Sunday, August 5, 2007

Test Drive to Work

Since traffic is really light on Sunday morning, I drove to work (15 miles away) and back to see how the 914 would handle.

Shifting was a real pain on city streets since the electric motor changes RPM very quickly during acceleration or regeneration. The RPM stays constant when I "coast" unlike a gasoline engine which tends to slowly drift back to "idle." This makes synchronization of the gears rather difficult.

As expected, power was rather lacking on the freeway. It took me 30 seconds to get up to 60 mph. After analyzing the AC controller logs, it turns out that I only took the motor up to 3000 RPM and it is spec'ed at 12000, so maybe I'll try a lower gear next time to get more torque at the wheels. I just have to get used to the concept of driving at higher RPM, since I got better gas mileage by keeping the RPMs low on my old car (which is 19 years newer than the 914 :).

If my ammeter from West Marine is correct, then just cruising takes about 50 amps and every time I accelerate even slightly, the current goes up over 200 amps. I'm not sure how much current these golf cart batteries are designed for. The battery voltage sunk from 152 volts at rest down to 132 volts after awhile during heavy acceleration. The controller cuts current to the motor if the voltage drops, so I definitely lost power on the uphill exit ramp off the freeway. I think people experience cognitive dissonance when they see a little red Porsche sports car being the slowest car on the road.

I'll be happy to e-mail anyone logs from the AC controller if you'd like to parse the data with Excel to see how the controller behaves. If I can capture a screen, I might even post a picture here.

5 comments:

Ross Cunniff said...

Interesting data (I'd be interested in the spreadsheet, too...). This is one reason why, someday, I'd like to replace my lead-acids with a higher-voltage (and lighter) exotic battery pack. I bet it's peppier at 216V...

pjorg said...

TIm,
I am not convinced that there isn't something wrong in the battery string. If you are using 180 AH 8 volt batteries then a 200 amp discharge is approximately a C1 rate. At this rate and at a 20 % capacity I don't think the cell voltage should be dropping down to 1.8 volts per cell. I will try find some typical lead acid battery data to confirm. I would hook up a multimeter across some different batteries to see what the voltage per battery is dropping to while accelerating. ( Connect to the post not to the strap) There could also be some high resistance battery straps somewhere.
More later

Paul

Roger Daisley said...

Tim: Although our setup is different, I'm using a brand new set of sixteen US125's (6-volt) Since I just finished my car, I'm doing a series of short runs and recharges. I just now finished a 10-mile run ... I now have a whopping total of 17 miles on the batteries! My 6-volter's are running about 2.13v per cell, after charge. After the last 10-mine run, the SOC meter showed 80% and the cell reading only dropped to 2.10v/cell. I'm now pumping 20a into the pack with a cutoff of 2.59v, then float at 4-5a. for an hour or so. (I'm using a Rudman PFC20)

In a previous EV I used the US 9-volt batteries and routinely pulled 400a. (I'm seeing 500a on my current pack.)

I don't know if this is any help.

Here are a couple of helpful references:

http://www.usbattery.com/pages/usbspecs.htm

http://evbatterymonitoring.com/

Look at the on-line Book 1, Chapter 3.

TimK said...

Hi all, thanks for the comments. My next toy to install is a PakTrakr which monitors the voltage on all the batteries and logs the data to a serial port. I'm hoping this will help me find any high-resistance connections in the system as well as help me more accurately see how individual batteries are behaving. If Ken Hall (designed the PakTrakr) is right, the kit should ship early next week and I should get it in a few days.

Mike said...

Tim said "Shifting was a real pain on city streets since the electric motor changes RPM very quickly during acceleration or regeneration. The RPM stays constant when I "coast" unlike a gasoline engine which tends to slowly drift back to "idle." This makes synchronization of the gears rather difficult."
Aside from the clutch problem you found already, part of this has to be turning regen off until you press on the brake. I'm guessing the motor would slow down in neutral if it was set to regen as it was originally.
Part of the fun of driving a Porsche is getting the RPM's up and using the throttle to slow the car for corners, then using the throttle to steer IN the corner (a 914 speciality).