I do not see where anyone else has reported on this so I will jump in with my experience. Needed to replace the battery in my '05 Elise. I saw that Battery Tender was offering a new line of Li batteries. I got model #BTL35A480C with 480 CCA for $249 from Stored Energy Products (SEPBatteries.com) through Amazon. It weighs something like 5 lbs. I called Battery Tender tech support and asked if I could use my Battery Tender Plus to charge the battery and they said no. I had to buy a Battery Tender Junior for $19. They said that they are coming out with a specific Li charger later in the spring.
Installation was perplexing me until I realized that the battery's motorcycle type terminals would connect directly to the cables from the Elise after I removed the large clamps that connect to battery posts (Duh). Used a large cable tie to secure it although I need to do something more secure.
Left it on the Jr. charger till it turned green and then took the car our for some errands. It was low 20's F here in Jersey and it started strongly every time. When I got home I plugged it back into the Jr. and the light was red which means that the car was not keeping it fully charged. These were 3 short 20' trips so I do not know how well the charging system of the Lotus will charge the battery. I guess as long as it will not destroy the battery I will be happy to take it home and throw it back on the charger.
I will see how it goes when this crappy winter is over and the car gets more use.