profdant139 wrote:
Some of the preceding discussion is over my head, so let me bring things back to a more basic level. It looks like I will need an insulated battery box and a heater to keep the lithium battery above freezing. But the heater draws current, of course.
So I would have to budget a reserve of power in order to make sure that the battery is warm enough to accept a charge.
This may be a deal killer because we are almost always away from the trailer during daylight hours, hiking or snow-shoeing or skiing. So we would not be in a position to monitor the battery and the solar charger in real time, to make sure that we are not damaging the system and that there is enough juice to run the fridge, etc.
Hmmm. We may not be ideal candidates for lithium, until they work out the temperature issues. That is really too bad. But I am not ready to give up on snow camping.
Valid concerns.
Before you throw-in-the-towel, you may want to look at Battleborn's (BB) heated 100ah lifepo4 battery. It's an off-the-shelf, plug and play solution for folks like yourself that have batteries located where they're exposed to sub-freezing temperatures. This battery contains an internal heating element that draws 1.8a (powered via the battery itself or charge current). It heats the battery to 35f---just above the 32f threshold for safe charging.
BB says the 1.8a heating element will keep an uninsulated battery at 35f, with ambient temperatures of 0f, using only a 30% duty cycle. They claim the heating element will keep an uninsulated battery at 35f, with an ambient temperature of 0f, for approx. 185 hours (almost 8 days)! This, of course, assumes a fully charged battery, with no other load or charging current applied.
If you're concerned about the 1.8a draw to power the heating element, there's no need to be. Not sure what you leave on when you go out for the day, or how long you're gone. But, let's say you leave on a propane fridge (2.5a; 50% duty cycle; 1.25ah), propane heater (7a; 50% duty cycle; 3.50ah), and your RV has a 1.5a parasitic load (1.5ah). The BB battery's 1.8a heating element would draw .55ah based on an ambient temp of 0f (COLD!!) and a 30% duty cycle. .55ah is a very negligible draw relative to the fridge, heater, and parasitic. Additionally, we won't even factor in any solar input during the day just to be conservative with our calculations. In any event, everything totaled together brings us to 6.80ah. If the 100ah BB battery was fully charged when you left, you'd have 18ah remaining if you returned 12 hours later---that's a *long* day of skiing, hiking or snow-shoeing---especially when it's 0 degrees outside! Also, this is all calculated based on the use of one 100ah BB battery. Most people who boondock typically have at least the equivalent of two 100ah batteries (200ah). With 200ah you could leave your RV for 24 hours and still have 36ah remaining.
The BB is pricey at $949, but it's a proven, reliable battery with a 10 year warranty. It uses 100+ cylindrical cells in a parallel/series configuration vs. the 4 prismatic cells in a series configuration found in your typical budget battery on Amazon. If the BB experiences several weak/bad/unbalanced cells it won't significantly impact the battery's output. With series-connected prismatic cells, one bad cell directly reduces the ah output of the entire battery---sometimes dramatically.
Lastly, we built a 200ah lifepo4 battery (mounted inside a small Group 24 battery box) which uses a 2a heating element similar to the one used in the BB battery. Switchable control (thermostat or BMS). It's capable of keeping our cells a toasty 60f while exposed to ambient temps as low as 0f (using no insulation). Like the BB, these heating elements provide an efficient, realworld solution that enables lifepo4 batteries to be safely used in sub-freezing temperatures.
Good luck!
