Battery Charger for the 144 volt Pack of Optima Yellow Top Deep Discharge Batteries

The 12 individual Yellow Top batteries have until now been individually charged using a Sears battery charger.  This process, although adequate for testing, is not conducive to using the car as a daily ride.  Consequently, different battery chargers were investigated to see which one might work for this project.  The design criteria included that the batteries and the charger must at all times be isolated from the chassis and it must be able to provide about 171.6 DC volts.  For the initial charger tests an Eltek 67.115.0 was selected (Picture DSC04736) from evolve electric  ( http://evolveelectrics.com/ ).  The charger ($1261.48 including shipping) can also be purchased with a liquid cooling chill plate ($314 extra).  Since the charger will be located in the trunk and for the moment there are no cooling lines available, the charger was modified in David's shop with the addition of an Aluminum heat sink and twin 12 volt fans (Pictures DSC04743 and DSC04731).

Picture DSC04736 showing the original Eltek 67.115.0 charger (6.29 kg after adding wiring connections on the left side).

Picture DSC04737 showing the name plate specifications of the Eltek 67.115.0 charger.

Picture DSC04743 showing the charger (on the left) after the addition of a 6.5" x 14" x 1.375" Aluminum heat sink.  The control cover (right side) has twin fans, a control box with output display, and a computer port for connection to laptop and programming.

Picture DSC04731 showing the end on view with the heat sink and control cover in place.  The air is pulled through the heat sink and exhausted away from the unit.  The complete charging system was 7.3 kg.

Operation of the Eltek 67.115.0 Charger

The Eltek 67.115.0 charger is a 3 KW charger and it is controlled via the CAN bus.  It can provide between 78 and 180 volts DC and a maximum of  23 amps.  The Eltek technical manual can be found here:  http://evolveelectrics.com/PDF/Eltek/Eltek%20Guide%20IP67.pdf   

Upon application of power, the charger begins continuously transmitting an identifier statement as it waits for instructions.  An 18F Series PIC microcontroller with CAN interface was selected by David due to his previous experience with the device.  The PIC under software control provides messages on the CAN bus that include current limit, voltage limit, power limit, and enable command.  The PIC sends the charger these commands several times each second, and if the Eltek charger does not receive a command within 1000 milliseconds. then the charger outputs an error condition, shuts down, and reverts to sending only its  identifier statement.  When the charger receives the control command, it then responds by sending (about 5 times per second) 3 different messages.  Status 1 (a lot of data which also includes voltage and current), Status 2 (more data which also includes power and temperature), and Errors.  The PIC uses these status messages to provide information to the display and to collect data that it can provide to an external data logger program.

Manufacturer Recommended Charging Cycles

The requirements for charging Yellow Top batteries (and many others) can be found at http://batteryuniversity.com/  or by review of some Xantrex charger's technical data sheets.  Each type of battery has a manufacturer's recommended charging cycle which must be followed to avoid causing damage to the cells, and the requirements for Lead acid batteries are very different from Lithium batteries.   In the case of the 12 volt Yellow Top batteries, each battery is initially charged at 14.3 volts followed by a trickle charge at 13.4 volts.

Algorithm for the charge cycle

The software was written by Chris in about 20 hours and is about 500 lines of C code.  With all the drivers and libraries the program is about 1300 lines.

The start of the charging cycle is called the Stage 1 Bulk Charging Mode in which the charger is amperage limited.   The Eltek 67.115.0 can provide 23 amps, but we elected to limit it to 20 amps.  The battery voltage gradually rises at the fixed amperage, and the Bulk Charging is continued until the voltage reaches 14.3 volts per battery, or 14.3 x 12 (batteries) = 171.6 volts for the pack.  After reaching the maximum voltage, Stage 2 (Absorption) charging begins where the voltage provided by the charger remains constant (at 171.6 volts), but the amperage drawn by the batteries gradually decreases.  Stage 2 ends when either the amperage falls to 2% of battery capacity ( Yellow Tops are 75 amp hour, thus 0.02 x 75 amp hours = 1.5 amps) or, a total of 3 hours has passed while in Stage 2 (this is a safety limit we elected).  Upon completion of Stage 2, Stage 3, or the Float Charge Stage, begins.  In the Float Charge Stage the Eltek voltage output is reduced to the manufacturer's recommended 13.4 volts per battery (or 13.4 x 12 = 160.8 volts for the pack).

Picture showing the bench testing of the display.  The 12 volts required to operate the PIC microcontroller and the display was derived from the J-1772 plug.  Two fuses were added within the charger, and then the power was supplied to a 12 volt power supply located within the control box. (Picture DSC01604).

DSC01604 showing the computer interface (black far left), reverse side of the display board (green), and the 12 volt power supply located within the control box.  (updated 9/7/2017)

Picture of the resistor banks that David used for the initial testing of the charger.

The next step will be to establish the wiring diagrams that will connect the J-1772 charging port (on the side of the car), the Eltek 67.115.0 charger, the batteries, and a J-1772 AVC2 board (to provide handshaking to public charging stations).   Finding a good location within the trunk and the fabrication of mounting brackets will then finalize the effort.