I've been investigating battery warmers for my homebrew Li setup (GBS LiFeMnPO4 100Ah / emus BMS). We park our van outdoors in our driveway; so even though the battery is mounted inside the van, it may see temperature extremes any time we're not driving it. Heating pads are a no-no due to uneven heating, so a battery warming blanket would be the natural choice. A common solution to the problem when one has access to shore power is to stick an incandescent work light in a box with the battery, which got me thinking.
I found a $10 PTC warmer attached to a small PC fan on Amazon, and my plan is to build a small foam-insulated box (or maybe repurpose a soft-sided cooler) and run this PTC heater/fan from the BMS' heater control output. Recirculating air inside the cooler, it shouldn't take much to keep the battery at a reasonable temp. The heater is labeled as 100W on Amazon, but at 12V with ~72F ambient in my house it only pulls enough current to equate to ~50W (as commonly mentioned in reviews). Current draw increases at higher voltages with a PTC, so the lithium setup will run 'hotter' albeit with cooler ambient temps--we'll see how it balances out once I have it set up.
"100W" 12V PTC Heater
I'm not sure how much this all really matters though, as I have a Webasto Air Top on its way from Russia. The battery is mounted inside the van, so the Webasto ought to keep the battery happy when we're actually using the van--which is when we actually need charging functionality anyhow. Really the only purpose this setup will serve is to allow the solar to charge the battery when the van is parked for an extended period without the Webasto running. Call me old-fashioned, but because of my upbringing I turn the lights off when I leave the room--so I shut off the van electrical if we won't be using it for a while, and Li will hold its charge quite well.
For $10 and a little time spent wiring, I figure it's a fun experiment though.
As a sidenote, if I were doing this all over again I would consider buying a Goal Zero Yeti Lithium with the 12V charger plugin so you can connect it to your vehicle's alternator. My setup has more capacity, flexibility, and bells and whistles, but at a cost of both money and space. With the GZ, you could just unplug it and carry it inside with you when not using the van. I was really kicking myself when GZ had a special on the 1000 Li w/ charger at Overland Expo West this spring, on our first trip after I had installed my setup. In my defense, GZ hadn't introduced the 12V charger when I started this whole thing.