First, the assumptions:
Battery capacity falls by about 1% per degree below about 20°C.
So at -5F = -20C we would see about a 40% degrade in capacity.
Starting with a 100Ah battery = 60Ah at -20C.
A 100Ah battery will reach 80% after only 12Ah are drained.
The question:
Can a heater be used that will provide more capacity back into the battery than it will use to operate?
The worksheet:
A 10w heater will use slightly less than 1Ah to run. If I use yesterday as an example day, I started generating power around 8:10AM and stopped about 4:20PM. This was 8 hr of at least meeting my paultry 2A load with sun power, which is also 16hr of battery run, or a 32Ah overnight draw.
Adding the heater to it, would increase the draw by something less than 16Ah. In theory, if I also insulate the battery a bit, the heater should be able to give me say 30Ah of extra capacity.
It would seem to be a win. Can anyone verify? What did I miss? I don't have a heated building (yet), so this is yet another opportunity to learn. I have cold cells.
Thanks
if you can wrap some insulation (batts or sheets) around the batteries, their self-heating while being charged, will help somewhat.
Maybe a generator powered heat mat under the batteries, to warm them up?
What you don't want is a death spiral, where an electric heater drains the batteries to the point, they can freeze.
It may be easier to add an extra battery to share the load, if you can support charging both.
try building a plywood box, make it high enough for an elevated placement (shelf) for the battery. put a 10w, or less, incandescent light on the bottom of the box. Use an indoor/outdoor thermometer, have an on/off switch on the outside, test the box for temp before adding the battery and see just what you get, you probably will need to add a thermostat to automate it.
ps, an old beekeepers trick for liquifying hardened honey using an old freezer a small lamp.. and time...
Funny you should mention that, Westbranch. The heater pad I was looking at is a 12v silicon 10w beekeeper hive heater. I would certainly put some controls around it, both for voltage and heating, and insulate the battery, as 10w isn't a lot.. The math seems pretty revealing as far as capacity goes for keeping the battery warmer. Could run it from the load terminals on my KID even, with voltage level turn off. The pad is about $20 on amazon.
The extra battery solves lots of problems, of course, except for cost.
I would not even put any insulation around it till you see what a small light does inside that small an enclosure...
Quote from: Westbranch on January 10, 2015, 12:06:52 PMput a 10w, or less, incandescent light on the bottom of the box.
Quote from: Westbranch on January 11, 2015, 12:19:40 AM
I would not even put any insulation around it till you see what a small light does inside that small an enclosure...
Big difference... is it mounted on (under) the outside the box, or is it inside the box? From a thermal design approach either can be made to work. But what about NEC... isn't there something about electrical sources of ignition inside a battery box? Would a licensed installer ever put an electrical heat source inside someone else's battery box?
--vtMaps
My first thought was just to put the pad itself under the battery. It is a flat silicon pad. My battery just sits in the open on a wooden workbench, no enclosure. I am not sure a 10w pad would put enough heat into a battery when the ambient temperature is around 0F though. If I did do a proper battery enclosure, I would not put the heat inside for the reasons you mention, whether called out in the NEC or not. Heating the outside would also reduce the effectiveness of the heating pad quite a bit, but be a much safer setup. There is likely a proper rated battery enclosure out there somewhere, that is insulated inside of a double wall or such. At that level, just going with a 2 battery setup might be the better choice. I am not going to spend a lot of money on the 12v setup as the plan is to go 48v. I am just having fun and learning.
VT makes a good point> I think a small piece of 'rope light' that could be used like your heating pad with all connections outside the box, would work. You wouldn't want to cut it up until you know how much eat you need. You can shorten it in about 12 inch segments
The light would be adjustable, but some of the wattage is of course radiated away outside of the IR spectrum. Given the price of the pad, and they both have to be shipped, I decided to go that direction to play with. Here is a link to what I am talking about if anyone is interested:
http://www.amazon.com/gp/product/B00F3P999K/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1 (http://www.amazon.com/gp/product/B00F3P999K/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1)
I will hook it to the load terminals on the KID to shut it off when the battery hits a certain level. It should be fun to see what happens. The only downside is it appears to be on the slow boat, literally.