Let's assume you wanted to run your 150W 12V heater on a timer that restricts its usage to 1 hour per day, and you are hoping that the 12.5 Amps this will take from your leisure batteries will be replenished by your 100W solar panel during the period when the heater is not running.


Whether this is a realistic expectation would depend on how efficient your solar panel is and, obviously, what the weather is like (and, of course, where the motorhome is parked). In principle, you might well be able to get away with it: in practice, you'd need to check the charge-level of the batteries regularly to begin with and, if the batteries don't seem to be losing charge, occasionally afterwards to make sure your solar panel is keeping up with your heater's once-daily demand.


You've provided no clue as to why you want to do this, but I'm guessing it might be an anti-frost (or anti-condensation) strategy. I'm doubtful that a 150W 12V heater running for one hour per day would be effective in either case, but who knows?


An equation to describe your inquiry would have have more than one unknown variable, which means it could not be resolved. Running your heater for an hour won't harm your batteries if they are in good condition and well charged and, if your solar panel can replace over time your suggested 1 hour per day heater power usage, that should be OK too. But, if your solar panel's output starts to lag behind your heater's power usage and you are not monitoring your batteries' charge state, you'll have problems eventually.

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