Tuesday, October 28, 2014

WHAT HOURS?

After weighing the Fiero yesterday, I decided to take if for a test drive to see if my watt hours decreased. As  274 pounds of batteries were taken out of the front of the car, I was expecting the watt hours per mile to decrease around 27.4 from the 288 to around 261 watt hours per mile. To my surprise, the watt hours per mile came down to 228, using 120 volts as the average. 

When I removed the batteries, not only was the weight reduced but the front of the car came up a couple of inches, possibly changing the aerodynamics of the car. Additionally, the belly pan was removed. The other issue is what do you use as voltage when figuring how many watts you used? Before I started the drive, the voltage was around 135 volts and as the drive progressed, the voltage was slowly declining. Below is a graph that illustrates the difference in watt hours depending on what you use as the voltage.


TEST DRIVE WATT HOUR TABLE

Another factor is that the entire test drive today was in forth gear, as the shift cables are out of adjustment again. As stated above, I was expecting the watt hours per mile to decrease by around 27 watt hours but it dropped anywhere from 31 to 60, depending on the voltage I plugged in. I can see a real need for a good watt hour meter that counts up and down and gives an average watt hour reading based on samples of current and voltage.




I thought I had a log file of the drive but when I opened it, there was nothing but zeros in the accelerator voltages, so I must have actually turned it when I was sitting in the driveway.

The garage is cleared out and ready to pull the car in when the weather turns cold this weekend and I have started getting contactors and wiring ready to bottom balance the other two packs.

Thanks for dripping by,

Randy

5 comments:

Joey said...

Do you think the change in ride height was affecting your wheel alignment. I know mine was out quite a bit, and I was only sitting about an inch lower than stock.

I use 3.2 volts per cell for my voltage when doing the energy consumption test. As long as you use the same value, and test with the battery always in the same state of charge, you should be good for making comparisons.

cztree said...

Joey,

The previous owner told me he just had the alignment done when I bought the car but then I took out 900 lbs of lead acid batteries and replaced it with 277 lbs just in the front of the car. The HiPower LiFePo4 cells had so much sag that the contactors kept tripping when the voltage was too low for my DC-DC converter to put out 12 volts, so when Jack Rickard offered the Better Place packs, I picked one up. Now I have all of the weight in the rear of the car until the other 2 Better Place modules are installed in the front but after that is done, I will take it in for an alignment. Did you have your alignment done with a minimal amount of tow-in or did you just have them align it to the original specification?

On the LiFePo4 cells I used 3.2 volts as the average but the LiMn2O4 seem to have a much higher average voltage, as when they are discharged and get down to 3.1 or so volts/cell, they are pretty much done. They are still taking a charge at 4.3 volts/cell and the charge curve is just starting to go vertical. For the LiFePo4 cells, I only charged to 3.65 volts per cell and the graph of the end of the charge was almost vertical when the charger started tapering the amperage at the end. For the 32 half cells, 3.2 volts would be 102.4 volt for the pack but as my table demonstrated, the lower the voltage you use as your average, the lower your watt hours per mile appear, so I want to pick an average voltage that will get me close to the actual watt hours per mile that the car is using. I guess as long as I use the same value each time it should be a good number for comparison of changes made to my car but may not be a good comparison against some one else's car. How may watt hours per mile are you calculating for your Porsche and how much does it weigh?

Thanks,

Randy

Joey said...

I had the alignment done to the original specs, no experimenting with minimum toe. I guess if ultimate efficiency was the goal, I'd have done for a different donor.

I'm getting about 280 watt-hr/mile and I'm at 3150 lbs vehicle weight (about 200 over stock). You want to use the average voltage for your cell chemistry. On the internet (so it has to be true) I see a lot of people using 3.7 volts for nominal.

cztree said...

I think I will keep the alignment stock also but may ask them to keep the tow in to the minimum of the specification.

Your watt hours per mile are really good.

Joey said...

I don't always get that high efficiency, because it is difficult to keep my foot that from going too low on the accelerator pedal. Also, I originally had H49 brushes that got 320 watt-hr/mile, and wore out in 4000 miles. The new H60 brushes are harder and should last a lot longer. As a bonus they get better efficiency.