Miev Minicab – Air Conditioning

How it works and how to use it.

Background

There is a certain amount of mythology out there on the web about the heating and air conditioning in early electric vehicles (iMievs and Leafs), some of which is reasonable, and some of which seems a bit OTT. The basic idea is that you can’t use your heater or air conditioning because it will reduce your range by a very large percentage, reducing the short range of your car to an impractically short range. While there is no doubt a resistive heater and an electric air conditioning compressor are going to use power and lots of it, over the last 4.5 years I have begun to doubt some of the scary stuff. I have resisted using air con or heating in all but the most extreme circumstances, and to when I knew I was going to recharge sooner rather than later. This can be quite dangerous, driving with a windscreen that won’t clear, or very cold hands is not fun or safe.

When you turn the heater on on my van, the remaining range drops by up to 25km, and goes back to normal when you turn it off. Air con goes down a little less, but not much. What made me start thinking was that it didn’t matter whether you had the heater on the lowest setting or the highest, or the air con on ice cold or the lowest cooling setting, the range drop is the same, and it doesn’t change over time (its not a warm up or cool down thing).

I was researching another subject (how to install an inverter that runs off the traction battery – another post later on) and was looking for images and diagrams of how the air conditioning and heater are connected into the high voltage system, and was looking at anything that popped up. Eventually I came across a paper delivered by Engineers from Mitsubishi to an Automotive Air Conditioning conference, which had no images that were useful, but was incredibly detailed about the development and workings of the heater and air conditioning in the iMiev, and by inheritance, the Minicab Miev.

The original document and be hound here https://docplayer.net/54900352-Air-conditioning-system-for-electric-vehicles-i-miev.html but in case this link dies, I have downloaded the PDF and put it here. Air-Conditioning system For Electric Vehicles (i-miev)

How it works

The document above explains how the  system works, but here is a summary as I see it.

The petrol i car, and probably the Minicab have an air con/heater system in the cabin, with controls on the dashboard. Hot water from the engines cooling system (which runs at about 90 degrees C) is fed through the heater element when required and air blown through it to the selected areas of the van. Alternatively the air conditioning compressor driven by a belt off the crankshaft of the engine compresses (or condenses) the refrigerant to a liquid, which is pumped into the evaporator in the air conditioner which cools the air which is blown into the selected areas of the van. When a fluid condenses from a vapour to a liquid form, heat is given off, this is dispensed with by a radiator at the front of the vehicle. When the fluid evaporates, it needs a lot of heat to do so, and the air con extracts this from the air around the evaporator. The fluid in the air conditioning system is very volatile, and is compressed to a fluid and evaporated to a vapour very easily by compression and by decompressing, but the heat is still required to be gained and released. This heat was called the “Latent Heat of Evaporation” when I did Physics 50 years ago, and is why getting scalded by steam is so bad, the water condenses on your skin, releases a lot of heat very quickly. Likewise, a pot of water takes a long time to go from hot to boiling, as all that extra heat has to be put into it.

In the electric iMiev or Minicab MIEV, there is no ready source of near boiling water, the cooling system for the controller and motor does not get anywhere near hot enough. So underneath the car (not in the cabin) is a water heater, which is connected directly to the high voltage traction battery. There is a water reservoir and a pump, which pumps water through through the heater, then via pipes into the heater unit in the car. For the air conditioning, the compressor cannot run off the traction motor, so it has its own brush less motor and a controller which runs off the high voltage battery too. This also sits under the car, not in the cabin. and provides compressed refrigerant to the air con unit in the car. The pictures below show the orange high voltage cables.

The air con unit in the car is very similar to the ones in the petrol versions, saving on design, tooling and parts cost. Keeping the air con unit in the car, and the heating element and compressor pump under the car, separates the 360V power away from the inside of the car for safety reasons.

The heating elements in the heater are a little unusual in that they are Positive Temperature Coefficient elements. This article describes how they work much better than I could, but a key feature is the self regulating ability, making control of temperatures easier. More on this later.

Analysing the issues

As I said above, the first issue is that the remaining range meter seems to inaccurately report the affect the air conditioning or heater has on the power usage. This will not be a surprise to any EV owner or driver, the Remaining Range meters other name is the Guess-O-Meter (GOM) . But “seems” is not very scientific or accurate, so I spent a little time watching what happens when the heater or air conditioner is used. On the MIEV and iMiev you can get a bit of an idea what is going on using the power gauge around the outside of the speedometer. Sit in the car with the car in ready mode and in park (do this inside you garage if you like, with the doors shut 🙂 ), and start with the temperature control on “ventilation” (the white part at the top of the control), and the fan on low and air con off.

Turn the control up to the first notch and you may see the power gauge move slightly, wait a while and you will see it go up into the green area quite dramatically, and then after a few minutes start dropping down the gauge again, and probably going back to no power usage. A bit of experimentation with higher heat settings will see the gauge going higher and staying higher for longer, but always eventually dropping down to lower power usage for a while. This crude test shows us a few things. Firstly the initial heating of the water in the heater takes a lot of power  for a while,  but when the water reaches an operating temperature, the power usage drops. There may be some thermostatic control of the heater, but some of this at least will be due to the PTC heating elements. The GOM does not appear to be related to any of this power usage or lack thereof.

Try this with the air conditioner too and you will see a similar but low power usage.

We can look at this in more detail and actually see some numbers (cos numbers are scientific) by using some readily available, reasonably priced software and hardware. I have covered how to do this in this post https://glover.gen.nz/wordpress/miev-minicab-software-for-looking-at-state-of-battery-etc/ but I will be using my cell phone, an OBDLink LX dongle, and an app called Car Scanner, for which I created a custom dash board showing any details connected with the heating and air conditioning.

Below is a gallery of screen shots showing various stages of power usage during use. What isn’t shown is how the power usage trips in and out when it is up to temperature, at lower temperatures it will oscillate between 0W and 1200W, and at higher temperatures up to 2400W in order to maintain a higher temperature.

What I deduce from this is that there is some thermostatic control, using a ‘step’ control. The PTC heater appears to be a number of plates (as seen in the diagrams in the first document) and it appears plates are turned on and off in sequence. At the start of use, when 6000W is used to initially heat the water from cold, most plates will be in use, but as it reaches temperature, the power usage drops in increments of 1200W to 1200W. Plates are turned on as required to maintain temperature.

At low temperatures, after initial heating, the system will tick over using 1200W and 0W alternately. Over a 9m50s driving period, where the air con air temperature rose from 20.5 to 22 degrees, 75% of the time was at 1200W and 25% at 0W, averaging about 900W over a normal journey. That is not a huge amount of energy, so used at lower temp settings the van can be kept comfortable without draining the battery.

I have ideas for how to refine these numbers, so I will come back to this area.

How to use it

This may sound like a silly thing. Everybody knows how to use the air conditioning controls don’t they. Well it appears I didn’t for the last 4.5 years. In my defense, there is no manual in the car, previous cars had a ‘Climate” button or Auto setting, and I am male, which means I know everything from birth. In this case I was a little less correct about the air conditioning than I usually am on such subjects.

So firstly heating. This is easy, start with the heater control on ventilation (the white sector at the top of the control). Turn it slightly to the right to warm the van up.It will warm up  quite quickly, and power usage will be quite low as it will go from high power usage (4.8kW or so) down to 1.2kW quite soon, and then flick between 0 and 1.2kW to maintain the air temperature at about 20 to 21 degrees. Turn it up a little more for a bit more warmth, but with higher power usage.

Do not wrench it over to the right as far as possible, this will put the heater into maximum power usage, from which it will rarely drop. Top power usage is about 6kW, and this will get the heater up to about 28 degrees, but in this setting it won’t go below about 2.4kW most of the time.

To turn the heater down, move the dial down to the ventilation setting, or turn the fan off. In my MIEV, to totally turn the heater off I have to turn it past the middle and then back to the middle.

Air conditioning is not that different. The air con button has to be on, and the fan on. Power usage is less, up to about 600w as it gets going, then down to 240w as it ticks over.

Don’t keep the temperature control on the coldest setting, its a real waste getting the air temperature down to 3  degrees!

Obviously direct the heated/cooled air where you need it, feet, face, windshield depending on requirements, using the left hand dial.