For the people who say "ChromeOS doesn't see my new battery!"... make sure the connector is correctly attached. Installing mine, there was no subtle positive click in place, and Chrome wouldn't say anything about the battery. (I was feeling antsy about the connector and didn't screw the back down.) Sure enough, I removed the back again and inspected the connector very closely. Mine was upside down. When properly oriented, you can actually see more copper on the top than the bottom where it mates with the socket on the motherboard. And there will be two very subtle clicks as you push it into place; the second click is almose imperceptible.

On ours a T5 was needed. Maybe some machines were built with T4s and some with T5s.