In my post about the chassis-stiffening strut tower brace, I mentioned I would cover how the battery pack design too, further strengthens the chassis.
Warning: lengthy content, broken into parts.
As with any EV conversion, as battery capacity grows, so does the difficulty in design - such was the case here. Back in 2011 before there was a Tesla Model S, the Tesla Roadster’s acceleration performance and 244 miles range had already set the bar, but I knew I could convert my Insight to do at least 350, maybe even 400. It would have been fun to be the first 400 mile road legal EV back then, but life’s curve balls and setbacks altered the course, and it’s taken a shameful 12 years to finish my EV2. Its 400-450 mile range (under ideal conditions) isn’t anything special today, with several high end luxury type 4-5 place seating factory EVs making it look easy… oh well, better late than never.
Part (1) The Front Module:
It’s pretty obvious that adding ~ 1100 lbs. of battery pack (988 lbs. of cells, aluminum modules, buss interconnects and cabling) to a sub-1900 lb. car without proper design might stress out its chassis. We had to figure out how to fit it all in without compromising the interior functionality, while achieving good weight distribution and a low center of gravity (CG) to preserve or even improve handling. Unlike purpose built factory EVs whose hollow flat floor section lowers over a single flat ‘skateboard style’ battery pack, conversions do not have the luxury of one single area of real-estate, where all the cells can be kept together in one enclosure. Conversions with more than say, 25 kWh capacity, usually have two modules apart from each other - typically one under the hood and the other either in the trunk or in a better fashion, dropped through the trunk floor between the frame rails. I’m my Insight’s case with such a large capacity pack (71.6 kWh), there are three enclosures (modules) to find room for. Additionally, we felt each module had to contribute to the structural integrity of the chassis, not subtract from it. Safety in the event of an accident was also addressed. Cell temperature needed to be controlled to keep all cells as close to the same temperature as possible, especially considering their different locations with different ambient temperatures. Lastly, as in all my conversions over the years, although it would have been easier to position the front battery module differently, the electric motor could not be obscured - especially an ultra-rare genuine EV1 motor! It had to be in clear view!
Working again with Marko, we studied every aspect of the car, learning that for such a light vehicle the Insight’s chassis is remarkably strong! Forget about its plastic fenders and rear skirts, the thin stamped interior panels and those styrofoam pieces - all part of Honda’s impressive efforts to reduce weight, because when it comes to the body frame parts, Honda used very thick beefy castings, extrusions and weldments in unique ways that make the chassis very stout! That said, we found that the Insight lacks the lower transverse rectangular tube frame rail that is part of the radiator core support typically found in most other cars that is part of a body-on-frame type, or completes the frame portion of a unibody car. In fact, up front there aren’t even the two left and right lower side rails, either. Instead, it has robust aluminum castings that form a triangle at each side of the motor bay, much like steel brackets that mount to a wall and extend outward perpendicular at a right angle and go under a closet or wall shelf to support it. Two more strong castings are midway height-wise and extending forward for more strength and to accommodate bumper attachment.
With that unique structural design, the front battery module could not sit on top of side frame rails or atop a terminating transverse radiator core support frame rail, because there are none. We decided to ‘hang’ (bolted-to) the front module off the triangle supports and also bolt it to the vertical radiator core support sheet metal. With 126 lbs. less engine/motor/transaxle weight being supported by the triangle supports, the 275 lb. front module actually only presents 149 lbs. extra weight loaded to the supports. Again, they’re quite beefy and could handle much more without issue. The addition of the strut tower brace making its own triangle tie-in to the rest of the body further strengthens things.
The front module, though considered as one of three battery modules, is actually a two piece affair, with 57 of the large format cells in the main rectangular box, and in what I call the ‘piggyback pack’ - a smaller box atop the main one housing, containing another 9 cells… it was the only way to fit that many cells under the hood!
The entire 318.2V, 225 Ah battery is a 3P86S, meaning 3 cells paralleled, then 86 of those in series. The front module is a 3P22S, with the main box at 3P19S (57 cells) and its top portion at 3P3S (9 cells). The front combination module’s location achieves nearly every design parameter, with the exception that it is vulnerable in the event of a substantial impact at the front of the car, though there is a degree of crush zone. As such, the main enclosure (box) is a bit larger than it had to be, with the interior lined with thin but strong electrically insulating Lexan polycarbonate. It has that extra space to accommodate interior dense foam sheets that surrounds the cells - serving the dual purpose of an impact absorbing barrier and as thermal insulation. The military grade Kokam cells have very low internal resistance, and so do not heat up even at the highest current levels, while they also retain most of their capacity even at colder temps, so the insulation isn’t necessary in that regard, but it does play a role in cell temperature equalization, as previously mentioned.
As aluminum has terrific heat-sinking qualities, the box itself serves to surround the cells at the same temperature. Because it’s up front in the airstream, if it’s cold outside while driving, the box will be cooled, and if it’s warm either from ambient or exhausted warm air from the motor cooling radiator, the AC radiator or both, then the box will be warmed. Knowing that, I designed a closed loop air circulation system that while being driven or under charge, constantly shares air between all modules. If the front module is getting externally cooled down, such as wintertime driving, that colder inside air the box is mixed with the warmer inside air of the mid-modules (boxes) as they are then being brought down in temperature, while their still warmer-than-the-front air is circulated up front to bring the temperature up. It effectively keeps all the cells very close in temperature. During sub 40 degree outside temperatures (if left outdoors) while charging, the BMS turns on a 600W PTC heating element to warm the cells.
The main rectangular-shaped aluminum ‘box’ has solid corner seem welds, a right angle stiffener, beefy support brackets and a thick aluminum lid with a high count of perimeter screws. It’s mounted low that improves CG and its rigid construction bolted at four points ties body parts together making things even more robust.
To make it fit into the tight squeeze, the thin vertical stamped aluminum piece that bridges the transverse top radiator support/hood latch to the light gauge beam down low had to be cut away, but even this mod ended up making things stronger. Now, a stubby section of that piece bolts to the battery box lid’s thick gauge aluminum and right angle bends, noticeably firming up the hood latch and radiator support!
The completed front module’s main section
with other components mounted pretty much disappears, with just the 9-cell portion in clear view: