Electric Porsche – Charger design

The basis of the charger design is 90 quarter brick DC/DC converters, so I got 100 just incase.  I may use the same converters to charge/balance the 12v system battery as well.  Each DC/DC converter will accept 36-75v input range and by default puts out 3.3v, however this output will be adjusted up 10% to put out 3.65v which is the perfect CV charge voltage for many LiFePO4 cells.

The 630 cell battery pack will be broken down into 6 individual battery boxes each containing 105 cells and one charger board which will contain 15 dc/dc converters.  I’m still deciding if I should use connectors, or wires soldered directly to the board to connect from the board to the cells.  The inital board design is shown below, which will be a 4 layer PCB roughly 6″ x 12″ which will be mounted above the headway cells, with very short connecting wires from the PCB to each buss bar.  There is no longer the need for a BMS because each cell is effectively charged individually to the preset 3.65v.

Each battery box will be roughly 6″ x 12″ x 24″ which will make it fairly easy to mount and integrate into the car.  The DB25 connector will connect each battery pack to the Arduino which will monitor each cells voltage.  There are fuses onboard that will protect the pack and wiring from shorts or other problems.

The power supplies can accept between 85-264v AC so I simply need a 3 prong grounded plug for the charge port, the standard connection for this voltage and current range is the NEMA L6-20, so behind the fuel filler door will be something like this: 

This will allow me to plug into any available 110/220v outlet to recharge, it would also be technically possible to plug directly into a 48v DC power supply or battery (36-75v actually due to the wide input range of the dc/dc converters).  This would bypass the AC power supplies and require a high current 48v DC connection.

3 Replies to “Electric Porsche – Charger design”

  1. Doing a great job here. The 944 is one of my favorite vehicles. I’ve had 3 RX7’s (2nd gen) which was very similar in appearance to the 944 -classic design! If you need any help from us at NetGain – just let us know! All the best!
    George

  2. How are you going to protect from cell level overdischarge i.e. where’s your LVC?

    the chargers create a well balanced pack, but only on the top balance. If they discharge unevenly, you wouldn’t know it.

    1. Hi Travis,

      Very good question, I have an arduino based voltage monitoring system that integrates with the PCB that will hold the dc/dc converters. It will allow real time monitoring as well as programable LVC or even multiple LVC points, and the ability to ignore high current LVC events. Initially this system will just be an alarm, however I may tie it into the Soliton1 to engage the low battery voltage limiting that the Soliton1 already does. I plan to have my main display show the highest cell voltage, lowest cell voltage and the average cell voltage at all times, so if one starts to drop away from the rest I will be able to see it, this would be a future feature, but I’m sure I could also have an alarm for variance, IE the maximum I would allow cells to vary at rest or under load and have an alarm for that too, that way I can see if a cell is drifting/faulty/etc before I ever reach the LVC point.

      For safety all voltage monitoring is opto-isolated, to eliminate any dangerous voltages running through the car. Each dc/dc PCB will connect to to the Arduino via a single Cat6 ethernet cable. The power supply for the charger will be connected in a similar way to allow voltage and current monitoring during the charge cycle and provide a safety lockout so the car can’t be started when the charger is on.

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