calculating battery size

DomB · Jan 20, 2012

Chaps,

This may be a little off the solar topic but maybe an off-grid expert can help?

I've been asked to provide a battery powered heating system for a kennel.

The heater is a 60W 240V device. He wants to run it for 10 hours a night and recharge the batteries every couple of days

Can anyone sanity check my calculation on the Ah of batteries rqd:?

60W @ 12V = 5A.
5A * (10 x 2)hrs = 5A * 20h = 100Ah
Add 20% for inefficiencies = 120Ah.

So if I hook up a 135Ah battery like this one
EXV135 Enduroline Calcium Marine and Leisure Battery 135Ah 1100MCA - Leisure Batteries - Enduroline Ca+ Leisure Batteries

to an inverter like this one
300 Watt Numax Inverter 12 Volt - Inverters for Batteries - 12 Volt Modified Sine Wave Inverters

Then all should be well?

He just charges the battery up every couple of days..

Any thoughts much appreciated!

solarfred · Jan 20, 2012

Draining a battery will shorten it's life and taking too low will kill it so AFAIK you should aim to only deplete it to 50% of capacity.Which is why good battery based systems are expensive to set up. Others will know more but I think you need 2 x batteries or recharge more often.
Just thinking aloud - what about a heat mat based system? They use them in reptile cages etc.

sharpener · Jan 20, 2012

Leisure (deep discharge) battery is a good choice but even so you have not got much margin for degradation.

If you could find a 12V heater (?car/caravan accessory ?headlight bulb in a biscuit tin) you would save the cost and power loss of the inverter (even if the heat does end up in the kennel).

But 60W strikes me as a lot for heating a kennel anyway, I used to use a 60W tubular heater to keep the damp out of a 32ft boat! Maybe a room stat would keep the consumption down as well as improving the comfort of the mutt. N.b. if you are using a mains tubular heater beware they have surprisingly high surface temperatures and need a guard.

MarkieSparkie · Jan 21, 2012

Allowing 20% for conversion inefficiencies is fine for the low power inverter you propose, but you should allow a further 25% battery capacity to avoid repeated deep discharge degradation that will shorten the life of even the best leisure battery.

12V heat mats are available for reptile cages and would remove the need for the inverter. Or you could consider the use of a mains therapeutic heat pad. I think the dog will appreciate this type of solution more than an inefficient tube heater heating the ambient air, where most of the heat will be wasted in the kennel roof space.