Adventure EV

Brrr, it’s cold. What to do?

by on Dec.08, 2009, under EV Land Rover, Heater

OK, back to the project…

It’s been unusually cold around these parts lately.  For a few days the daytime temperatures never raised above 20F.  The other day it looked like this:

Snow Panorama

Mmmm frosty…  If there was ever a reasonable need for 4WD, this might be convincing:

Snowy Road

Heater

This brings me the topic of today’s post, heat.

As we know by now, a conventional engine’s real party trick isn’t motivating a car down the road, it’s actually generating heat.  Since only 15-25% of the energy in petrol gets converted to kinetic energy, the rest goes into heat.  Most of the year, the heat is transferred into the atmosphere through the cooling system, but in the winter it gets used to heat the cabin through a little radiator (heater core) in the ventilation system.  An ICE is very effective at this, along the lines of producing 60,000 BTU.  For comparison, a 1500 watt room heater only produces around 5000 BTU.

We also know that an EV is far more efficient in converting potential energy into kinetic energy.  Not much heat is generated by the electric motor at all.  I don’t need a liquid cooling system.  I ripped the radiator and all the associated hoses out.  So, what to do in the winter?

I could do what I used to do when driving the Land Rover through the winter… bundle up like a spaceman…  Err, nah…  That was so 1996.  Let’s try something else this time.  In addition to adding an interior (carpeting, foam, rubber) and sealing all the air-gaps, I’m going to try an experiment.

There are a few options with heaters in an EV.  One method is to use a form of fuel burning device such as a kerosene or gasoline fired heater, the kind used in big trucks.  But I don’t like this idea because it just means another system to fill up.  I also wouldn’t exactly have a pure EV, either, though it would probably be the most powerful type of heating system.

Another idea is to use an electric boiler element to heat a liquid circuit that replaces the original car’s cooling system.  This is popular with some converters because it doesn’t require modifying the existing conventional heater core.  I’m going to stay away from this method because it means adding a liquid heating circuit with a pump, etc, and I just don’t have the room.

I could use a 12V 150 watt heater that can be found in automotive catalogs or Walmart.  Getting, err… warmer.

No, not powerful enough.  Instead, I’m going to try modifying a $20 120V ceramic room heater.  Actually, I’m going to use two ceramic heater elements, and instead of being a piddly 150 watts, they’ll be running about 1500 watts each, or around 10,000 BTU for the two.  It’s a far cry from the 60,000 BTU the ICE can put out, but how often do you run your car heater on full tilt?  I’ll bet not much.  Maybe to initially warm the car, but after a few minutes I end up turning it way down.

So I went to Big Lots and picked up two of these ceramic heaters.  The contents of these heaters will be used to replace the innards of my Land Rover’s heater box.

Ceramic Heater

First things first.  Grab a screwdriver and rip it apart.

Inside HeaterThe guts contain a bunch of things I don’t need; fan speed control, temperature control, crappy fan, casing.  That all gets chucked.  The parts I’m after are the ceramic heater core, the plastic core shroud, and the thermal cutout switch.

Heater PartsHere’s a closeup of the ceramic core with its original wiring compared with a quarter to give you an idea of element’s size.Ceramic Element

These should be plenty small to fit in the Rover’s heater box.  Here you can see the old radiator core in the box.

Old Heater CoreOne of the benefits of using a high-voltage, ceramic heater in an EV is that the EV is usually running at high voltage anyway.  I’ll be powering the cores directly off the 205V traction pack.

But wait, 205V is not 120V.  The ceramic elements will burn up!  This is true, so I’ve rewired them for 240V.  Each ceramic core actually contains four elements each rated to run at 120V.  Normally, you get two settings with these heaters, and this is accomplished by running the elements in parallel pairs.  Choose the ‘low’ setting and two elements heat up in parallel producing 750w of heat.  Select the ‘high’ setting and the other pair of elements join in to provide 1500 watts.

I’ll be running each of the two cores separately for two heat settings, and the individual elements have been rewired in series pairs.

Theory

According to Ohm’s Law, the current through a conductor between two points is directly proportional to the potential difference or voltage across the two points, and inversely proportional to the resistance between them.

Huh?

I’m gonna have to get into some math here…

OK, I’ll simplify.  The key elements of basic electricity are Amps, Volts, and Watts.  Their relationship is expressed by the equation, Amps x Volts = Watts.  If you know any two values, you can derive the third.  Another handy piece of information is that wiring things in parallel doesn’t change the voltage, but changes the current draw (amps), whereas devices wired in series change the voltage, but not the current draw, the inverse.  Witness:

Series vs Parallel Wiring

Notice that the batteries wired in series in total have the same capacity (Ah) as a single cell, but multiply the voltage produced by the number of cells.  The parallel wired batteries retain the voltage of a single cell, but the capacity is multiplied by the number of cells.  Both circuits produce the same amount of power.  In the series case, 48V x 10Ah = 480 watt hours of energy.  In the parallel, 12V x 40Ah = 480 watt hours of energy.  It’s the same amount of power, just produced differently.  This also works for any device consuming power, like the ceramic heater elements.

In my case, I know that I’m using four elements rated for 120v and in total produce 1500 watts when all four elements are run in parallel.  I’ve split the four elements into two pairs, each pair wired in series.  This way the ceramic core will happily run at 240v but consume half the amperage.  The wattage will stay the same, and it’s the wattage that provides the heat.

Element Rewire

Here the two elements on the left are wired in series and run in parallel with the two elements run in series on the right.  It looks confusing because the elements are already connected to each other internally, so the bare tab (there are five tabs, three of which have wires running to them; left outer, middle, right outer) between the sets of wires is the connection between the two back-to-back elements.  Power for the two sets of elements comes in at the edges (red wires) and share a common ground in the middle (white wire) At least, I hope it’ll work that way…

I may not get the full amount of heat out of the setup since I’ll only have about 200 volts to feed it, but hopefully it should do.  Again, I’m running two of these cores in connection with the blower fan’s two different speeds.  Low speed will trigger only one core, while high speed will add the second core (in parallel, separately wired).

These ceramic cores are self regulating resistors.  As the cores temperature rises their resistance increases limiting the amount of electricity it consumes (inverse-resistance).  It’s a nice, safe design because the core can’t get too hot.  If it does, it just starts reducing the amount of electricity that flows through it.  Less electricity, less heat.

A side effect is that when air is run over the core (like the air from a blower fan) the core cools down, the resistance decreases and more electricity flows… more heat is generated until the core heats up more than the air can cool it, then resistance limits the current again.  Because of this inverse resistance behavior, within limits the more air available to cool the core will mean more heat output.

Enough theory, let’s get back to the heater box modification.  I was lucky.  Most people have to completely rip apart their dashboards to get to their heater boxes, a painful process that can take hours on some vehicles.  My Land Rover’s heater box sits in the engine bay attached to the firewall, fully expose.  It doesn’t get easier than disconnecting the blower hose and undoing six bolts!

I was also lucky because the heater box is plenty large enough to house the two ceramic heater cores.  And the dimensions of the box meant that I could bolt the ceramic cores in their original heat-proof, plastic shrouds directly to the box with the airflow having to pass through each core.  I didn’t have make any internal baffles or special mounts, I just used long #6 stainless screws to bolt the cores the box walls.  The lower core uses a set of stand off nuts to position the core one inch inside the output door.  Then I used some plastic, automotive wiring conduit to clean it all up.  That’s pretty tidy!

Final Heater Wiring

And the final result all buttoned up after repainting.  You can see one of the cores inside the blower hole.

Final Heater

AC vs DC and Safety

Controlling this whole process requires a few other goodies.  The major issue here is the use of DC (Direct Current) voltage in the EV traction pack.  Unlike the electricity coursing through the walls of your house, which is AC (Alternating Current) voltage, DC requires a whole different set of controls.

AC voltage turns on and off 60 times a second.  If you touch AC current you will get electrocuted, but your muscles will involuntary spasm in response.  This makes AC relatively safe because it tends to throw you away from the electric source.

High voltage DC is much more dangerous.  DC doesn’t alternate like AC, rather if you’re electrocuted by DC voltage your muscles will contract and stay contracted for as long as you’re touching the voltage source… or until you die.  Unfortunately, this also means that if you happen to grab a DC voltage source, your involuntarily contracted grip may not let you release the voltage source.  Bad news.

0511-0709-2016-1841_Careless_Electrician_clipart_image

For this reason, high current circuits are often remotely switched for safety.  You may operate a switch to turn a device on or off, but it’s only powered by a small amount of current.  The switch actually actuates another switching device designed to isolate the high current load from the switch you operate, keeping you safe.  These switching devices are known as relays.  It’s like those movies where the Captain of an ocean-liner uses a big brass throttle to signal the engine room.  The guys in the engine room actually operate the engine, while the Captain only operates a signal device.

A circuit is “under load” when a device uses electricity.  The higher the load, the more electrical current moves through the circuit.  Relays are used anywhere a high current load is used, whether the circuit is designed for AC or DC power.  But the two forms of power operate differently.  When a circuit is broken, like when a switch is used to break the power flow, a circuit under a heavy load can actually send power through the air between the switch contacts, or arc.

In the case of AC power, the alternating pulse of electricity minimizes the effect because the flow of electricity basically switches off for an instant, long enough to break the connection.

DC tends to hold on.  There’s no alternating break in the current flow, so as the circuit physically separates an arc of electricity forms in the air gap between contacts.  Welding equipment takes advantage of this behavior by sending high current DC power through the air to create an arc hot enough to melt metal.  If the air gap gets big enough, the current will be cut off, but because the tendency to arc with DC power is much greater and stronger than with AC, switchgear for DC has to be much more robust, with larger gaps between contacts, or devices like magnets that extinguish the arc.  It also means that DC switchgear for high-current use can get expensive.

Relays

To safely control my heater cores I have to use relays, and I have two choices here.  Electromechanical relays are the most common type of relay.  They’re used everywhere in cars.  Basically, a low current circuit connected to a switch operates a little electromagnet.  The electromagnet pulls a heavy duty circuit closed to complete the high load circuit.  Simple and cheap… usually.  An electromechanical relay capable of handling the voltages and current required by my heater cores requires the use of a “magnetic blow-out”, which makes them really expensive.Electromechanical Relay InnardsMy other choice is a Solid State Relay.  These are transistor devices that safely switch currents without using physical contacts.  No contacts means no air gap.  No air gap means no arc.  Being transistors, they consume more power than electromechanical relays, and this generates heat.  No much, but enough to need a heat sink to prevent device damage.  I ended up purchasing these Magnecraft (#6312AXXMDS-DC3) Solid State Relays from Newark, an online electronics vendor for about $30 with heat sinks.  These are rated for 200VDC and 12 amps, more than enough for my needs.  The equivalent electromechanical device would have been about $85.

Solid State Relays

Fuses

So, a two position heater switch on the dashboard will control low-current 12V power to signal the Solid State Relays to trigger which will safely control the 200V heater cores.  Meanwhile, the dash switch will also control the 12V blower motor.  Simple and safe.  To round out the circuit, the high voltage heater circuit passes through a fuse block with 12A time delay fuses rated for 600VDC.  Often a resistive-load circuit, like the heaters, can current spike when firs energized, so time-delay fuses were chosen to handle the initial current burst.

Fuses

And that’s about it for the cabin heater setup in my EV.  Only time will tell whether the two rewired cores will be enough to at least be reasonably comfortable in the winter.  I purchased three solid state relays in case I need to install a third core.

A nice aspect of this heater system is that it’s pretty much instantaneous.  No more waiting for the engine to heat up.   Sweet.

Those especially sharp in the audience, will note that any power used to heat the EV will reduce range.  Correct.  In this case, if I were to run the heaters for an our they’d consume 3 Kw power from the 33 Kw pack.  That’s about equivalent to five miles of range out of 50, a 10% hit.  Not too bad, especially considering the heater won’t need to be on full the whole time anyway… hopefully.


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