How many batteries are needed for this system?

[morton.bia]

I'm making a teardrop for weekend trips. We need a small heater to take the chill out on freezing nights. Only needs to work for 1 night then will be back home to charge it. Using a walmart 400w inverter and a 400w tiny ceramic heater. Heater only works for a few minutes per hour. I was calculating as follows

400w x 12hr = 4800wh
4800wh x 1.3 = 6240wh
6240wh ÷ 12v = 520ah

with walmart deep cycle batteries only being 105ah I'd need 5 of them? Is this correct?

[SunEagle]

I'm making a teardrop for weekend trips. We need a small heater to take the chill out on freezing nights. Only needs to work for 1 night then will be back home to charge it. Using a walmart 400w inverter and a 400w tiny ceramic heater. Heater only works for a few minutes per hour. I was calculating as follows

400w x 12hr = 4800wh
4800wh x 1.3 = 6240wh
6240wh ÷ 12v = 520ah

with walmart deep cycle batteries only being 105ah I'd need 5 of them? Is this correct?

You might be better off using 6V batteries. You can get them in a higher Amphour rating and wire them 2S2P. That should get you closer to the 520Ah system you need and only use 4 batteries.

My concern is that your inverter is only rated for 400W yet your load is also rated for 400W. While a bigger inverter could drain your batteries faster you might want to look at a 600W inverter.

My other concern is that you could drain your battery system if you do use that heater for 12hours.

[sdold]

Heater only works for a few minutes per hour. I was calculating as follows

400w x 12hr = 4800wh
4800wh x 1.3 = 6240wh
6240wh ÷ 12v = 520ah

Maybe I missed something, but if the heater only runs for a few minutes per hour, wouldn't the numbers be much smaller? For example, if it only runs ten minutes per hour, in a 12 hour period that would only be 800 wh. Size your batteries so that they don't discharge more than 50% (flooded) or 80% (AGM).

Those sound like they would be really fun to build. Feel free to post pics!

[SunEagle]

Maybe I missed something, but if the heater only runs for a few minutes per hour, wouldn't the numbers be much smaller? For example, if it only runs ten minutes per hour, in a 12 hour period that would only be 800 wh. Size your batteries so that they don't discharge more than 50% (flooded) or 80% (AGM).

Those sound like they would be really fun to build. Feel free to post pics!

Thanks. I missed that data about only running a few minutes an hour. I just went along with the OP's 4800wh calculation.

I agree with sdold. The OP will need a much smaller battery system, but I would not go with 5 parallel wired batteries. Find out what you really need and go with those 6V batteries.

[morton.bia]

Thanks. I missed that data about only running a few minutes an hour. I just went along with the OP's 4800wh calculation.

I agree with sdold. The OP will need a much smaller battery system, but I would not go with 5 parallel wired batteries. Find out what you really need and go with those 6V batteries.

Problem I face when going 6v is space and weight since they pretty much double quantity but dont halve the weight lol.

[NorthRick]

You'd be better off with a small propane heater and thicker blankets. Run the propane heater before bed and when you first get up in the morning. Turn it off for when you are sleeping and rely on body heat and a good blanket or two to kept warm in bed.

[bcroe]

Propane is the choice of RVrs. But if you must go electric, how about a small heater
that operates directly from the battery voltage? Bruce Roe

[sdold]

Problem I face when going 6v is space and weight since they pretty much double quantity but dont halve the weight lol.

They don't double the quantity. Say you need 200AH at 12V.

A 100AH 12V battery is about the same size and weight as a 6V 200AH battery. So you have two choices, both use two batteries:

1. Use two 100AH 12 batteries in parallel, or
2. Use two 200AH 6V batteries in series.

Both setups take up about the same space and weigh about the same but #2 is better for cycle service because the batteries will charge and discharge equally.