Inverter & NASA BM-1

Sy-Revolution

Sy-Revolution

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Hi folks,
I'm about to upsize my current inverter so that I can use it to run the imersion heater in my calorifier. I also want to connect it to the NASA BM-1 100amp shunt, so assuming that the heater element is 800 watt what's the largest (in Watts) inverter I should go for whilst not exceeding the shunt rating.

I was thinking 1500w would be ok, the only other items I might use would be reletively low wattage, small power tools, laptop, the highest perhaps a hair drier. 1000w might be a little too close to the 800w required by the calorifier for comfort. A 2000w model I'm looking at has better features (remote switch/auto switching) Could I get away with that?

I suppose I could just be very carefull about what people plug in......

Any ideas?
 
If the inverter was 100% efficient then the maximum would be 100A x 12V = 1200 watts but as it will only be about 80% efficient then the maximum output power would be about 960 watts if the input current was not to exceed the 100A of your shunt. If you use a larger inverter at a fraction of its rating then the efficiency could well be lower but on the other hand the reliability would be better as the components would not be as stressed.

My gut feel is that a 1200 Watt unit would probably meet your needs but you would need to keep an eye on the current being drawn. As far as the shunt is concerned a short term, slight overload is unlikly to do any harm and a 1200 W unit at 80% efficiency would still only be drawing 125A (ie a 25% overload).
 
If you plan to draw in excess of 100A from your batteries, you will need 200AH of capacity for every hour that you run the load. You can pro-rata this figure. You will also need some way to replace the charge. Unless you have a 'large' alternator, again in excess of 100A output, you will be running the engine for longer than you will be running the calorifier.
Factor in also the wear and tear on the batteries, you may be lucky to get 300 recharge cycles. This means you'll buy new batteries every year.

Just some thoughts. The figures are approximate but you should get the idea.
 
Recusantz might be a little rude but he is also correct in that the use of an inverter to drive an emersion heater is crazy. A said you will need to run the engine or a separate generator to drive the inverter. With reasonable sized batteries. So the calorifier should heat the water.

Alternatives might bhe to fit a 240V alternator to the engine or fit a a 12v heating element to the water tank. However it is unlikely you could get a heating element for 12v at any decent power because the current becomes too much at 12v.

Bottom line forget it. 12v High power inverters are not really practical for 12v except perhaps for short running time jobs. olewill
 
In terms of sizing of inverter and calorifier as inverter load then 800 watts from the 1200 watt inverter is OK. Its not like power tools where the start load is much higher than the average.

But the battery capacity needed to store the energy will be large. Quick estimate : to heat 25 litres of water from e.g. 10 to 60 deg C. 4200*25*(60-10) Joules or 5.25 megajoules. Expressed in terms of pure battery discharge at 12 volts that is 120 amp hours (energy/(12*3600)) : That means running the engine for several hours to restore the charge.

Why dont you simply use the engine cooling water to heat the calorifier directly - it will probably take less time than the battery charging engine run time .

Or consider something like the Eberspacher Hydronic water heater. (costs similar to inverter + batteries + alternator upgrade)
 
Thanks for the replies, I have 5 x 110amh AGMs plus 275 amh cranking batteries, 170w solar and a rutland 913. The calorifier is connected to the engine cooling water but I wanted a backup way of heating water so that I didn't have to burn diesel every time. Unfortunately I only have a 50 amp alternator.

I don't have room to fit a hydronic thingy. It takes about 30 minutes to heat the calorifiey when on shore power, 120 amp hours sounds a lot though......
 
Have you thought of using a portable generator rather than the main engine, most are quite cheap and will probably be OK to run for an hour to heat the water
 
Again, yes I have one but don't want to burn fuel if at all possible. Also they make a racket (Honda 1k suitcase).

I'd rather add another solar panel or two...........

Bit more background, we're heading down to the Med in July to liveaboard there so there should be a bit more sun available to us.
 
A thermo syphon solar heater may be an option, and can you modify your solar and wind generator so that they dump the excess energy into the water heater?

You probably won't need as much hot water in the med anyway. You could always leave a couple of 20 litre containers (painted black) on deck, coiled hosepipes and solar showers work too.
 
Good ideas there, I was indeed thinking of setting up a thermo syphon system. My calourifier has twin coils so thats definately an option. We'll be aboard all year round so my thinking was how to heat the water in the winter, without having to run the engine every day.

Looks like I'm going to have to think again.......
 
Bit more background, we're heading down to the Med in July to liveaboard there so there should be a bit more sun available to us.
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Use the sun to heat the water directly and forget the immersion heater when away from shore power.
 
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