JWST uses "ABSL™ 8s44p rechargeable Li-ion batteries", that's 8 series by 44 parallel (352 total) 18650 cells.
The particular Sony 18650 Hard Carbon (Mandrel) or HCM cell has a "nameplate capacity" of 1.5 Ah (5.4 Wh) . Each 8s44p battery unit has a "nameplate capacity" of 66 Ah (1900 Wh).
It's unclear how many of these battery units are on JWST. ABSL parent company Enersys says this in a press release on the launch on JWST:
EnerSys was selected by Northrop Grumman in 2012 to provide ABSL™ 8s44p rechargeable Li-ion batteries with disconnect relays for Webb, and then awarded a second contract in 2018 for an additional 8s44p battery, tailored to incorporate alternate cell chemistry.
While a 2020 JWST (draft) update  says this about batteries:
FLIGHT BATTERY CHANGE
Results from recent contingency analyses indicate the amount
of time to work contingencies in the event of post-launch solar
array deployment (t +30 min) anomaly has fallen under 3 hours
with the current ~53 AH battery (HCM Li Ion).
Fortunately, we procured a spare (Moli-M Li Ion) battery that was of
considerably larger capacity ~106 AH – two years ago and it is ready
to go. This more than double capacity will provide more that sufficient
time to complete all possible contingency operations to recover the
For these reasons, the project has decided to switch to the space
battery. GSFC Engineering Directorate and the Standing Review Board
have been briefed on this decision.
It should be noted that 53 Ah is almost suspiciously 80% of 66 Ah, which is not an unreasonable "depth-of-discharge (DOD)" limit in a contingency scenario.
These sources seem to be talking about the same thing (timelines match), though they are conflicting on implied battery quantities on JWST.
- Pearson et al. "The Use of Small Cell Lithium-Ion Batteries for Small Satellite Applications," (2004)
- Troutman, J. "ABSL Performance Comparison," (2011) (slideshow)
- Smith, E. "JWST Update (DRAFT)," (2020) (slideshow)