Time for summer tires

It's been a long wait for the days to warm up as summer gets closer. Hoping for no more freezing days this spring I changed summer tires.

The car has worked great! No brakedowns and the only maintainance has been keeping the tire pressures high to keep the rolling friction low.

Corollaefx has now done 3900 km electrically and it saves me about 100€/month compared to a gasoline driven 2002 Ford Mondeo that I used to drive. These savings are based on calculating gasoline vs. electricity cost and smaller insurance and tax costs.

The co2 emissions have most likely dropped only to a fraction of the Mondeo's 185 g/km as most of the battery charging is done at the moment at home using hydropower purchased from Suomen Energiayhtiö.

Winter came eventually

The temperature has been between -15..-22degC for a week now and I got the cold driving experiences I was waiting for. I installed a simple indoor/outdoor temperature sensor in the middle of my battery pack to see what happens there while driving. I learned that the temperature rise was about 15..20degC so the temperature would climb up just above 0 on a -15degC day.

With such low temperatures the battery pack was struggling to keep up the voltage during uphills and I had to be patient not to give too much throttle or the battery voltage would have dived below 50v and the BMS would have cutted off for sure. As there was less than normal power available from the batteries due to their temperature it was hard to keep up with the traffic at some points.

To improve the situation while the cold weather came here to stay, I decided to seal the engine room and insulate the battery pack as well as I could.

I used bubble wrap to insulate the battery pack. Then I blocked the airflow from the engine room using foamed plastic (the gray sheets in the pictures). I also covered the bottom of the engine room from all bigger holes.

This had a dramatic positive effect for winter driving! Today morning the outside temperature was -22degC and the battery temperature was +4degC when I started my every day work trip from garage. When I got there the battery pack temperature showed +22degC and the battery performance was close to what it was in summertime!

Another positive surprise was that when I went to lunch four hours later the pack's temperature showed still +15degC while the outside temperature was 30 degrees less!

I think the rest of the winter will be easy driving unless any super freezing days will be ahead.

Almost forgot; when I reached work the car's cabin temperature was -12degC.. To fix this I ordered a new warmer hat and a pair of gloves ;)

Cold batteries, weak performance

So far 1400km of driving with electricity has been fun and easy! This winter has been quite warm and the lowest temperature has been -10C while driving. On that temperature the battery performance is weakened remarkably. Their internal resistance is increased lowering power output and efficiency. I assumed that the batteries would heat up internally to a more efficient working temperature level but this has not seemed to happen. For that reason I decided to try to decrease the batteries cooling by blocking the airflow through the battery pack. The pictures above show how the airflow was blocked through the front mask of the car. Also the original motor covers were installed below the batteries which guide the airflow away from the batteries.

I do not yet know if this modification helps to increase the battery pack working temperature as I still do not have temperature sensors  equipped. Also the winter got warmer so I'll have to wait for more cold driving experiences.

A combined ampere-hour / temperature meter is under development using an Arduino and DS18B20 temperature sensors. After all needed components have arrived I am optimistic to get them installed soon as we got a flying start with the application thanks to my brother's Arduino experience!

Corollaefx perfomance specifications

Here is a list of the car's performance specifications learned so far:

Rated motor power: 11,5kW
Weight: 940kg
Nominal traction battery capacity: 4,6 kWh (76,8V 60Ah)
Top speed: 84km/h reached at 5th gear
Operating range: 37km reached at 0 celsius temperature, driver and one passenger, 50km/h average speed.
Energy consumption: ~ 19kWh/100km measured using a energy meter on the charger supply cable.

Preparing for winter

The Corolla has been very reliable for the 850km that I have driven so far. During this time I have not had to use my gasoline powered car at all so I took it off from street use. The trips outside corolla's range can still be made by my wife's diesel car, so no problem there.

I'll try to use the corolla on my everyday driving through the whole winter. It will not be easy though. So far -7 degrees celcius is the lowest temperature that the car has faced and that time it survived from work to home. The low temperature increases friction due to gearbox oil which had a remarkable effect on driving. To minimize that effect I searched for the lowest viscosity oil at cold temperatures from local shops filling API-GL4 requirements. After a careful search I decided to use Redline D4 ATF oil that can be used in both automatic and manual gearboxes. After the oil change a good improvement could be noticed at below freezing temperatures while no difference could be noticed on warmer days. I do not know which oil the corolla had before, but most likely it was mineral oil that gets very stiff on cold days.

As the temperature drops so does lifepo4 battery performance as well. The unloaded cell voltage is not affected much, but the internal resistance of the cells is increasing remarkably as temperature decreases. The battery pack is so well cooled in its current location that I am thinking of insulating the battery pack thermally so that their internal losses would increase the cells temperature hopfully to a better working level. Otherwise I am expecting the PCM to shut down the operation at around -20 degree celsius due to too low cell voltage on a long uphill where 200-300 amps are drawn from the batteries for quite a long time.

Despite the very cold driving experience I very much enjoy driving the corollaefx. The 300W windshield heater has been great to keep the windows clear but it definitely does not warm the cabin!

Warning sign on its place

A warning sign was placed on top of the battery pack lid. It also states the nominal voltage of the battery pack (76,8V) to easily tell what voltage level is used in the car.

I have driven 409 km at the moment and no big problems have occured. The weather has been mostly rainy with temperatures around 5 celcius. The 300W heater for the windshield has also been good enough so far, but I am starting to get the feeling that it might be underrated on sub zero temperatures.

When I changed the winter tires I noticed that one front brake made quite a lot of unnecessary friction. I removed the brake pads and cleaned them and their surroundings. As a result both front wheels felt similar to rotate. This brake fix had a very positive effect on driving. The battery current reduced approximately 15A at 60km/h and now also the fourth gear seems to be useable! This increased the useable speed range up to 80km/h on even road. With a low powered vehicle such as this one it is truly advantageous to minimize all losses possible.

First electric driven day at work

As the corolla is not highway capable, I drove my 18 km trip to work through smaller roads. The speed limits on the way were between 40-60 km/h which the corolla handles nicely. Only the 2nd and 3rd gears are needed in normal driving. 2nd gear works well on 0-40 km/h and third gear gives a speed up to 65 km/h.

I found four workmates to join me for lunch and we drove there all five of us packed into the corolla. The car's low power was easily noticeable for the driver as full throttle was often needed for a normal acceleration. It was fun though!!

I am not yet sure if the corolla would make it back home without charging. Until it is sure I'll charge the batteries a bit to make it safely at home for a full charge.

Corolla eFX is now street legal!

The compliance with electrical safety regulations was shown with a commissioning inspection record made by a local qualified electrician. All high voltage cables and their feedthroughs were inspected visually. The charger input cables were tested for proper PE- continuity and insulation resistance. A warning sign to be placed on top of the high voltage enclosures was planned and a proper one will be installed as soon as possible. The approved commissioning inspection record was brought to the vechicle inspection station and then the Corolla was able to be approved for road use.

The picture above shows the car's new changed information after the electrifying project. Drivetrain is changed from gasoline to electricity. All five seats are left in use, new weight is 940kg which is 50kg less than original. The change percentage of this car is now 16%, 14% came from the changed motor and 2% from removing the radiator.

This Corolla made 176202km using gasoline. I hope it will do many more with its new electric propulsion!

Inspection results and new high voltage cables

The car itself was approved as it was and it was weighted to be 50kg lighter than its original weight! However its new electrics need to be proved to comply with safety regulations (link to the applied regulation in finnish: www.finlex.fi/fi/laki/alkup/2011/20111064). Once I have the required documentation available and delivered the Corollaefx should be ready to hit the streets!

Driving the car to the inspection was fun! It was a rainy day so the heating could be tested to see if the windshield stays out of fog and it stayed very clear! The performance was quite like expected; not too much power available. 2nd gear felt as the best choise to get the car moving from a stopped situation. I reached 80km/h with 4th gear but as the needed currents were over continuous current levels I decided to slow down and drive 60-70 km/h using 3rd gear. With this drivetrain and chassis 70km/h looks to be the continuous maximum driving speed at the moment. I can't wait to get the car into everyday driving to get more experince and testing:)

The picture above shows new high voltage cables that were installed. The old welding cables used earlier were removed as they were not in a very good shape and I did not know what type they were. The new cables are 50mm^2 H07V-K type purchased from a local hardware store IKH.

All set for inspection

Shield were made to prevent accidental touching of battery terminals and powered connectors. They were made from clear acryl sheets that were painted black on the bottom side. The shields also block some of the spashing water from reaching the battery terminals.

Also the motor controller assembly got an enclosure which was modified so that it can be opened by a single butterfly nut.

As everything works now I think its time to have the Corolla inspected to make it street legal as an electric car!

Updated main circuit diagram

Quite many changes were done to the initial circuit diagram along the way. The changes were governed by the idea of not needing to alter the original wiring. The circuit diagram shown above shows the current state of the Corolla eFX electrical connections. I am currently happy with it as everything seems to work well.

The biggest change to the original circuit diagram are the six parallel connected power MOSFETs that are located between the 12V battery negative terminal and the chassis. They were added to boost the discharge current capability of the PCM circuit from 20A up to about 100A. The gates of these MOSFETs are controlled by the same gate drive that controls the 12V battery pack PCM discharge MOSFETs. This way when the PCM notices an undervoltage situation the MOSFETs are turned off  protecting the lifepo4 cells from potential damage.

The other major change was the addition of the 72V to 12V isolated power supply. This change made it possible to use a common 12V relay to wake up the motor controller. It also provided an isolated 12V power supply to be used by the panel meters. The 72V to 12V isolated power supply chosen is actually a low cost plug type offline AC/DC power supply that needs only about 50Vdc to start up. Not all of them do, but many of them do start with a lot lower voltages than the typically rated voltage range of 96Vac...250Vac. Of course, if used with a lower than specified input voltage the output current can not be expected to reach rated values.

Electric brake booster mounted

The electric vacuum pump and the vacuum operated microswitch were mounted close to the vacuum brake booster. The pump was mounted using a rubbery tube clamp to damp some of the pump's vibrations. This electric vacuum pump is quite a shaker when it is on.

12V PCM tested

After successful tests with the 72V battery pack's PCM branded as Signalab, I was expecting similar behaviour from the 12V battery pack PCM. I do not know the manufacturer of this board which is shown in the picture, but it is still available in eBay.

When I first connected the batteries to the 12V PCM I had my multimeter connected to measure the resistance between the P- and B- terminals. I was expecting the resistance to drop close to zero when the last cell was connected. To my disappointment the resistance dropped before the last cell was connected to the B+ terminated. My first assumption was that the board was faulty. A new PCM board would take weeks to arrive so I though that I would examine this one a bit more before giving up with it.

I disconnected the PCM and took it to my lab room to examine it with a laboratory power supply. I found out that the PCM worked as it should with the cell over- and under voltages, but what was left undocumented is that if any cell voltage would be less than about 0,7V the under voltage situation is no longer detected and the P- terminal is enabled again (if the other cell's do have a valid voltage between 2.0 to 3.85V). This happened to me during the first test of the PCM when the last cell was disconnected and still the P- was enabled.

Now that I know how this circuit works I decided to install it anyway although the circuit can not be expected to disconnect the load if for example a wire gets disconnected between the PCM and the battery pack.

Placing the battery chargers

My original plan was to place the battery chargers under the hood, next to the batteries. It turned out that waterproof fanless chargers were very highly priced and thus did not fit in this low cost project.

The chargers that were chosen for the project are fan cooled and ment to be used indoors so they had to be mounted in the cabin. The most suitable place for them was found from the front passengers feet space.

Mounting the electric heater

The heater assembly was finished by adding a second aluminum plate on top of the power resistors using a clamp made from steel. The five 2.7ohm resistors were connected in parallel using 4mm^2 wire. After testing that each resistor and temperature switch worked the heater assembly was painted using black exhaust paint that withstands temperatures up to 650C.

The heater assembly was then mounted on top of the air vents that blow air to the windshield. The heater assembly is only 30mm high so it does not block visibility much. After it was painted black it fits quite well into corollas interiors. I hope there is now enough heat available to keep the windshield clear of fog.

12V system testing

The 12V 60Ah LiFePO4 battery pack was mounted on its place with a simple sheet metal frame and a belt. I wanted to test if all the original electronics still work after the original motor was removed. It was great to notice that all lights worked and only the cabin fan did not work. After some research I found out that its power wire was in the motors cable harness and got the fan also working just by connecting it.

Current consumption was measured for the different utilities and the main consumers were found to be the cabin fan taking 16A on full power while the driving lights took 14A.  It's good to know these so I can rate the new fuses and relays correctly. Some changes have already been made on the circuit diagram that was posted earlier. I'll post its updates along the way.

The Corolla eFX is now in a driveable condition with its lights fully functional. The following steps will be wiring the 12V battery pack protection electronics, finishing the heater and mounting the electric brake booster and chargers. After that I'll show the car to the local authorities and hopefully get the car street legal!

Controller programming for full power and release throttle regeneration

The traction battery is now ready to be driven with full power so the motor controller needs to be programmed to give its maximum motor and battery current. I also wanted to try the "release throttle regen" mentioned in Kelly Controller's PM72401B features. I had to contact their support in order to get the release thottle regen to work as I could not find the parameters for it in the configuration program. I case some of you have the same problem "release throttle regen" can be activated by setting the desired regeneration value to the parameter described as "Tps Mode And Max Allowed Regen Current[3]" shown in the picture above. I do not know what "Tps" means, but setting a value to this parameter certainly started regeneration when the throttle was released.

The release throttle generation was set to an amount of 30% which in practice felt a bit too powerful for easy gear switching and too weak to actually stop the car by regenerating. Also there seems not to be any ramp when regeneration is started resulting in a fast torque change which can be felt as a distracting thump in such a light car as the Corolla.

I am thinking of testing a combination of perhaps 10% release throttle regeneration and maybe 40% of brake switch regeneration activated by a microswitch that would be installed on the brake pedal. I am happy over the ease of Kelly Controller's programming so testing different alternatives is only a matter of time and ideas.

First smoke - CBM overheating

 

 

It was a good thing to follow closely through the first charging of the traction battery pack. When the first cell voltages reached the balancing threshold I started to be worried of the CBMs possible overheating as I had only tested the CBMs in free air in a vertical orientation with a very small charging current. I consentrated for a while into other things and when I checked the charging situation again about 20 of the 24 CBMs were active and the whole enclosure was clearly too hot to survive. I stopped the charging but it was too late. The heat shrink tube used as insulation between the CBMs melted resulting in a short between two CBMs. Smoke started to rise and I started to remove the fuses to disconnect the batteries from the smoking CBMs. Finally all 24 fuses were removed and no flames were seen. The molten heat shrink tubing and smoked CBMs are shown in the first picture. This was a good reminder for me  about how easy it is to underrate cooling.

Luckily the PCM board was not harmed. As seen in the picture the CBMs were removed from the enclosure dropping the balancing current to just 60 mA provided by the PCM board only. While charging the battery pack with 10A the charging was now stopped into a cell over voltage as one of the cells reached a voltage of 3,9V. Some other cells were clearly not yet full so I made a manual balancing for the battery pack by sinking current from the fully charged cells and charging the ones with lower state of charge. After a while of manual balancing I got the cells balanced enough to make the charger to stop charging normally; the charging voltage was about 87,6V and the charging current was reduced into about 1A.

Now the first traction battery charging cycle is done. I am curious to see if 60mA top balancing current is enough to keep the cells balanced. Anyway I am so far very pleased with the PCM board!

Traction battery charger close up

The charger was purchased from www.kellycontroller.com. It is an engineered product made for customers specs. I ordered this charger with the following specs that were needed for the order:

Mains voltage: 230vac
Output voltage: 87,6V (24 cells x 3,65V)
Battery capacity: 60Ah

This charger is ment for indoor use only so I cannot place it under the hood where it would be exposed to moisture and dirt. Therefore I decided to mount it in corolla's trunk.

The chargers manual does not say anything about its leakage current (current drawn from the battery while the charger is unpowered) so I measured it to be 1,8mA with 80v battery voltage. This is lower than what I expected so its well suited for onboard use.

The chargers manual does also not mention the chargers power factor which is claimed to be 0,96 according to www.kellycontroller.com. I will measure its power factor to find out if it really has an active PFC or not. Anyway I'm a bit dissapointed with the chargers documentation.

First time traction battery charging

Before connecting the motor controller to the traction battery pack I wanted to test the connections made so far as well as the PCM board and the charger bought for the 72V battery pack.

I first inserted the mini blade fuses in their fuse holders at the battery terminals. The fuses were inserted starting from the cell closest to the pack's negative terminal and then proceeding to the next cell connected to it. This connecting procedure was adwised by some PCM's and I decided to follow it as the PCM I received did not come with any manuals.

Before inserting the last fuse I checked that the mosfets in the PCM board were turned off. Once the last fuse was inserted the PCM's mosfets were turned on which is correct as all cells had a voltage of about 3,3V.

The charger was plugged in and a charging current of 10A was measured. I was beforehand a bit worried about the PCM's mosfets heating, but the heat dissipation with 10A charging current was measured to be just 0,5W divided by two TO-220 packaged mosfets so they only warmed up barely noticeable. I do not know which mosfets have been installed on the PCM as all of their markings have been sanded off for some reason.

I did not make a full charge as I want to be around with my multimeter learning about how the top balancing behaves and how the CBMs and everything else heats up. So I'll continue charging while working in the garage ready to stop charging if something fails.