This page discusses design and installation of the cabin heater in my 2004 scion xB EV conversion.
Luxury or Necessity
On an ICE vehicle, there is a massive amount of waste heat from the engine. Some of this is recaptured from the engine cooling system to provide heat in an ICE vehicle via the fluid heater core.
Heating may not be necessary in an EV conversion if you live in a dry climate or if the car is an open top design or you only drive it in nice weather. If however you plan on driving the car year round and in less than perfect conditions a heater is necessary for comfort and defrosting of the windshield.
A cabin heater in an EV runs directly off the traction battery, much as baseboard electric heat works in a home. This does mean that there is a range penalty for running the heat in an EV ; typically this is around 10% of overall range, but will of course vary depending on how powerful the heater is and how much it is used and the driving conditions.
Assuming you live in a fairly moderate climate, a 1 to 1.5 kilowatt heater will provide adequate heat. Conveniently, this is about the wattage of your typical ceramic core space heater. If you live in a very cold climate you may need more heating capacity.
A car with a small cabin, such as a two seat sports car, might get away with a smaller heater, and conversely a van with a lot of interior space should probably have a larger one.
A 1 kilowatt heater running on a 320V battery will only draw about 3 amps of battery current. The same heater running on a 100 volt battery will draw 10 amps of battery current.
There are several basic heater designs that can be used in an EV conversion.
- You can buy EV specific fluid heaters and pumps that can be plumbed into the vehicle’s existing heater core. This method preserves all the function of your car’s existing heating system and ventilation box under the dashboard, and does not necessitate tearing apart the dashboard. Major downside is these can be rather expensive and they are much more complex than forced air heating.
- You can replace the car’s existing fluid heater core with an electric forced air heater core. This approach gets rid of the necessity for a fluid heater and is much cheaper as the heating element can be sourced from a variety of small and cheap home appliances. The best option seems to be the ceramic heating element out of modern space heaters, but other appliances such as hair dryers and toasters can also be used. The major downside of this approach is simply the labor and hassle of completely disassembling the dashboard to get at, and replace, the heater core. This is often the hardest part of a car to get at. Some creativity may also be required to make the original heating controls function appropriately and control the new heater core.
- You can simply use a space heater or other heating device as is, completely bypassing the original heating and ventilation system of the car. This approach is cheesy (envision a hair dryer duct taped to the dashboard) but can work in a pinch.
In all cases above, the heater must be designed to run from a similar voltage to what the traction battery takes. A car with a traction battery near 120 volts, 240 volts, or 360 volts will have many options available. Cars with traction pack voltages in between those values may require more creativity.
It is worth noting that most ceramic core heaters contain wiring that allows different heating levels, and often this takes the form of multiple parallel heating cores in one element. These can be rewired in series instead of parallel for example, to make a 120v heater core run safely from 240V.
Switching DC current
The only major gotcha with using heating appliances is that they will be powered from DC, not AC in an EV conversion. This does not affect the heating element itself (a watt is a watt) but the switching components must be rated for DC voltages and current. Most relays that can handle high voltage and current are AC rated and can contact weld or otherwise fail to switch properly if used with DC.
The heating element high voltage supply should be fused with an appropriate type and size of fuse, and this fuse should be independent of fuses for other high voltage loads such as the inverter and DC/DC converter.
It is also a good idea to ensure the heating element never runs unless a circulation fan is moving air through it. This can be the car’s original vent fan or the original fan of the heating appliance. A thermostat switch should also be included to ensure the heating element does not get too hot. Many options are available but I find that water heater thermostat switches are cheap, plentiful, adjustable, and work perfectly. Simply mount the switch in the heated air flow near the heater core and adjust to 120 degrees, and integrate into the control circuit.
I chose to modify an electric space heater and replace the original heater core of the car with it. This required the arduous task of tearing apart the entire dash board to access the heater core. This was very annoying but allowed access to a lot more smoke and nicotine fouled recesses of the car which I cleaned as appropriate.
I was able to find a six element ceramic core heater at a local thrift shop. The six elements each take 120V and make 300W of heat. I threw away everything from the appliance but the heater cores and their support structure and rewired them to make two parallel strings of three cores. This allows running the core at 320V with two heat settings (one string or both)
I modified the supports for the element to fit in the space originally taken by the Scion xB fluid heater core. In this manner I was able to replace the original heater core with this new electric one. I placed a water heater thermostat switch in the housing to prevent the heater from getting too hot.
I grafted some circuits appropriately to the original scion heater controls to create a heater core control circuit that turns on the heater when the heat knob is turned all the way up, the fan is running, the thermostat switch is cool, and the vent levers are in a position to force air through the heater core. The circuit, when energized, closes a 60 amp, 400V DC rated relay with a 12V coil. The relay is located in a box in the engine compartment along with a safety fuse and the high/low heat switch that I was too lazy to integrate with the cabin.
The heater is not really powerful but given a few minutes it will start to make a noticeable warmup in the cabin, and is reasonably effective at helping to defrost the windshield.
Heating while Charging
Productions EVs often have the capability to pre-heat the cabin while charging, and in this way not impact driving range at all. This can be done in a variety of ways in a conversion as well. I did not try it however.
Waste Heat Sources
Unlike the huge amounts of heat an ICE engine produces, there is not nearly as much waste heat in an EV. There are a few sources though that depending on the configuration of the car may be possible to utilize.
Some heat can be recovered from the charger while it is operating. This is easily accomplished by just seeing that a fan cooled charger vents its waste heat into the cabin when operating. Obviously this does not help when driving, but it may allow for the cabin to be pre-heated to some degree prior to driving away.
The motor and inverter/controller will both make some heat, but not very much compared to an ICE vehicle, and it may be rather difficult to capture much of it. However, there have been a few cases where an approach like this has been tried.
The batteries themselves will make some heat during driving, but unless that driving is very aggressive or the batteries are very inefficient, the amount of heat produced will be minimal and difficult to capture.
The brakes also make heat when applied, but it is likely impossible to capture and direct a useful amount of that heat to the cabin.
I am not doing this in my car, but I will touch on it briefly. Batteries perform more poorly (less capacity, and less power) when too cold. If the batteries in an EV are enclosed and insulated, then they can also be heated to mitigate the effects of cold temperatures on their performance.
Production EVs often include a fair amount of climate control (both heating and cooling) for their batteries. The Nissan Leaf has some problems in hotter climates because it does not do enough cooling.
It was not too uncommon for lead acid conversions that were intended to be driven in cooler weather to include insulated battery boxes and battery heaters due to the sensitivity of lead acid batteries to cold temperatures. Application specific battery heater pads can be purchased, or planter box heater wire can be used with considerable success. In a car used regularly, the battery heaters can run while the car is parked and charging, and thermal mass and insulation can keep the batteries significantly warmer than ambient temperature for many hours between plug ins.
In a lithium powered conversion it is still a decent idea if driven in cold weather to insulate the battery boxes, and if driven in extreme conditions battery heating capability is probably a good idea.
I did insulate my battery boxes minimally (one or two layers of corrugated plastic) but I did not include active battery heating. This is still a considerable improvement over the completely exposed and uninsulated batteries in my 1985 Toyota MR2 EV conversion, and was worth the extra effort. So far I have found that the batteries will warm up noticeably over 20 to 30 miles of driving in cooler (50 degrees or so) weather.