Electric Porsche – Batteries, Chargers and Weight

There is no question that it’s December up here in Canada, the donor car (Dark Grey) is under a pile of snow. The parts car (Green) has made it’s way into the garage where it will be receiving it’s new heart. The 2.5L engine out of the donor car has had a few Porsche required things replaced and will give new life to the parts car very shortly.

As usual space is at a premium, but I believe the process will go reasonably smoothly.  Although this is my first time putting the engine INTO a 944, after removing two of them I know the basics off by heart now.

Many of the stock parts on the donor car are noticably lighter than the parts car so where ever possible I will use the lighter parts, including the starter, radiator, exhaust, headers etc.

Now that the parts car has made it in the garage I took the opportunity to remove the stock battery for comparison to the Headway 12 cell LiFePO4 battery that I assembled.  This let me add one more real weight into the spreadsheet of removed parts, and I was pretty close.  I had estimated 30lbs for the stock battery and it came in at 34lbs.  I also did a few comparisons between the two batteries related to power output, although this isn’t quite fair because the Lead Acid battery out of the Porsche is about 1 1/2 years old with minimal use over the past 6-8 months.  So I charged them both up, 14.4v for the lead acid, and 14v for the headways.  About 20 minutes after coming off the chargers the Headways were just shy of 14v and the Lead Acid had come down to 12.72v.  Since my background is in Car Audio I decided that a real test for the two batteries was to power a good size car audio amplifier, although I have a few fairly efficient amplifiers that will be used in the electric car, I decided to use a much less efficient, current guzzling but great sounding Coustic Design Reference 510DR.  For this test I simply used a pair of load resistors so that I could turn up the power without making too much noise. 

I started with the Headway pack, setup the volt meter but didn’t measure current this time around.  I played the same tracks in the same order for the same length of time using both batteries.  The amp/resistors were given time to cool down between tests.

510DR load test (Dec. 13, 2010) Lead Acid 50AH?  (34lbs) Headway 30AH  (9.8lbs)
Charge Voltage 14.40v 14.00v
No Load 12.72v 13.95v
Amp On 12.65v 13.73v
Amp ½ Volume 12.56v 13.63v
Amp Full Volume (After 30 Mins) 11.60v 13.00v
Amp On (5 Min later) 12.25v 13.25v

This was an informal test, and I just wanted to evaluate the 4 dc/dc converter Headway charger, but some simple conclusions can be drawn. The Headway battery is not only a much smaller, and much lighter solution, it will also maintain a higher voltage longer for the headlights, wipers, and controller if the dc/dc converter ever fails.  It is however much more expensive, the Lead Acid battery was essentially free since it came with the car, the total cost of the Headway pack will be over $300 with buss bars and enclosure.

 

I used this exersize to test the dc/dc converter charging method a few times, as you can see the hardwired charger attached to the Headway cells.  I assembled the pack with very unbalanced cells to see how the system would deal with it.  The pack is made up of four series sets of three parallel cells (4S3P), so there are 4 dc/dc converters used to charge the pack, with each parallel group needing it’s own dc/dc converter.  One of the four groups was fully charged, two of the groups were about 90%, and the final group being about 70%, they all came in line nicely with the 70% group drawing the most current (the main current loop is through the heavy wires, and the smaller red wires only carry current if there is a pack imbalance).  The pack charged up nicely with all cell groups finishing at 3.50v

I’m very happy with these first tests, the power supplies and dc/dc converters are performing exactly as expected.  The power supplies work equaly as well on 240v as 120v and the current draw vs power output scales as expected between the two voltages.  A 240v 20A line should provide quite reasonable charge times.

2 Replies to “Electric Porsche – Batteries, Chargers and Weight”

  1. So exciting…

    I’ve been driving my new ’83 944 on fossil for a few months now. What a fun car.

    Can’t wait to start the conversion (although I _am_ going to be waiting as I need to save enough cash to make it happen) but I’m really enjoying seeing how you’re approaching your conversion.

    Cheers,
    Timothy

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