Thanks for all the discussion.
There's absolutely no point in trying to fool anyone by asking for a GVM that does not include the mass of all the things/people that might sometimes be in the car at the same time. If you have an accident and you're so much as one gram over the GVM that you've applied for, been tested for, and had stamped on your mod plate, then you can kiss your insurance good-bye, and maybe a fine or your license as well.
Folks talking about taking out power steering, or unnecessary wires in the loom, seem to be missing the fact that the problem is excessive weight in the rear
. The front is OK. But replacing the lead-acid accessory battery in the boot, with lithium, is definitely of interest now.
And yes, we're aware that battery chemistry has advanced in the 3+ years we've been working on our BMS (oh yes, and its test platform
). But it hasn't advanced as much as I thought it would. Basically we can have 6C now for about the same price we could have 4C when we started, but they don't seem to have got any lighter or more compact or less expensive for the same amp-hours.
Actually, we could have had 6C for about the same price back then, but it wasn't from a company we trusted, and we would have had to take a 25% capacity hit.
Some reasons we bought the cells only 6 months into the project:
1. We had no idea it was going to take us more than 1 year.
2. It gets very tedious making fake cells and trying to use them to figure out how many cells we can fit where. We needed 4 separate battery boxes just to make use of the fuel tank space -- the prime real-estate nearest the center-of-gravity of the MX-5.
3. To hedge against unfavourable exchange rates.
Remove the highest, most front / rear overhang cells.
Will do. At least in the rear, where the problem is.
Even if you can get present axle weight and GVM accepted, the performance and handling will suffer with the present vehicle weight.
You were chasing sporty performance.... and we needed you to get there !
I agree that handling suffers for every kg increase over the original kerb weight. But straight-line performance improves
with every additional cell we can shoe-horn in (assuming the same cell life). Halving the number of cells will halve the power, but will only decrease the mass by about 12%
You will get good range with the present battery capacity however it is getting back to the old lead sled EV days in weight.
Yes, but with a lot more power.
Edit: and don't be affraid to 10C the SE cells. If they last 12 months and you have fun then at least you have some spares while you work on the next battery pack. After all, if they last 10 years so what ? I think you will move on to other cells long before that.
I'm afraid I can't come at that idea. It just seems too wasteful. Yes, we'll move on to other cells, but with a later model MX-5 (or a completely different vehicle). There's no reason why someone shouldn't enjoy at least
another 4 years out of these SE cells while driving this
Yes, the under-boot battery box is out of the car, but I haven't weighed it yet. The scales are still (usefully) under the front wheels and I have to do other things for the next couple of days (paying work -- eek!). I haven't touched any other boxes.
Jeff Owen, who prefers to phone rather than post, has suggested we remove 26 cells from the under-boot box and change the connection to the frontmost box of 13 so it becomes part of the "B" (blue) half-pack. The effect is to remove 13 cells from each half-pack while getting more weight off the rear axle than would happen with Mark's and my earlier suggestion to remove 14 from the under-boot box and 14 from the rollbar box.
Jeff suggests removing the rear row of 14 completely (already done) and removing 3 from each end of the other two rows (leaving 17 in each of the two rows). The battery box could then be correspondingly reduced in size to save more weight and eliminate the tight squeeze we have at the sides.
I like this suggestion and propose to remove the 26 cells from the rear box and reinstall it, re-weigh the two axles, and propose the result to the engineer.