Hello! I am trying to heat an outdoor cat house at my workplace and do not have access to an outdoor electric outlet. The set-up I have now is less than ideal and I would appreciate feedback. I have two, deep cycle 12 volt marine batteries that I attach an 800 watt 'mobile power outlet' inverter to, and the heating pad for the cat house is plugged into the inverter. I have a battery charger also. When it gets below 25 degrees F I start heating the house. I charge the batteries during the day while at work, then before leaving for the night, I hook the batteries up to the inverter. All well and good during the work week. I had hoped that by using two batteries that I could get away with turning the heating pad on on friday evening and it would all be good until Monday morning. BUT, it does not hold a charge that long so I have to go into the office on the weekend to re-charge. So the question: is there something else I could be doing to operate the heating pad during cold weather that would last at least 60 hours? Or am I dreaming? Thank you!

 

I think you are dreaming. A 12V deep-cycle storage battery has about 60Ah, which means it will supply 6A for 10h or 3A for 20h. 3A at 12 V is only 36W. A 12V automotive taillight lamp is about 30W, so one battery would keep one lamp lit for ~20A.

If you could use a really well insulated box (like an Ice Chest Cooler), and it was equipped with an automatic door that is kept closed most of the time, You might be able to keep the cat from freezing with just one 30W lamp...

I remember solving a problem like this... I had an unheated aircraft hangar, where the winter temperatures would get to 20degF for days at a time. I had a 'fridge filled with canned drinks which would freeze inside the 'fridge because even though the compressor never switched on, the inside of the box would eventually cool to the outside ambient temperature. I solved the problem by putting a 3W Night Light inside the 'fridge. 3W of heat was enough to keep the drinks from freezing, but that was a very-well insulated box..., and I only had to raise the temperature in the box by a few degrees.

 

I don't see any obvious solution except higher capacity (or more) batteries.
The batteries would also last long if you could use a 12V heating pad to avoid the inefficiency of the "mobile power outlet".

 

passive solar heating during daylight hours then electric as needed at night?

 

The factors in heating any container are the outside surface area, the insulation quality, the required difference in temperature, air velocity, and the power required to change the inside temperature.

You can insulate the walls better, locate the cat house where it is less exposed to the outside air temperature and wind, or use larger batteries.
Your best insulation rate per inch of thickness is foam board. Your best insulation value per dollar is fiberglass. You could bury most of the cat house to keep it away from the air temperature or put it inside a larger box, like a dog house. You could lay pine branches on top of the cat house to break the wind velocity.

Just some suggestions.

 

I think you are dreaming. A 12V deep-cycle storage battery has about 60Ah, which means it will supply 6A for 10h or 3A for 20h. 3A at 12 V is only 36W. A 12V automotive taillight lamp is about 30W, so one battery would keep one lamp lit for ~20A.

If you could use a really well insulated box (like an Ice Chest Cooler), and it was equipped with an automatic door that is kept closed most of the time, You might be able to keep the cat from freezing with just one 30W lamp...

I remember solving a problem like this... I had an unheated aircraft hangar, where the winter temperatures would get to 20degF for days at a time. I had a 'fridge filled with canned drinks which would freeze inside the 'fridge because even though the compressor never switched on, the inside of the box would eventually cool to the outside ambient temperature. I solved the problem by putting a 3W Night Light inside the 'fridge. 3W of heat was enough to keep the drinks from freezing, but that was a very-well insulated box..., and I only had to raise the temperature in the box by a few degrees.

Thank you MikeML. I don't quite understand all of what you were saying but I appreciate it. The cats have one of those Igloo houses which is most definately not as insulated as an Ice Chest cooler but I have it surrounded by hay bales and foam board insulation. Seems like the light bulb idea, even a 30W, would wind me up in the same predicament of having to come in daily to charge the battery. Thanks for helping me brainstorm!

 

passive solar heating during daylight hours then electric as needed at night?

Great idea except I don't have access to electricity at night. Only during the daytime for re-charging the batteries. Thank you though.

 

I don't see any obvious solution except higher capacity (or more) batteries.
The batteries would also last long if you could use a 12V heating pad to avoid the inefficiency of the "mobile power outlet".

A 12V heating pad? Hmmm. How would I hook that up to the batteries? I've seen the kind for in a car or RV that have those round plugs. And, do you think I am losing enough capacity due to the inverter to make it worth trying out a 12v heating pad. In other words, would it get me another 24 hour period of juice? Thanks for your suggestions. Brain is pondering now.

 

The factors in heating any container are the outside surface area, the insulation quality, the required difference in temperature, air velocity, and the power required to change the inside temperature.

You can insulate the walls better, locate the cat house where it is less exposed to the outside air temperature and wind, or use larger batteries.
Your best insulation rate per inch of thickness is foam board. Your best insulation value per dollar is fiberglass. You could bury most of the cat house to keep it away from the air temperature or put it inside a larger box, like a dog house. You could lay pine branches on top of the cat house to break the wind velocity.

Just some suggestions.

Thank you #12. All very practical ideas. The cat igloo is as protected as possible and still have access to an electrical outlet (during the daytime). It is surrounded by hay bales and foam board but I can always do more to insulate it for sure. Thank you.

 

A 36W tail light gets very hot because all the power is dissipated at a small point. A 36W 12V heating pad (if you could get one) distributes the same amount of heating (36W) over a an area of a sq. foot or so, so the cat can lay on it and not get fried.

A thermostat (that runs the heater only when the temperature of the heater is less than say 50degF) would also make the battery last longer.

Ah (Amp*hours) is like gallons of fuel in a car's tank. If you are lead footed, you use the fuel faster, and drive for fewer hours. If you drive slow with a tailwind, the fuel lasts longer. 60Ah in a battery is not much fuel. In fact, for the electric car fans out there, it would be about two table-spoons of gasoline...

 

To start with you need to calculate (or estimate) the heat flow into the house minus the heat flow out of the cat house. If you get a positive number you are indeed heating the house.

I'm no expert in these areas, but I do know you can buy 2 inch insulating foam for very little at any home goods store. That makes damn good insulation, especially if you combine it with some with the metal foil cover. That minimizes the heat losses.

Next is heat input. The first question is how many watts of heating power does the cat itself generate?

 

Is the floor insulated also?

 

A 12V heating pad? Hmmm. How would I hook that up to the batteries? I've seen the kind for in a car or RV that have those round plugs. And, do you think I am losing enough capacity due to the inverter to make it worth trying out a 12v heating pad. In other words, would it get me another 24 hour period of juice? Thanks for your suggestions. Brain is pondering now.

You just buy a readily available 12V socket that matches the round (cigarette lighter) plug on the heating pad and connect it directly to the battery.
But the efficiency of the converter somewhat depends upon the design and how heavily it is loaded. You probably wouldn't gain more than perhaps 20-30% in energy by eliminating the inverter, so that may not be enough to help you.

 

I know it may be a bit more complex to implement than you want but it sounds like getting the heat from chemical energy directly may be a better approach. A greenhouse heater will run for many days off a fill of paraffin. You could use the heater outside the cathouse to heat water that is then circulated through pipes to the cathouse by a small electric pump

 

Blue sky thought - have you considered a heat storage system? A storage tank filled with rock, brick or even water can be heated during the day while electricity is available. Then, the storage medium releases it's stored thermal energy at night. A discussion of such a system for a house can be read here. It has calculations for a 3-4 day reservoir. They calculate the BTU requirements, and it looks like they end up with 1 cubic foot per 1000 BTUs. Your application would be much smaller.

Just a wild idea, trying to think out of the box.

 

Blue sky thought - have you considered a heat storage system?

Exactly what I was going to suggest. There's one simple reason: Passive solar heating with a reservoir can capture near 100% of the energy that falls on it. A solar-cell battery is stuck at ~10% efficiency if you're lucky. So passive solar starts with a 10X advantage right off the bat.

Greenhouse people will use a black plastic drum filled with water as a heat sink. The water is warmed all day in the sun and delivers heat slowly and evenly (and safely) all night long.

With good insulation and attention to harvesting the heat that falls on it, you can make a cozy arrangement. Just stopping the wind and stopping the radiation to space at night will make a huge difference.

 

Get a better cat. Where I grew up and live our cats are all good to at least -30F and even colder if they are grouped together.

 

You just buy a readily available 12V socket that matches the round (cigarette lighter) plug on the heating pad and connect it directly to the battery.
But the efficiency of the converter somewhat depends upon the design and how heavily it is loaded. You probably wouldn't gain more than perhaps 20-30% in energy by eliminating the inverter, so that may not be enough to help you.

Thank you for the suggestion!

 

I know it may be a bit more complex to implement than you want but it sounds like getting the heat from chemical energy directly may be a better approach. A greenhouse heater will run for many days off a fill of paraffin. You could use the heater outside the cathouse to heat water that is then circulated through pipes to the cathouse by a small electric pump

Certainly an interesting idea and one to ponder and check in to.

 

Blue sky thought - have you considered a heat storage system? A storage tank filled with rock, brick or even water can be heated during the day while electricity is available. Then, the storage medium releases it's stored thermal energy at night. A discussion of such a system for a house can be read here. It has calculations for a 3-4 day reservoir. They calculate the BTU requirements, and it looks like they end up with 1 cubic foot per 1000 BTUs. Your application would be much smaller.

Just a wild idea, trying to think out of the box.

Definitely a cool idea to consider for other possible projects but this one is at work where it needs to be very discreet (no animals allowed type thing). I like the thinking though!