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Post by Skandi on Jan 2, 2017 19:54:52 GMT
If i've dumped this in the wrong place, please move it  I'm looking to make a backup battery system for my heating pump, it doesn't need to be automatic, or constantly connected, as the furnace is totaly manual anyway, and can't possibly be on without me knowing the power has gone. I need to support 40W 0.4amp at 220-240V for 2-3 days, 6 or 7 hours a day. I can easily add a plug to my pump, and have done before when I needed to plug it into a different circuit when water got in to the electrics.. Can I get away with just a battery and inverter? and if so what size battery am I going to need for this, and if my school electrics were correct those W A and V numbers do not work, but that's what's written on the motor! |
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Post by spacecase0 on Jan 2, 2017 21:44:20 GMT
AC motors take lots of power 40W for 3 days is 2880 what hours an inverter looses a bit of power, so that is likely at least 3KWH that is a pretty big battery set likely easier to add some solar or wind power to reduce the size of the batteries a bit, the other way is to use a 12V backup pump not sure what your flow rate is, but you can likely replace your 240V pump with a 12V one that uses 1/4 or less the power with the same results is it a hot water pump you are looking for ? if so, go look at this www.amazon.com/gp/product/B01G305PK0/just a few valves and you can change form one to the other... can give a proper review of it when it shows up in the mail (already ordered) if you are looking for an air pump (also called a fan), there are plenty of 12V ones that work fantastic with not lots of power required |
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Post by Skandi on Jan 2, 2017 22:46:01 GMT
Solar wouldn't be any use since all the hours we run the pump in winter it is dark. And wind requires planning permision to install, and with my dodgy 1950's electrics, I suspect they would demand a full rewire. but The pump only runs 6 hours is a day so that cuts the requirement down to under 800Wh for three days and I would settle for two.
I cannot tell precisely as the pump is not manufactured anymore, but the equivilent pumps from the same manufacturer are rated at 50 L/m so I do not think I can go to 12 V there's only 2m head but it's 171ft of piping and 10 radiators that it has to go round, and probably some condiderable amount of sludge!
What is interesting is how cheap those 12V pumps are, gives me ideas about solar water heating.. hmms. (not for heating for showers.)
We've been here three winters now and not lost power for over four hours, but it would be nice to be prepared, heating is the major issue for us. cooking is sorted, candles are always avaliable in this house.. but heating, nope nothing without power.
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Post by kkbhf on Jan 2, 2017 22:53:14 GMT
The numbers may seem odd until you take into account two things.
1) Surge demand: A motor draws a lot more current starting compared to when it's running.
2) Watts gets a little tricky with motors because there is something called a Power Factor due to the inductive nature of motors. Simply stated the formula would be V*A*PF=W. Power company charges you by the W but you, with a battery and inverter, have to be able to supply VA. Typical PF for a motor ranges 60% to 70%.
Anyways...
Let's budget 0.4A @ 240V = 96VA(The inverter you choose should be rated at least 100W continuous, 300 to 500W surge. 150W continuous recommended)
96VA * 7hrs/day = 672VAhrs/day
672 * 3days is a total of 2016VAhrs output from the inverter.
Inverters place their own demand on a battery, usually called Conversion Efficiency. Let's budget for a 90% efficiency.
2016VAhrs / 0.90 = 2240VAhrs from battery to the inverter. (VA out = VA in * Efficiency)
Inverters commonly can be had that run on 12V, 24V, or 48V batteries. Let's figure out the Amp Hours drawn at those voltages to deliver 2240VAhrs. (VAhrs/V)
@12v = 187Ah
@24v = 94Ah
@48v = 47Ah
drawn from the battery to run the pump 7 hrs/day for 3 days.
But wait! It gets worse!
You'll wear out batteries super fast if you fully discharge them, not to mention inverters don't like low batteries. Rule of thumb is never draw more than 50% of your battery's capacity. Double your required amp hours for the minimum battery capacity you'll need:
@12v = 374Ahr
@24v = 187Ahr
@48v = 94Ahr
To act as a backup for your grid look into something called a Charging Inverter. They keep your batteries continuously charged and automatically switch from grid to battery when the grid is off.
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