Saturday, February 6, 2010

Efficiency differences

When I bought this Gizmo in August 2006 it came with 6 Trojan T-875 8v batteries which were 1-2 years old. One of the things I did from the beginning was to use a Kil-a-Watt meter to record the energy I pulled out of the wall. I wanted to know how much it was costing me to charger my car and to see what the efficiency was. My regular commute is 4.4 miles with an elevation change of about 450 feet, most of that is in the span of about a mile. Sometimes my Wife drives the Gizmo to work and her commute is 9 miles. I mention this because with lead acid batteries and the Zivan NG1 charger I have, every charge cycle includes an equalization phase were the batteries are gently over charged to help all of the cells reach the same state of charge. This means that with shorter drives the wasted energy is not spread out over as many miles. Furthermore, the hill climb at the end of my commute where I would routinely see 250 battery amps, a less efficient current to pull since so much energy is lost to heat, would also not be spread out over as many miles as a longer commute would give.

From August 2006 through August 26, 2009 I used the Trojan T-875 batteries. On July 29, 2009 I didn't realize the range of the pack had diminished so far and ended up reversing a cell in one of the batteries. I never was able to revive it. At that point I took the Interstate Batteries U2200UTL pack out of Gizmo #26 I purchased last summer from Galactic Pizza in Minneapolis, MN and installed it in Gizmo #31 and used them until January 18, 2010 when I installed the TS-LFP100AHA pack. (Gizmo #26 needs extensive restoration.) BTW, if you are in Minneapolis go to Galactic Pizza and get some pizza. It is excellent! Tell them that the guy from Washington who bought one of the Gizmos sent you. Besides, you will be supporting a business which uses EVs to deliver pizzas as long as weather permits.

Note that the energy values below are all as measured from the wall so include charging inefficiencies.

Total Miles: 967
Total kWh: 272.81
Average miles/kWh: 3.54
Average Wh/mi: 282.1

Total Miles: 2364
Total kWh: 682.15
Average miles/kWh: 3.47
Average Wh/mi: 288.6

Total Miles: 497
Total kWh: 138.53
Average miles/kWh: 3.59
Average Wh/mi: 278.7
(On March 28 I sent the NORM Circuit, which reads the hall effect throttle sensor and sends the appropriate signal to the controller, to Black Sheep Technology to get a replacement built. I didn't realize at the time that I could have kept using the old one with my manual override switch until the new one arrived so I was without a Gizmo until April 2009. The new interface was worth the wait!)

2009 (8V T-875 pack)
Total Miles: 1180
Total kWh: 315.47
Average miles/kWh: 3.74
Average Wh/mi: 267.4

2009-2010 (6V U2200UTL pack)
Total Miles: 1180 (This number isn't a typo. I had to double check it, too.)
Total kWh: 294.24
Average miles/kWh: 4.01
Average Wh/mi: 249.4

2010 (TS-LFP100AHA buddy paired pack)
Total Miles: 823
Total kWh: 135.37
Average miles/kWh: 6.08
Average Wh/mi: 164.5
(January 18, 2010 through March 29, 2010)

There some interesting things I notice about the data. First 2007 showed slightly more energy use per mile than the previous year. The Gizmo came with a 30 tooth drive pulley and in August of 2007 I had to have the motor rebuilt by Jim Hustead of Hi-torque Electric in Redmond, OR. This is the Jim of White Zombie fame and the builder of the Jim-Pulse line of Warp motors. With a 30T pulley the motor was turning too slow to keep it cool. I went to the smallest pulley I could find which was a 22T pulley. This changed the gear ratio from 3:1 to 4.09:1. The motor has been much happier. I think that the build-up of carbon dust and wearing & cracking of brushes may have had something to do with the lower efficiency.

I think it is interesting that even though the T-875 pack was aging that the efficiency showed an improvement over the previous years (except for 2007). When I look at the energy used to charge after my 4.4 mile commute, however, the energy consumption was up slightly from earlier years and similar weather conditions.

When the 6V battery pack was installed I saw right away a drop in energy consumption. At first I was expecting to see the energy consumption increase because I added over 120lbs to the weight of the Gizmo. After some thought, I think the reason that the energy consumption decreased is that the significant increase in plate area meant that the 250A draws actually reduced the current per unit of plate surface area so not as much energy was lost due to resistance. This is definitely a variable one might consider when choosing a battery pack. Lighter is not necessarily going to be more efficient. Of course I only have one data point to support this hypothesis.

The biggest shock :) came when I looked at the energy use with the TS batteries. To go from a best case of 250Wh/mi with the 6V lead acid pack to a tiny 165Wh/mi is incredible! I am including all the energy I've put into the TS pack so that if I don't completely charge the pack on one charge it will be made up for when the pack gets fully charged later. I keep thinking something is wrong. I'm using the same Kil-a-Watt meter I have been using all along. Maybe I should hook a second one in series with the first to see if they both give the same results. I'm going to periodically update the data as I get more use on the pack. The only things I've come up with as to why the efficiency is so much higher is that there is very little wasted energy when charging a Lithium Ion pack. I don't equalize each time. I plan on doing that this summer to see how far out of balance the pack has gotten. I'll probably only equalize once or twice each year since the BMS will alert me to an out of balance cell. One other possibility is that with the higher voltage I went to, the controller and motor are more efficient. I really don't know how much more efficient they are but I assume a little more. On the other hand, I don't drive full throttle as much either so the switching losses would be higher, I'm guessing. Time will tell.

(edit: updated energy consumption & distance values March 31, 2010)


Jack Rickard said...

I might suggest that with the TS cells, you have somewhat dramatically decreased the weight of your vehicle. This should have the effect of decreasing the necessary watthours per mile to move it. Could this explain the increased efficiency?

Jack Rickard

Gizmo said...

I thought that too except an approximately 20% decrease in weight shouldn't translate to a 50% increase in efficiency. Remember I'm measuring Wh from the wall not the battery. I really don't know the efficiency of the Zivan NG1 charger. Also notice that when I went from six 8V batteries to eight 6V batteries that my efficiency improved even though I had about 120 lbs more weight. Also, weight has little to no effect when moving at constant speed on level ground. It only has an affect during some form of acceleration (slowing down, speeding up, climbing, descending). Even long runs on level ground show the TS cells have significantly higher efficiency. I think the biggest factors are Pukerts, equalizing, and chemical differences. I believe Pukerts is largely responsible for the increase in efficiency when going from the T-875s to the US2200. Both sets of batteries are at their end of life but I think the larger surface area on the US2200 reduced the current draw/cm^2. At least my hypothesis matches the results. Second, equalizing the lead acid batteries wastes energy. I stop charging before my BMS modules reach their shunting voltage of 4.00V and if they do start shunting they shunt only 500mA. Finally, someone on EVDL mentioned that there was a secondary chemical reaction in lead acid batteries during charge which doesn't return the energy used on discharge. I don't know what it is and I haven't looked for it but it is plausible. I wish I had a Wh counter so I could compare efficiencies independent of the battery and charger inefficiencies. I think it could show the effect of weight differences. If I get ambitious this summer I may tighten up my coil-overs, load an extra 200 lbs in the back and go for a drive. If I do, I'll report back so maybe we can get at least one data point on the "weight effect."