Cheap Battery Heaters

Last Updated:01/28/98
Note: This was a feature I came up with for my Jet 007 Omni conversion.  My Civic EV conversion is fitted with more conventional battery warmers:  These can be found on the web at:

Last Updated:01/28/98
(I cannot take credit for this idea, it was proposed on the EV Discussion List.) 

My battery heating system consists of three water bed heaters located under the batteries. I used standard waterbed heaters from Walmart. They sell two kinds, one has the heating elements sealed inside a kind of plastic bag, and the other type has the heating elements molded inside a solid rubber mat. I used the solid type heaters. My carís batteries are housed in two large steel boxes, five batteries in the front box and fifteen in the rear. I used one heater under the front five and two under the rear fifteen. 
After cleaning and repainting the boxes I covered the floor with quarter inch blue foam insulation, the type used under flooring. I drilled a hole in the box near the bottom to route the wiring through. I notched the insulation for the wiring and the terminal block where the wire joins the heating pad. Next the heating pads were laid out over the insulation, with the terminal block dropping into the recess cut in the insulation. I used duct tape to tack the pads into position. I had to cut the end off of the power cords for each pad to fit them through the hole in the box. 
On the front box I mounted the thermostat on the outside of the battery box and reattached the end of the cord. On the rear heaters I used a small circuit box to hold a 110 volt AC relay to switch the pads on and off. The relay is controlled by the thermostat that came with the pads. This allows me to set the desired temperature for each battery box independently and permits one thermostat to control two pads (the rear pair) without exceeding its rating. 
After the pads were installed, I lined the boxes with two layers of heavy construction type plastic sheeting. This was also secured with duct tape. The sensors for the thermostats were placed vertically between a couple of the batteries and taped in place. This keeps the sensor from dropping down in contact the heating pads. I insulated the sensor leads with split loom. The power leads to the thermostats were joined together and routed to a "motor base" plug on the front of the car. Motor base plugs are weather proof male outlets, commonly used on campers as a power in port. To monitor the battery temperature I installed an inexpensive inside/outside thermometer from Parts America. 
The weather has only recently gotten bad enough to use this system, but the results have been very nice. I have the thermostats set for about 75 degrees. The extension cord for the car is fed from a timer that switches it on at midnight. At 7 am I depart for work with an indicated battery temperature of about 70 degrees. My car sits outside all day (30 degree temps) without an opportunity to charge or run the heaters. Eight hours later the temperature of the batteries is usually down to the high 50s. So far I haven't had to insulate the outside of the battery boxes. 

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