The question is how do you control them. Do the make the type of heaters with wire that automatically controls itself or do you have to use a controller? I have only used thermocouple sensors and temp controllers. Very effective but difficult to adapt to battery boxes.
Rick Miller
The approach i took to thermally manage my batteries was learned from an experienced Canadian, Rick Lane. His approach was simple, to equally insulate all batteries and apply the same wattage per battery. This assures equal heating regardless of how long the heaters were on (VERY important). In my current configuration (v1), i just manually turn on the heaters prior to and during charging and off before driving away. The amount of wattage per battery (12w) is low enough to not worry about cooking the batteries if i forgot to turn them off (or heated too long). i typically plug in the heaters the night before the day i plan a long (summer-range-like) drive.
i plan (v2) to install heat sensors to get a more quantitative understanding of what temperatures the batteries are at based on ambient temperature, and the various charging, discharging and heating cycles. Once the temps are known, creating a control to switch them on and off is a no-brainer. i'm still deciding how/where to put the sensors, ie, how many sensors per battery box and where to put the sensors. My current thought is to "over sense" during the data-collection phase, with perhaps 4 sensors per box, 2 towards the top of batteries (one between batteries and one on edge next to insulation) and 2 towards the bottom of the batteries. These extra measurements should show temperature gradients as the heat rises from the heaters and is lost out the tops, as well as the internal (between two batteries) and the edge temperatures. It will be interesting to see the results of such experiments... :-)
i plan to build a temperature sensor slave circuit for each battery box. The circuit will run off the 12v aux battery and be connected to the master computer via CAN bus. The master computer will poll each slave circuit at regular intervals and log the data for future analysis/action/display. The slave circuit will have a microcontroller (eg, Microchip 18F2480
http://ww1.microchip.com/downloads/en/DeviceDoc/39637c.pdf) to read the temperature sensors (eg, Microchip TC72
http://ww1.microchip.com/downloads/en/DeviceDoc/21743a.pdf).
