I'd go with the cheapest and easiest solution since this is a seasonal problem.

run dryer hose from the furnace outlet and to the battery cabinet

or I'm I missing something?

 

Thanks! That would probably work well in some ways. Though might have to funnel all of the heater output into the dryer hose, to get enough pressure to push through the battery box? Also hoping for something a little more "built in" and that doesn't put hoses right where I need to stand and walk.

 

Not trying to get into a technical discussion, just wanting to get some rough perspective on options, on the off-chance anyone has dealt with similar choices/solutions...

The situation:

• I have two 100ah Li batteries (Dragonfly) to power the van's RV features. They're inside the van, in a short, narrow wood cabinet on the floor. The cabinet also contains the rear wheel well, charger, and room for little else. The cabinet has two vents. The van wall and floor are insulated. (I can attach pics in a couple of days after emptying the van, but that description pretty much covers it.)
• When outside overnight temps drop to the teens and single digits F, temps in the cabinet reach well below 25F, despite my furnace thermostat being set at 45F. When this happens, it takes hours for the batteries to warm enough (to 25F+) to receive a charge, even after I've warmed the van interior, cooked a quick breakfast with the induction stove, and driven a couple hours with the van heater on. By which time, I may have reached my destination and missed my window to recharge the batteries from the alternator.
• Temps still fall below 25F with the cabinet lid removed. Clearly I'm not getting much airflow around the box, much less through it. The platform bed and gear stowed under it surely aren't helping, nor is the fact that heat rises.

The questions:

• Any guesses on whether installing a small fan in the front of battery cabinet would be enough to keep them above 25F, when the furnace thermostat is set to at least 45F (knowing that temps at floor level will be lower)? This seems less hassle and lower risk than installing battery heater pads, but it would be nice to gauge if it has much chance of success, before knocking holes in the cabinet, etc. I'm assuming a fan would use a comparable or lesser amount of juice than heating pads, but that's pure assumption. The front of the cabinet is near the furnace output vent.
• Or will I likely need to resort to heating pads for the batteries? I know Battleborn/Dragonfly used to sell them, and have also Heine's recommendations for car seat heaters. (Battleborn now also sells heated batteries, which sound cool, but my current batteries are still pretty new.)
• Or are there entirely different solutions I should consider?
• If a fan gets your vote, any recommendations for what type to look at? Automotive, computer, marine? Say about 3" diameter. 12v. Not too loud or energy hungry, but capable of pushing air past two batteries, the wheel well, and out the back of the cabinet. Something I can connect to the battery wiring, since the van's fuse panel is elsewhere. I'll at least add a switch and fuse, and will talk with Dragonfly about whether there's an option to tap into the temperature function of the BMS (which is a Victron 712). The cabinet is 3/4" plywood.

I'll also see if I can make any improvements to the insulation, e.g., throwing some Thinsulate over the wheel well, which appears to just have the factory plastic cover. I expect this will help a little, but not solve the problem by itself.

Thanks for any pointers!Go

The insulating the wheel well is an easy and minimally invasive place to start. Go with XPS or Polyiso much higher R value for a given thickness. It would be good if you can add some foam board against the wall and on the floor in addition to what you already have. Heat loss is a parallel resistance phenomenon. In English: any weak parts of your insulation envelope will have a disproportionate impact. The heat will "find" the easiest way out of the van.

 

There are quite a few self-heating options available now for cold climates. Remember that the temperature is mainly an issue for charging. Obviously the best solution is to keep the batteries in a warmed place, but if you must keep them in a cool place then a heater that's activated when you charge might be an option. That's how most of the commercial heated batteries I've seen work.

They don't waste power by keeping themselves warm all the time. Rather the heater kicks on when you start charging and it doesn't let the battery charge until it's warmed up sufficiently.

 

Also, I have a computer fan in the cabinet that contains my fridge. It barely sips power and is very quiet. Don't buy the cheapest or the most expensive and you'll probably be thrilled.

 

What's your induction top, 1800w?

If you can't get the thermocouple to register above freezing temps after running a sustained 80A+ load through them, then Battleborn really ****** up when placing the thermocouples in the battery.

 

In case you decide you want to replace the batteries: Lithium Battery for Low Temperature Charging | RELiON This is definitely the most expensive option, but also probably the easiest.

 

Are the Relion batteries using an internal heater?

That seems like the obvious solution. I added a heat-pad under mine and they worked even with 25F outside (and inside - no heater yet) but my confidence was/is marginal. I think I want to change from the "magical / automatic thermostat pad" to a one I control with my own relay and temp unit - and maybe higher power. But the gist of it seems pretty simple: heat up directly under them - like RVs have done with water tanks for year - to keep them above freezing. Good news is the output side (heating) works even if the batteries are well below freezing; it's the charging that doesn't.

 

Are the Relion batteries using an internal heater?

Yes.

 

Thanks everyone! Very helpful. (I'd written a longer reply, with follow-ups on some of your points, but the site isn't letting me post it for some reason.)