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Bring on the Heat · 2 December 05
There’s a number of things you need to do inside the electric car’s dash compartment. What that consists of depends on your donor car and the type of “stuff” you’ve decided to install in your EV.
First off there’s metering, which, ideally, would be in a visible spot. Either mounted on top of the dash (clunky and maybe unsafe if high voltage wires present) or somewhere in the instrument panel.
Our first EV had the meter installed down around the light dimmer switch, which meant that eyes were taken off of the road to read it. Not this time.
The other major dash item is the heater. Let’s talk about that and share a few pictures.
You have a few choices for heating an EV. The easiest choice, for the folks in southern climates, is no heater. Wouldn’t that be nice!
Some electric cars have propane heaters, which seems like a step backwards. Sure, there’s a lot more heat available, but now you have to “gas up” the EV from time to time.
Most folks opt for an electric heater. A newer model that I’ve seen is like an on-demand water heater that is hooked to the car’s old heater core and heats antifreeze. There’s a lag time of a few minutes as the liquid is warmed up and circulated.
I’m using an electric ceramic heater, in fact two of them this time. Ceramic heaters can be picked up at a nearby hardware or *mart store for around twenty bucks. The model I picked up this year was slightly more expensive at $24 because it “oscillates.” Woo!
Here’s the rough layout inside as I started taking it apart.
Lots of screws. At least thirty of them.
The base is held on by four screws and inside is a C ring holding the pivot pipe to the base, since it’s the oscillation point. Once you pop the base off there’s another dozen screws to remove before the plastic case will open up.
Be a little careful and don’t force things. We don’t want to damage the surprise, prize inside.
For twenty four bucks there’s an incredible amount of parts inside.
There’s a beefy muffin fan. As it turns out we need a couple of these to circulate air from our pellet stove and this should fit the bill.
There’s 30+ screws and bolts, a nice on/off switch, a multi-position switch, a variable thermostat switch, an AC lamp, and a small geared motor used for oscillating.
Right in the middle, with red wire leads, is the thermal cut-off switch. If you look at the previous image you can see it mounted in the black plastic. This interrupts power if the unit gets too hot.
And then there’s the star of the show, the Ceramic Heater Core.
The way these things work is that voltage is applied to the metal leads. The leads and the squiggly radiator metal that looks like xmas hard candy conducts electricity to the thin ceramic wafers.
Zoom in on the picture and you’ll see four layers of ceramic material. When voltage is applied this heats up and the metal draws the heat away. Air flowing through the metal fins is heated up and blown into the house…or passenger compartment.
The beauty of the ceramic heater is that is has inverse resistance. Resistance is like pinching a garden hose and holding back the flow of water. In electronics it limits how much current flows.
The more current flowing through our ceramic heater the warmer it gets. But since this has inverse resistance the ceramic’s resistance goes up as it gets hotter. More resistance means less current flow and heat output goes down. It’s still hot, mind you, but somewhat under control.
Blowing air through the fins draws away the heat, resistance goes down and heat production goes up. Wonderful science!
Now to figure out where to put all of this wondrous heat.
I have two choices: try to cram it into the original heater core position or into the box where the AC core used to be.
The heater in your car looks alot like the ceramic heater core, but instead of layers of ceramic it has tubing to route the hot radiator liquid through. In fact all of the time, Summer and Winter, there’s hot radiator fluid running behind your dash.
Well, that’s not always true. I’ve see cars with a flow control knob, but Eve didn’t have one.
One of my old junker cars had a radiator problem (radiator thermostat stuck closed) and the short term kludge was to drive around with the heater running full blast…in the Summer!
How do they keep you from getting cooked by all of this heat? Well, there’s a big ol’ flap that diverts air. The photo above shows the air circulation/heating chamber as viewed from the incoming air’s perspective. When you move the heat lever you are really opening up a flap and directing some of the incoming air through the heater core. Close the flap and the air goes another route. No heat.
It’s kind of cramp in that box and, to be quite honest (and safe!), we don’t want the ability to reduce air flow to the ceramic heater. A better location is inside the A/C core box shown here.
My plan is to use the ceramic heater’s original black plastic (which seems to be some type of heat resistant substance) and connect the two ceramic cores back to back, with a couple of inches between them. This will be mounted inside the box such that all air has to flow through the heaters.
As a reminder, here’s the heater schematic:
The first EV used the air conditioning on/off switch since it was in the right place and not being used. Probably do the same thing here. What might be interesting is to combine that with the variable thermostat that came with the heater: when a certain temperature is reached the relay is turned off.
Speaking of relays the schematic shows two of them. The first one turns on the big relay, supplying 144vdc to the heaters. It also grounds the fan.
The blower fan in a car works at different speeds by changing the amount of resistance between the motor and ground. On LOW there’s three resistors, on MEDIUM two, HIGH has one, and on MAX no resistors.
The resistors are kind of interesting. They are big rolled up wires mounted inside the blower fan box. Since they have to deal with a lot of current going through them they get hot, so they are placed in the path of the blowing air.
Yet another source of heat when running the fan in the Summer!