Lithium auxiliary battery and cold weather charging

hour

Observer
Good test occurring right now. Last night it was below freezing by ~5:30pm and the average of my two temperature sensors was 51*F at 7:30pm. I expected that heating would begin by like 8pm but it didn't drop low enough to kick on until 10:45pm, with ~188ah remaining at that time. Woke up this morning @8am to 182ah remaining. It's worth mentioning that I have a constant load from electronics amounting ~4 watts. A bit more consumption than previous weeks... makes sense, colder.. and colder earlier. But it's still not terribly significant if expecting to get even a little sun the following day.

It's not going to get above freezing today.. 27*F right now, high of 31*F. No sun to speak of, gray skies, pulling in a whopping 3 watts off the panels. I definitely won't make up what I took out overnight today, and I'll be using energy continuously to maintain set temperature since the sun won't be warming me up at all.

This would be a great day to just disable heating all together, start it up again at 3am or something.
 

SBDuller

Member
working up insulation system for the LFP. a case of measure twice cut once still get it wrong. need to visualize and hands on to build. scrap Alu plate for possible heating unit. resistance to cold flow. need to integrate battery restraint. just thinking outloud

jpON7TX5T2eN%go+yvo3wA.jpg

47* this morning per VE. sitting on shelf in garage. Temps outside down to 33* this am, and expect same overnight. For fun the LFP is placed within the incomplete insulation package, which
has additional layer of Thermarest mat for 'wiggle' room, once rigid insulation box is fully formed. warmed the LFP up to 53* prior to this wrap, and now left for overnight in garage.

fullsizeoutput_94.jpeg
 
Last edited:

hour

Observer
working up insulation system for the LFP. a case of measure twice cut once still get it wrong. need to visualize and hands on to build. scrap Alu plate for possible heating unit. resistance to cold flow. need to integrate battery restraint. just thinking outloud
That's going to be insanely efficient.
 

Alloy

Active member
I'm in agreement to the Polyisocyanurate ( hard word to spell) constructed battery box. When I have battery on hand, I'll start mock-up. Looking @hour container, needs a blanket at least
Isoboard isn't the best insulation for (below 40F) cold.

Below 40F the gas inside each of the air pockets condenses.
 

john61ct

Adventurer
Solution - thin layer XPS facing the extreme cold side, even if only 1/2

Rest being iso.

Test best for freezers on boats, not counting super pricey NASA level stuff.
 

luthj

Engineer In Residence
Polyiso uses a blowing agent which condenses in sub zero temps. So for those conditions, its best to use XPS. But when you have an exterior that's well above 75F, and an interior well below freezing, a hybrid breakdown using both can be ideal.
 
Last edited:

Alloy

Active member
XPS is the best for flat panel RV applications because works at all temps and does not require aditional waterproofing.

Paper/foil is used on Polyiso to prevent water from entering but water will enter the foam that is not protected.
 

Verkstad

Raggarkung
Polyiso uses a blowing agent which condenses in sub zero temps. So for those conditions, its best to use XPS. But when you have an exterior that's well above 75F, and an interior well below freezing, a hybrid breakdown using both can be ideal.
Whats the problem with it condensing ?
Does it create vacuum & tend to collapse the closed cell ? Perhaps as liquid overall insulation value reduced ?
 

luthj

Engineer In Residence
The blowing gas blocks Infrared so when it condenses it allows more to pass through each micro bubble. At least thats how I understand it.

Plus the blowing agent bleeds out over a period of years, so the R value drops a fair bit over time.
 

SBDuller

Member
Out of three relatively local home improvement/lumber yards, I have found polyiso board and white styro panels in a 4x8 sheets, various thickness/R-values. One of the three mega-stores has XPS in a 2x2 panel 1 inch thick. Actually a convenient size for experimental hobby build for me. I'm thinking XPS is not a preferred insulation material in this region.
 

john61ct

Adventurer
Online sources of styrofoam abound, if you don't mind them cutting down from 4x8', shipping can even be reasonable.

In a space-constrained use case, I would only use it for a 1/2" facing the extreme cold side myself
 
Top