Hello to All,
Continued from pt. 2…
With the background of this tale now properly set up, and continuing with my tradition of sharing technical info as freely as possible so that others can take advantage of it and help advance EVs by pushing the performance envelope, as promised here are the details of the high power Kokam LiPol pack:
I had hoped that when I got a lithium sponsor, that along with a big weight reduction for the car and the high power output needed to fully feed a hungry Zilla Z2K, that a properly configured lithium pack (read that 300-400 lbs.) might also give the Zombie more than the 30-35 miles of range the lead acid pack gave. I had gotten a sample of the range capabilities of lithium with the borrowed 175 lb. A123. Even though the 880 A123 cells in it weren’t ‘energy density’ types, that tiny pack could still out-range 852 lbs. of lead acid batteries. What could say, twice that weight in lithium do? How cool would it be to have a street legal EV that burns from 0-60 in less than 3 seconds, runs a high 10 second 1/4 mile, ‘and’ had 75 miles range? Do you think that might get the public’s attention? I figured that about 350 lbs. of good lithium power cells would be about right. Anyway, these were the target performance parameters I was hoping a lithium pack could provide for the Zombie, but as I studied the specs of these ultra high power cells and spoke with Kokam’s EE who confirmed that the specs were, if anything – conservative, I began to realize that ‘these’ cells could not only meet my dream specs…they would far exceed them!
The Kokam ultra high power cells really live up to their name, and are seemingly made for EV drag racing. They are considered large format type cells, but at about just 7.5 inches square they’re still smaller than other LiPol type cells. Each is about 1/3 inch thick, and weighs just 1.8 lbs. At 3.7V & 30 ahr, they have a beefy continuous discharge rating of 20C (600 amps) but it’s their <10 second rating of 40C (1200 amps) that puts them in killer territory! Unlike the small cylindrical type cells that have to be paralleled anywhere from 8 – 12 wide in order to achieve the high currents needed for EV drag racing, it takes just two of these Kokam cells in parallel to crank out 2,400 amps for 10 seconds – I’ll repeat that… 2,400 amps for 10 seconds!
Paralleling pairs of cells does more than just making the extraction of extreme currents possible – the pair also effectively creates a 60 hr cell. At the higher C2 discharge rate however, their capacity actually increases to 32 ahr. The cells pass the nail puncture test without exploding (seen it done), without catching on fire (seen it done), and without spewing fountains of chemicals (seen it done)…read that ‘they are safe’. If all this doesn’t already sound terrific, check this out. They have been tested at 100% discharge for 1450 cycles and still had 81% capacity remaining! At 80% DOD they are good for 2500 cycles and still have 91% capacity remaining.
The Zombie started life as Datsun 1200, a little economy car from the early 70′s powered by a tiny 1200cc 4 cylinder gas engine that sipped fuel, and with the body’s small frontal area, its low mass, and its fairly good cd, the Datsun 1200 was rated as America’s highest gas mileage car in ’73. As a high powered electric car, though it can suck the amps when called on to dispatch a 500 horse Vette, the Zombie’s got a Jekyll and Hyde personality that turns it from a track terror into an efficient EV when driven ‘nicely’.
Before moving on to my predicted specs for the Kokam-powered Zombie, looking back at things (White Zombie History pages makes this easy) will put it into perspective. As we go through this together, forget that White Zombie is primarily a drag racing-focused EV, and rethink of it in terms of usable range per charge. You’ll begin to see why we are so excited about these cells!
The 60, 12V batteries that made up the lead acid pack weighed 852 lbs. and in terms of ‘power density’ could output 1500 amps at an initial sag to 220V at launch, then they would sag lower and lower towards 180V at the end of a run. That’s a max output of 330 kW dropping to 270 kW, or 442 battery hp declining to 362 battery hp. Of all the lead acid batteries I’ve tried over the years, from Optimas to Orbitals and including the various models of Hawkers, the combination of 60 small 16 ahr Hawkers gave the highest ‘power density’ of any lead acid battery, and power density has always been the focus with the batteries used in this car.
In terms of ‘energy density’ with twin strings of 360V @ 16 ahr C20, the combined pack was 360V @ 32 ahr C20, but lead acid being lead acid, at EV currents the actual ahr the combined pack could deliver at the real world C2 was about 16 ahr, resulting in 5.8 kWhr of what I refer to as ‘usable EV capacity’. Even at its heaviest 2660 lb. lead acid state, with the drag radials pumped up hard to 35 psi, the Zombie rolls quite easily and is efficient to the tune of about 190 Whrs per mile @ 55 mph. With the lead acid pack hanging at 375V at 55 mph, the ammeter indicates just 25-30 amps of current being pulled from the pack, so this backs up the 190 Whr per mile claim. An hour of continuous driving at 55 mph would then, give 55 miles range if the batteries could produce about 10.5 kWhrs…but they couldn’t. At 5.8 kWhr, you could get 30 miles of range, and 35 miles at lower urban speeds, hence the 30-35 miles per charge I rated my car at. The battery to vehicle weight ratio (BVR) was 32% so it all fits with the accepted calculations for a lead acid powered EV. Though the Zombie’s primary mission has always been acceleration, it’s also been great to have a decent range per charge (for lead acid) that has allowed the car really be a functional street legal drag car, with the emphasis on the word ‘street’. Being able to drive to and from the track (16 miles one way) with opportunity charging at the track, for me, has always added credibility to the car’s mission. As the lead pack aged, that 30-35 miles became more like 25-30 miles. This was again, pretty much proven that Friday night in July of this year, when running on just half of the pack (covered in pt.1) the car did 13 miles of spirited 65 mph freeway driving, then started to fade away, stranding me about 2 miles shy of my place…call it 13 miles of good driving. If both strings had been working, that comes out to 26 miles range @ 65 mph.
Borrowing the crazy little A123 motorcycle pack was an enlightening experience on many levels. Like the Zombie’s lead acid pack, the 175 lb. ‘box ‘O batteries’ was designed with one thing in mind – power density! Bill Dube could care less about energy density when trying to push Killacycle ever quicker and faster through the 1/4 mile…it’s all about power density! 1400 amps from 175 lbs. of cordless drill cells is pretty amazing!
Lithium being lithium, even when the chemistry and the cell’s mechanical design is oriented towards power density, compared to lead acid chemistry you still end up with outstanding ‘energy density’. Point in case…just 175 lbs. of these cells made 6.9 kWhrs of usable EV power (880 cell pack configured at 8P110S – 2.3 ahr X 8 = 18.4 ahr – 3.4V X 110 = 374V – 374V X 18.4 ahr = 6881 Whr or 6.9 kWhr) You can look at it as 5 times the energy density as the lead acid pack, lb. for lb., or perhaps more entertaining, is that a little box 1/5 the size and weight of the lead acid pack, that fit inside the spare tire well area instead of taking up a large portion of the car, made ‘more’ kWhrs, at an impressive 6.9 kWhr vs 5.8 kWhr! To back this up, in an unplanned range test, I put on 32 miles without a recharge driving the Zombie back home from PIR after a night of racing, then drove it back the next night. After 32 miles the pack’s voltage was still ‘right there’…I probably could have made a few hard runs without recharging! Those 32 miles ate up 6 kWhrs of juice, but true to the reputation of lithium having a very flat discharge, with just .9 kWhr left in the pack it seemed quite ready to keep going!
OK, with this out of the way, here’s what I decided would be the best way to utilize the Kokam ultra high power cells to push the Zombie to the next level:
When going from lead acid to lithium, it takes a bit of adjusting one’s thought process on pack voltage. A lead acid pack comprised of 12V ‘nominal’ batteries is pretty easy to figure out, as everyone knows the ‘actual voltage’ of a fully charged 12V battery is about 12.85V or so…call it 13V. If you design a 360V pack, you know it sits at about 390V unloaded. You also know that at full charge when still connected to the charger and in the final constant voltage stage, each battery goes up to around 15V, so the pack rises as high as 450V. You also know that immediately after shutting down the charger, that the 450V rapidly goes away and the pack is below 400V in seconds. By the time you key-on, the pack is in the 390-395V range, safe for the Zilla, and ready to go. Things change with lithium. Knowing that I could pretty much name the number of cells I wanted, I thought of going higher in voltage for the pack. thinking that 208 cells (2P X 104S) would be perfect and would give 384.6V nominal. The problem is, these cells get taken to 4.2V at full charge, then off of charge only drop to 4.1V, for a 426.4V resting voltage after charging…Zilla, not happy I’d have to drop the pack voltage.
As mentioned in pt. 2, I wanted to assemble these cells into modules. I worked closely with Rich Rudman on this with many brainstorming sessions over pie and scribbled-on napkins, and also with him back in Missouri where we ran the concept past the Kokam engineering team. The idea was to keep the design clean, simple, and accessible. It’s the accessible part that dictated a modular design, because having a large assembly of cells all ganged together in the trunk space of the car, makes a package that although small compared to the lead acid pack, is still too heavy and bulky to work on. It also makes it difficult to quickly get to cells if there’s a problem. With a possible TV show in the works (more on this in pt.4) and with Kokam interested in being a supplier of cells for that project, I wanted the modular design so other packs could be configured by adjusting the numbers of and the placement of modules for a given vehicle. Other factors that shaped the module’s design were weight, physical size, shape, current carrying ability, and cell numbers per module. I wanted each module to not be too heavy, so 35-37 lbs. was the goal. Rudman’s newest BMS board is an 8 channel unit, meaning it can keep track of 8 cells (or 8 paralleled groups of cells). At 1.8 lbs. per cell, and needing to have pairs of cells in parallel, a 2P X 8S, 16 cell module made sense. Each module would be made of tough clear Lexan, and the cell’s output tabs would be tied together with high current nickel plated copper buss and clamp bars. With just shy of 29 lbs. of active material (cells) and the heavy 3-4 lbs. of copper interconnects (needed to pass 2.4 kiloamps), hitting that 35-37 lb. goal would be a challenge. At 29.6V, 64 ahr @ C2, and ~36 lbs. per module, and with pack voltage, space constraints, and a pack target weight including cabling and hold-downs of 450-460 lbs., I went with a 12 module, 192 cell design for a 355V nominal, 22.7 kWhr @ C2 power package capable of outputting 2.4 kiloamps for 10 seconds! The very low voltage sag at high currents is very impressive with these particular cells. Graphs provided by Kokam reveal that for every 5C rate of discharge, the cell sags ~.1V, so beginning at 3.8V if one were to extract 150 amps, the cell drops to 3.7V, and at 10C or 300 amps, it goes to 3.6V, so at its continuous rating of 20C or 600 amps, the cell drops and stays at 3.4V…this is very impressive stuff! In theory, at the 10 second rate of 40C -1200 amps, the cell still hangs at 3V! Do the math for our 2P96S pack, and this equates to a staggering 691 kW! It’s amazing, that 345 lbs. of Kokam cells will generate 926 battery hp! This is terrific power density.
As exciting as the prodigious power density of this pack will be, there’s that other side of things, energy density, that is equally exciting for my street legal EV. With a full 22.7 kWhr @ C2, and with 190 Whrs per mile efficiency, White Zombie’s highway driving range will be a dream-come-true 110-120 miles! Remember, it doesn’t hurt these cells to take them down 100%…they can be taken there 1450 times and still retain 81% of their original capacity! Being conservative and just using 100 miles per 100% discharge, and being conservative and stopping at the 1000 cycle point, this equates to 100,000 miles of driving!
Knowing that my Datsun is a Nissan, it seems very handy for comparison’s sake that Nissan is unveiling its new electric car, the Leaf. Larger than a Datsun 1200 and heavier by more than a half ton – the Leaf weighs 3400 lbs. vs the predicted Zombie curb weight of 2275 lbs., Nissan’s new electric car has a 24 kWhr pack made from 192 flat-shaped cells, and they claim 100 miles per charge. Other early testers have this to say:
“The Leaf sports a 24 kilowatt-hour lithium manganese battery…air-cooled battery provides enough juice to go 100 miles”
“The Leaf, an all-electric five-door hatchback, will have a 100-mile range…driven by a 24-kilowatt-hour lithium-ion battery pack.”
“…said the 100-mile range suggests the car will have a 20-kilowatt battery.”
As with any production factory EV, you are not allowed 100% discharges from the battery’s capacity, so it’s safe to assume those 100 miles are accomplished on anywhere from 20-22 kWhrs. I am predicting 110-120 miles in my smaller car with its lower frontal resistance and that only weighs 2275 lbs. and with close to 23 kWhrs of usable battery capacity. Coincidentally, my old Nissan uses 192 flat-shaped lithium manganese cobalt cells @ 23 kWhrs while the new Nissan uses 192 flat-shaped lithium manganese cells @ 24 kWhrs. …it makes for a very interesting comparison:
3400 lbs. curb weight
mid-sized, 5 passenger
100 miles per charge
0-60 in est. 8.5 seconds
est. 1/4 mile ET 17 seconds
top speed 87 mph
Nissan / Datsun 1200 ‘White Zombie’:
2275 lbs. curb weight
small, 4 passenger (2 now due to 6 pt. roll bar)
110 – 120 miles per charge
0-60 in est. 2.5 seconds
est. 1/4 mile ET (at 1/2 power level) 10.8 seconds
top speed est. with ratio changed to 3:50, 135 mph
345 lbs. of these Kokam cells are 59% lighter than the out-going lead acid pack that was comprised of 60, 14.2 lb. Hawker 12V batteries that weighed 852 lbs., but considering module packaging with their built-in copper interconnects, cabling and hold-downs for the 12 modules, as stated, the Kokam pack as installed in the car should be 450-460 lbs. The lead acid pack had 852 lbs. of batteries, but with the copper interconnects, long cabling between the rear seat and trunk areas, the twin string bridging contactors, compartments and hold-downs, the actual lead acid assembled and installed pack weight was 906 lbs. On the surface, it seems the car will lose 450 lbs. or so, but other structural mods to the car adds back some weight. You’ll have to tune in to pt. 4 to get that and other interesting info on how things are progressing.
See Ya…John Wayland