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2000 Honda Insight, AC Induction Electric
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Warning - verbose post!

As requested, here are some details about my electric Insight’s Tesla-sized 1100 lb. (978 lbs. raw cell weight) 71.6 kWh, 318.2V battery pack. I can hear groans already about adding that kind of weight to an 1887 lb. car :) A lot of thought has gone into this, though.

Cell info... The Insight/EV2 uses 258 large format 75 Ah Kokam pouch type cells, HP NMC. The ‘HP’ stands for high power - rated at 8C continuous and 15C < 10 seconds. In its 3P86S configuration the pack is capable of 1800 continuous amps, and a potent 3,375 amps for 10 seconds, if you want :) Of course, in this application these specs are the definition of over-kill. But, it also means that they have very low internal resistance, and at the currents the drive system draws they do not warm up. Even under the extremes of high current electric drag racing, the smaller flat 30Ah Kokam cells in White Zombie barely get warm with up to 1500 amps pulled from each cell! Unlike large flat cells, cylindrical cells by nature, heat up under discharge/charge because the plates are all rolled up, so they require cooling. The hybrid Insight’s cylindrical NiMH cells have forced air cooling, and in a clever move by Honda, the fan draws air conditioned cool cabin air. Tesla’s huge packs of cylindrical cells require extensive cooling, actually using refrigerated liquid to keep the thousands of cells from overheating. None of this is required with these large flat Kokam cells, though maintaining equal temperature between the cells as they are placed in different areas of the car, is, and in regard to charging during cold temperatures, battery heating is needed.

At 60 mph cruise, the drivetrain should only draw between 28-31 amps at a pack nominal 318 V, for about 9.8 kW (13 hp) average power. That’s just 10 amps drawn from each cell at 60 mph. At the full power level of 150 kW (available during the first 30% state of charge), only 150 amps is drawn from each cell - that’s just 1/4 of each cell’s 600 amp continuous rating, so these cells will be on ‘easy street’ powering this car!

All of the cell modules are insulated and temperature equalized via a closed loop air circulation system that takes advantage of the great temperature-wicking attributes of the aluminum and copper plates of these cells. It’s important to keep all the cells in a large battery pack as close to the same temperature as possible to keep them equalized.

Because this is a conversion, available space was assessed with considerations to weight balance, CG, safety during impact, and retaining the full utility of the car. This dictated that the pack needed to be divided into three main modules, as shown in the diagram.

Safety... One concession in regards to impact protection for the batteries, is the front module. It does have a degree of crush zone protection, but not much. The enclosure is quite sturdy though, with welded aluminum and beefy support brackets, and it has an underside armor plate, but it is vulnerable in the event of a heavy hit to the front. The module is also subjected to warm air exiting the radiators, while also being heated by summer hot temps or cooled off by cold winter temps. The lower gas tank area module is a bit more isolated from the ambient temps. The under/hatch floor module is not subjected to outside temps, and is warmed by cabin heat in the winter and cooled by AC in the summer. The air circulation system combats the varying influences from outside and cabin temperatures by constantly moving the air above each module’s flat aluminum cell tab clamps/buss bars and passing it through each module. The temperature sinking qualities of the aluminum and copper cell plates extends up to the top through their cell tabs and into the aluminum tab clamps. Cool air passing over these clamp pads will also cool the cell plates, and warm air

passing over these clamp pads will warm the plates. With the fan speed set to a low rate, it draws less power, is nearly inaudible and is quite effective at keeping the three modules at the same temperature.

Dealing with cold cells is another matter. Wintertime ambient temps require cell heating during charging. The air circulation system has a 600W ceramic heating element. When the BMS detects temperatures dropping below a threshold, the element is powered on and the air circulation fan is kicked-on to full speed.

Addressing structural integrity... Sturdy welded aluminum module enclosures don’t just contain lithium cells- they also contribute to increased chassis rigidity by bridging open areas. They are bolted into existing factory threaded bosses, as well as created ones. An example of this, is lower-mid module #2. Mounted down low into the midship located large hollow expanse where the non-structural plastic gas tank and other fuel related components were, it’s ridgid and reinforced shape ties the the perimeter body walls together, stiffening up that area. This 407 lb. enclosure contains 93 Kokam 75 Ah pouch cells in a 3P31S configuration. The floor of this enclosure is on the same plain as the stock (and added) areo panel’s, so a 1/4” thick wedge-shaped armor plate is mounted at it’s leading edge to protect against hitting road debris (think early Tesla Model S impacting a trailer hitch on the freeway). It is designed to actually lift the car over an obstacle if that ever occurs. It added 7 lbs. of weight, but it is good insurance. Both the under-hatch floor module (3P31S) and the low-mounted front module (3P22S) also contribute to structural strength as well. The under-hatch floor module is attached to the extruded aluminum beam that ties the sides of the car together just behind the seats, making it even stiffer, while heavy gauge welded lower module braces are bolted into structural areas, adding to body rigidity.

With consideration to the battery weight up front, today we test-fitted a 3 point strut tower brace we are making, that will greatly stiffen the front end. As can be painfully acknowledged, the otherwise beautiful construction of the Insight is marred by an under-hood unpainted aluminum square tube transverse ‘fork’ that suspends the 12V battery. It looks out of place with its crude welds and high school shop class presentation. That lead acid 12V battery is gone, replaced by a stronger and much lighter 40 Ah, 13.3V lithium battery, now located in the rear of the car. And so, that unsightly weldment was jettisoned. It has recently occurred to me though, that it ‘might have’ been more than a bad-looking battery support device, as it was securely bolted transversely across the engine bay between the firewall and the strut towers. The extra battery weight ahead of the front wheels is easily supported by the car’s robust heavy aluminum frame extrusions, but I was thinking that ‘maybe’ that ugly square tube affair might have been more than just a battery holding g device. And so, I decided additional structural stiffening was a good idea, in the form of a 3 point strut bar. Stay tuned for pictures of it when completed.

Hope some find this interesting :)
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