Cyberface, Glad to see you're getting some useful information out of that thread
(it's replicated on here too).
I did this conversion in my S1 Elise when I had it after my standard battery packed up, as I said I'll be doing this conversion on the Exige.
I got my battery from "Battery Town" here in NZ which is a car and industrial battery supplier over here. Their website is www.batterytown.co.nz
and the battery I specifically went for was a SLA-AGM (Sealed Lead-Acid - Acidic Glass Mat) battery, they supply the "Synergy" brand, but I know big international companies like Yuasa also do similar products. On this page www.batterytown.co.nz/sla_agm.htm
, the one I went for was the A12-18 (12 volt, 18 amp-hours battery). This is a non-vented, sealed lead-acid battery where the acid is saturated into a glassfibre matting inside the battery.
As you can see the battery is 4.9kg compared with the standard Elise battery which I think is just over 12.
This battery is the sort they use to run computer UPS modules and the "S" (cyclic) version is used in golf carts and mobility scooters (as part of a bank usually). I calculated 18 amp-hours as being what's needed to sensibly run an Elise that doesn't have a lot of electrical gizzmos.
Bear in mind that to meet type approval, a manufacturer must factory-fit a battery that will start a car both in the desert heat and in wildly subzero temperatures after a period of time. Given that my Lotuses are (as you might have guessed) rather prized and pampered, they will never realistically need to perform in these conditions. You might want to move up a size or two if you've got more goodies on board (satnav/ top-end ICE, leccy windows, aircon) but for an S1 with a basic-ish (non-amped) stereo and the Cobra alarm it seemed to do the trick in the year I had it installed and continues to do fine duty for the new owner a further year down the track. As I'm aware it's done many hundreds of cranks and never once failed to start.... when the new owner (who was also the original owner of the car) got the car back, he commented that in the 6 years he originally owned it, he went through 2 new batteries, so it would seem that the real performance of the A12-18 isn't so bad. Add to that that when you DO have to replace it, it's a doddle as you can get the battery out without non-surgically removing your fingers!
Bear in mind that after a week with the alarm set, the engine becomes slow to crank and after about 2 weeks you'd be lucky to start it (with the A12-18) so I wired in a harness to quick-connect an intelligent, cyclic charger (similar to what you guys have over there as an accumate/optimate?). Whenever I knew the car wasn't going to be used for a week or more, I'd just plug it in and leave it connected and it was always as fresh as a daisy.
The only other thing I had to do was to fabricate up a new battery bracket to hold the smaller battery in place. I did this by laying up a fibreglass captive "cap" and bolting that to the OEM bracket, which made a tidy job of it.
Pics of the A12-18 battery in place below (in the front compartment of an S1, obviously):
In summary I'd say that if your Lotus is a weekend/summer toy, then go for it, it's a great mod that saves a shedload of weight and (at least in the S1) makes the battery MEGA easy to replace compared to OEM. I'd be cautious though if you use the car year-round as a daily driver and you need to start the thing out in the snow after sitting for a few days and rely on the car for transport. Certainly it's a mod that I found VERY worthwhile and as I said, I shall be doing it again early in the new year and it will be appearing in my "Rebuild" thread when I get around to it