The Battery Pack is the main fuel source in all Tesla Motors cars. In Model S it is mounted beneath the floor of the cabin and forms a structural member; in Roadster it is mounted behind the seats and ahead of the rear axel.
Model S originally came in 3 battery pack variants: 40 kWh, 60 kWh and 85 kWh. Roadster was originally available with a 53 kWh battery, but in 2014 Roadster owners were contacted about plans to offer a 90 kWh battery as an upgrade.
The battery packs in both cars are comprised of thousands of rechargeable Lithium-ion cells officially named as 18650 or 168A and which resemble an AA battery in physical size.
There are Battery Management System (BMS) boards on each module that are wired to monitor each group (brick) as well as the module itself. There is a main BMS for the overall pack, mounted in the back of the pack near the contactors.
Approximately 25% (250lb) of the weight of the pack is in the frame/housing, with the remaining 75% being in the battery modules.
Details for the 60kWh pack are not as certain. One view is that it is made of 60 cells per group instead of 74. As with the 85 kWh pack, 6 groups are arranged to form a module, and 14 (not 16) modules make up the pack, for a total of 5040 cells.
Another view is that there are 15 modules, not 14. A further view is that there are 14 modules, but three of them have fewer cells. Lastly it is possible that there are in fact 74 cells per module like the larger pack.
Whatever the configuration, it results in a pack with a lower maximum voltage. In turn this means the battery can deliver a lower maximum power (than the 85), which explains why the S60 accelerates slower than the S85 despite weighing less.