24v200ah lifepo4 with no solar. Thinking about solar panels & diy heating for safe charging

nyoffgrid

Hey all,

I have an off grid cabin in Sullivan county NY. Very shaded land with tall 50ft trees. I have a single 24v200ah lifepo4 battery that runs my cabin (fridge,rv water pump, ductless ac, and so on) using a victron multiplus II. Been going great. When I am in the cabin my SOC goes from 100% to about 65% at the end of the day. I then charge with my generator - 2200w honda for about two hours and it goes back to 100%. This works well to me. But.. I came across some good quality used panels near me. 260w each for $50. So assuming they are good.. 

Since I don't have any power draw while I am away from the cabin, so let's assume - in summer - that I stay there for two days ( weekend) battery will go down to 30% soc and then I leave for one or two weeks.. If I use two of those panels plus a victron 100/30 mppt then it might be able to fully charge my battery while I am away, or close to it, right?

Also, if I set up a solar system like this then I might charge a little while I am there so when I leave it won't even be at 30% soc and be higher? 

Really rookie question but is there any issue with using cabin, so inverter will draw from battery, during the day but the mppt will also charge the batteries at the same time? is that an issue? 

In winter... I cannot charge using panels when cold. While I am there the cabin is hot from woodstove and pellet stove but when I am away.. it will be frozen. I am thinking of using a thermostat controller module (like the Dwyer TS-88T) to then feed a relay (like the Bosch 0986AH0081) that then connect to those stick on heating pads (maybe 4 of them 30w each for a total of 120w) - will need buck converter i know) and then set it up so when battery is cold - the dwyer will send power to the heating pads only - only drawing from solar panels, and then once reaches a safe temp - it will switch off heating pads and send power to mppt to charge.. Sounds possible? 

Wheelman55

OP. You’ll need a space for the array (solar panels) that is sunny, with zero shade, for a couple of hours each side of solar noon. 

Even a leaf or single branch will severely limit solar production. 

The Honda is a good solution if you don't have the sunny space. 

BB.

Do you have access to a 'sunny' spot within a few hundred feet of the cabin?

Using a high voltage array and MPPT charge controller (MPPT upwards of 400 VDC), you can send the power pretty efficiently over smaller AWG copper wire.

-Bill

nyoffgrid

Hey Bill, I do but I think on my cabin is also a decent spot. I currently a small 100w 12v panel with 12v battery for toilet fan and it gets plenty of sun. 

Since the set up with these two used panels is so cheap I will try to see how it works this summer. And then maybe set up that system for winter.. any thought on my heating plan? 

BB.

Li Ion batteries are so efficient, that (from what I can tell) they cannot "self heat" during charging and discharging cycles... Unlike Lead Acid batteries that when installed in an insulated box/area, once they have been heated (from a  winter "cold soak", they can keep warm enough from normal cycling.

For Li Ion, installing in the batteries in an insulated box, and using heating pads to keep warm should be an interesting solution.

That really leaves the question of heating the batteries from a "cold soak". Using your genset would be a good start... Looking at your specific Li battery specs... Many (some, most, all?) can discharge when sub freezing.

For example from a "random" LiFePO4 battery:

Recommended Temperature

Charging: 32°F ~ 122°F / 0°C ~ 50°C

Discharging: -4°F ~ 140°F / -20°C ~ 60°C

Storage: 14°F ~ 122°F / -10°C ~ 50°C

They certainly can be discharged to/below 0F to run heaters... And you only have to heat to ~32-45F or so to begin charging.

Many Li Ion also come with BMS (battery management system) installed. So you have to check out the operational range for the BMS, what happens if BMS shuts off, how to reset to "on" again, etc.

Do you need DC power during winter "storage" (nobody there, run alarm/cell phone etc.) or is this just needing to bring the batteries to operating temperature when you come for a visit? 

-Bill