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.

New meters behind Corolla's steering wheel

Three led panel meters were installed to be easily seen by the driver. The panel meters were earlier introduced in the post titled "Configuring panel meters".

The enclosure for the meters was made from a piece of white plastic 30x30mm cable trunking which was covered with black duct tape to better match with Corolla's dashboard.

These meters will be giving readings of the traction battery voltage and current as well as motor current. Only the speed meter will be left in use from the original meters, although rpm and motor temperature metering might also be nice in the future.

Battery pack connected to BMS

The next layer of wires connect the traction battery pack to its battery management circuits. The connections were made using 1,5mm^2 wire with a mini blade fuse holder as close to each cell terminal as possible. The mini blade fuse holders were soldered on each of the wires.

Traction battery power wiring

The 72V battery pack was wired keeping in mind that the wiring should be able to provide up to 400A for short periods. Most of the connections were done using power terminal connectors purchased togheter with the batteries from www.ev-power.eu. As the batteries were not assembled side to side, most of the power terminal connectors were too short and their holes had to be slightly modified to make the connections possible.

A 400A 75mV shunt resistor was mounted on the negative terminal of the battery pack to allow battery current monitoring. A 400A ANL type fuse was mounted as close to the positive terminal as possible. The cables used have a cross section of 50mm^2.

The battery management electronics will be wired next followed by the first charging of the traction battery pack.

Mounting electronic enclosures

A mounting stand was made for the enclosures including motor controller, battery management system and relays. The stand was mounted between the original motor mounting point shown on the left and the gearbox frame.

The stand was made using suitable steel rods that were welded together. It was then painted with zinc spray and a thick layer of black steel paint.

The wiring for the 72V battery pack can now be finished which will allow a first test drive with decent batteries! The wiring of the 12V system will have to wait until its battery pack and battery management system enclosure have been fixed on their places properly.

Wiring the battery management electronics

Both 12V and 72V lifepo4 battery packs will be managed by cell balancing modules (CBM) and a protection circuit module (PCM). Both are easily available and simple to connect. One CBM will be connected parallel with each cell and the PCM needs a connection between every cell. Although these connections are simple, the amount of them make this step quite demanding.

A half way wired 72V battery management circuity is shown in the picture. The PCM is shown on top and below it lies 24 CBMs. Once this is fully wired it will be mounted in a 240x190x90 mm enclosure. The connections to the battery pack will be done using 1,5mm^2 wire with a fuse located close to the terminals of the battery pack.

72V battery pack mounted

The finished 72V battery pack base plate was painted first with zink- paint and then with black paint to slow down corrosion. It was then mounted on its place where the radiator used to be. Next the battery pack was mounted on it cell by cell. The easy to reach location of the 72V battery pack will be handy while making the electrical connections on the cell's terminals.

Mounting the cells on the base plate

All of the cells of the 72V battery pack found their place on the base plate. The cells are sitting tightly on their places as the cells do nothave room to move in any direction on the base plate. The cells are tied togheter using bent washers that are shown in the close up photo below the wing nut and the rubber washer. When the wing nut is tightened, the bent washer presses both cells towards the base plate and also makes sure that the cells can not escape sideways.

Battery base plate under construction

The base plate for the 72V battery pack is being built from different steel materials that I had available. Most of the base plate is done using 25 x 25 mm steel square tube that is cut and welded. A tight place is made for each of the cells and the cells are fixed on their placed by pressing them against the base plate with a threaded rod and a wing nut.

The cells are slightly separated from each other to allow a better airflow which improves cooling. While the maximum continuous current of 160A is drawn from the battery pack every cell dissipates approximately 20W of power according to my interpretion of the datas given by the cell manufacturer Winston. If the cells would be tightly packed next to each other I would be worried that the ones in the middle of the pack would overheat. After getting the Corolla on the road it will be necessary to learn how the batteries, motor and the motor controller heat up so that any unpleasant suprises due to overheating could be avoided.

Heater assembly

A simple heater system was built from 5pcs of 2.7ohm 100W aluminum case resistors and 5pcs of temperature switches, bought from China through ebay. The resistors and temperature switches were mounted on a 1m long aluminum plate that will be mounted on top of the air vent that blows air to the windshield. The idea is to have just enough heat to keep fog out of the windshield.

Temperature switches were mounted next to each of the 2.7ohm resistors. Their job is to cut the power off from the resistor next to it if it reaches the temperature switches temperature limit. The temperature switches that were ordered have a limit of 55 celsius.

The heater will be powered from the 13,2V 60Ah LiFePo4 battery pack and all of the 2.7ohm resistors are connected parallel. This gives an initial heating power of about 320W.
If the heating power of the heater turns out to be too low I'll need to either improve the heat sink, install more resistors, or install temperature switches with a higher temperature limit. Practise will tell what will need to be changed.

In addition to the windshield heater, seat heaters will be placed on front seats. Anyway while travelling in this car during cold days, some extra clothing will definitely be needed as only a fraction of the original Corolla's heating power will be available.

72V Battery pack test fitting

The batteries for the 72V battery pack arrived today! They were nicely packed in a wooden box with a good amount of padding.

While waiting for the batteries to arrive, I had welded a frame for the batteries to be mounted on. The frame is attached to Corolla's chassis under the same bolts that attach the Corolla's front bumber. I was eager to see how the batteries would fit on their frame and made a test fitting for them right away. After trying different possible layout alternatives, the solution shown in the picture above felt like the way to go. Now that the place for each cell has been decided, next thing to do is to fix them securely into their places.

To make room for the batteries the motor controller was decided to be moved slightly. There seems to be enough room for the battery chargers to be mounted right next to the motor controller. The 12V battery pack is planned to be mounted on the same place where Corolla had its original 12V battery.

Configuring panel meters

I wanted the Corolla to have simple yet informative metering to be shown to the driver at all times. Three digital panel meters will be mounted below the original speed and rpm meter to monitor the 72V battery pack voltage, 72V battery pack current and motor current. CX-102-B digital led panel meters were chosen for the job and they were ordered from www.circuitspecialists.eu.

These panel meters are easy to configure by external resistors to give a desired reading from a measured dc input. The battery voltage meter was configured to show the voltage with a 100mV resolution. The battery pack current is measured using a 400A 75mV shunt resistor. Therefore the meter showing the battery pack current was configured to give a reading of 400 when the input voltage is 75mV.

The motor current will be measured from the current meter output of the Kelly Controls motor controller. I contacted the Kelly Controls support to get more information about their controllers current meter output and learned that it is generated with a 5V 8-bit pwm output which should be readable using any 5V panel meter. I configured the panel meter to give a reading of 400 when 5V is applied to its input. An isolated power supply is needed for the motor current metering as the panel meter connected to Kelly Controls current meter output is intented to be connected between +5V and the current meter output. Once the meters are installed it will be interesting to see how accurate current metering the Kelly motor controller can give.

12V battery pack initial charging

Before buying the LiFePO4 cells for the 72V battery pack I wanted to make sure that the cells for the 12V battery pack were ok. Each of the cells were measured to have a voltage of exactly 3,30V after shipping which indicates that the cells would be in good condition. 

The cell balancing modules were connected parallel to each cell and a laboratory power supply was used to charge the battery pack. The cell balancing modules will be located in a separate enclosure and the connection to the cells is made by 1,5mm^2 wires. Each of the wires have a fuseholder that is located as close to the cell terminal as possible. The fuses will act in case a short circuit occurs and they are also helpful when connecting electronics to the balancing circuits and protection circuit module. The connections can be made safely first and after the connections are ready the fuses can then be placed in the fuse holders.

After two days of charging with a current of about 0,8A the battery pack was fully charged up to 14,4V and the balancing circuits seemed to work as expected. I will not make any deeper tests to the batteries at this point. They look to be in good shape and I am ready to order the cells for the 72V battery pack.

Electric brake vacuum booster testing

The brake booster used to get its needed vacuum from the manifold of the original gasoline engine. Now that it is removed vacuum needs to be generated using electricity as it is the only energy source that will be used. I bought a dismantled electric vacuum pump from some model of VW Golf MK4. The pump was measured to reach a vacuum of 0.8 bar while drawing a current of 8 amps.

A simple vacuum switch is used to control the pump so that it will only be turned on when more vacuum is needed. The vacuum switch was made many years ago by my father who used a vacuum advance unit of a ignition distributor and a microswitch. The working principle of the vacuum switch is very simple. The vacuum advance unit pulls a lever according to the vacuum applied to it. Once enough vacuum is applied the lever operates the microswitch. The operating point of the vacuum switch can be easily adjusted just by positioning the microswitch to be operated on a desired level of vacuum. Using the vacuum gauge shown in the picture a vacuum level of 0.57 bar was adjusted as the operating point of the vacuum switch. Once it is reached the microswitch cuts the power off from the vacuum pump. A power diode was placed parallel to the vacuum pump motor to increase the lifetime of the microswitch.

When the vacuum pump motor is stopped it leaks vacuum through it so a check valve is needed between the brake booster and the vacuum pump. The vacuum level to be adjusted by the vacuum switch needs to be taken from the brake booster side of the check valve. In this case the original Corolla's check valve was able to be used as it was working well without any leak and it was easy to mount an additional pipe to connect the vacuum switch between the check valve and the brake booster.

The test setup worked well. Pressing the brake pedal all the way down caused the vacuum to drop to a level of ~0.44bar and it took one second for the pump to reach the 0.57bar vacuum again. As the tests looked ok, this setup will be assembled to boost Corolla's brakes. Further braking experiences will have to wait for the first road test drives.

Corolla eFX circuit diagram

During the last weeks I have spent hours and hours in studying different batteries, chargers, LiFePO4 protection modules, cell balancing modules and battery management systems. I'm pleased to finally have made up my mind about the solutions that will be used in this electric Corolla. I'm even more pleased about the circuit diagram that turned out to be quite simple and low cost yet providing all the features I wanted.

LiFePO4 Protection Circuit Modules (PCM) will be used in both 12,8V and 76,8V battery packs to monitor that each cell's voltage is kept between 2,0V and 3,9V. If the voltage of any cell goes outside of this range the PCM disconnects the P- terminal from the battery packs negative terminal. If that happens in either of the battery packs, the Corolla will stop immediately. This feature is only the last resort to protect the LiFePO4 cells from severe over- or undervoltage. In normal conditions the driver is responsible to maintain acceptable charge level by charging the batteries often enough and not trying to drive too far.

The PCM's that will be used have integrated balancing circuits but the balancing current is typically only less than 100mA. I think it is quite low for 60Ah batteries and a 10A charger so I decided that 1,7A Cell Balancing Modules (CBM) will be installed parallel with each cell to help the situation.

Now that the plan is done it's time to purchase the rest of the components and complete the project. Let's see how many changes will be needed along the way!

First batteries arrived

I have recently spent hours of time searching for suitable batteries, chargers and battery management systems to be used in this project. I decided that the Corolla would be powered by Winston made LYP60AHA LiFeYPO4 cells. They are 3.2V 60Ah cells capable of 3C (180A) continuous discharge rate. Four of these cells will be used to make a 12V battery pack for lights and heating etc. 24 of these same cells will be used to form a 72V battery pack for moving the car.

The first four cells shown in the picture were purchased from www.ev-power.eu that is located in Czech Republic. Purchasing was easy and I was well informed at all times during the order process. After finishing up my plans for the 72V pack battery management and chargers I will most likely purchase the rest of the batteries from the same place.

www.ev-power.eu has also a very informative blog: gwl-power.tumblr.com that gives us readers interesting posts related to solar energy, batteries and everything around them.

Weekend test driving

Some more test driving was done on a beautiful winter saturday. A couple of visitors also made a short test drive with the Corolla eFX and we learned together that it is extremely silent. It was  sometimes hard to notice its movement because the wheels are the only thing making noticeable noise. On the video most of the noise is made by the train that goes by far away.

Some simple performance tests were also made. We learned that the corolla could climb the slope on our yard on its first and second gear, but on third gear it did not accelerate anymore uphill. On even road it started rather easily with third gear. We still had no voltage or current meters installed on the car so we did not learn about the used motor current or battery power.

I just received a pair of 400A shunt resistors bought through ebay that will soon be installed to measure battery and motor currents. With the shunt resistors and a battery voltage meter installed it will be much easier to evaluate the performance of the car.

The second test drive

A temporary ~72V battery pack was created using four 12V 12Ah lead acid motorcycle starter batteries and two 13,2V 6,9Ah LiFePo4 battery packs in series. The two LiFePo4 packs are the same ones used in my electric lawn mover introduced in my other blog.

The power on/off switch was also a temporary solution. A switch was brought into the cabinet through the driver side window.

After charging the batteries the controller was tested and the motor rotating direction was verified. The last job before driving out from the garage was to change winter tires to the front as our yard has a steep and icy slope that could not be climbed with summer tires.

It was about -10 degrees celcius when the Corolla eFX was reversed out of the garage for the first time. I had never driven an electric car before and it felt great to accelerate from 0 rpm just using the throttle pedal. I took the risk to reverse all the way down of the icy slope in our yard unsure if the corolla would make it back to the garage on its own. On the bottom of the slope I changed to the 1st gear and gave some throttle. The Corolla moved up the slope easily and silently! It was so fun that I drove back to the bottom once more before driving back to the garage.

On this second test drive I did not have any voltage or current meters installed so I do not know what kind of motor and battery currents were used. Once I have installed some meters I'll drive up and down the same slope many times to learn about the current levels used and if the regenerative braking works correctly or not.

Motor controller power connections

I had some 50mm^2 welding cable available and it was used for the motor power connections and for the shorter wirings inside the controller's enclosure. While connecting the power cables on the controller I noticed that the motor output connectors are very close to the round control signal connectors. I had to lower the head of one of the M8 bolts to prevent it from making a short with the signal connector body. The clearance between the connectors are now just 2mm. I will use some additional insulation between them for peace of mind.

Fitting the motor controller

A L-shaped steel rod was bolted between the original engine mounting and the gearbox. The motor controller enclosure was placed on top of the steel rod. In this position the enclosure can be accessed easily and the throttle cable reaches the modified hall throttle pedal.

Motor controller programming

The motor controller was carefully powered up for the first time using a laboratory power supply. Only the power cables and the serial cable was attached to the controller to program it. About 20Vdc was given to the controller and it drew less than 100mA current when turned on.

A RS-232 cable was provided with the controller. Luckily my laptop has a serial port which is getting rare on newer laptops. The laptop used for the programming runs Windows XP so there should be no problems.

I installed the programming software that is available from Kelly Controls web page. Then after powering up the controller and having the serial cable connected between the PC and controller the Kelly Controls software found the motor controller by itself!

The programming software was easy to use and the available options were quite well explained. Most of the default setting were accepted at this time but some were altered. I am preparing for the second test drive where I would use 6 small lead acid batteries is series. For these weak batteries I wanted to set the battery current limit to as low as possible (80A). I also adjusted the throttle voltage range to be 1.2V to 3.8V after testing the throttle pedal output with a multimeter. With 5V power supply the hall throttle signal range was measured very closely from 1V to 4V so I will start with about 0,2V margin on both ends.

Finally I altered the regenerative braking settings a little. I would like to have the "release throttle regenerative braking" feature enabled, but this braking mode was not as well explained as the other two available braking modes. The second test drive will show if I had understood the braking parameters correctly or not.

Motor controller assembly

The motor controller components were installed in a 2U rack enclosure. It is not a watertight box but it can be sealed if needed. The heatsink was mounted on the bottom of the motor controller so that it sticks out from the enclosure.

The rack enclosure will be mounted close to the batteries and the motor to keep the power wirings short. The hall throttle can be seen next to the motor controller.

Electric gas pedal

The hall throttle pedal was modified so that corolla's original gas pedal and throttle cable could be used to actuate it. The hall throttle will be mounted in the same enclosure with the motor controller.