ABSL Space Products (formerly AEA Technology) was responsible for producing the batteries for both the spacecraft and the lander. The project was a significant challenge as a result of the battery’s performance requirements, namely irregular charging and discharging cycles and an extended period of hybernation as well as exposure to radiation and severe cold. Ultimately a lithium-ion battery was produced instead of the originally envisioned nickel-cadmium battery, yielding significant savings in volume and mass.
Lithium ion battery pack for Rosetta. From Pearson et al. 2002, Proceedings of the Sixth European Space Power Conference http://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1700&context=smallsat