Confirming my suspicion from March 8. Battery arrived today from China (Covid delay) and worked out of the box without modification in my ARC-6120-T121. us$12 after shipping.
Digging up this zombie thread because it's the first result on Google. After significant research, the battery we are all looking for is a common 10450 battery with a JST PH 3p 2.0 plug (that's a 3pin with 2mm pin separation (aka pitch)). They are almost always available on Ebay for about US$9...
I have a 1214-4i with 4 (3TB) drives in a Raid 1+0, single volume configuration. The RaidSet Hierarchy shows a capacity of 4.5TB. Shouldn't it be closer to 6TB?