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.