Weird behavior of the replacement battery

I replaced the battery in my Apple Watch series 2 with one from iFixit. I'm getting a battery life that is all over the places, from 2 to 10 hours randomly. A typical scenario: I charged the watch overnight, but the battery level stuck at 17%. I rebooted the watch in the morning, and the battery life is now showing 66%. Left the watch on the charger, the battery level dropped to 49% after a while! :-( It further dropped to 18% afterwards. Rebooting the watch again while leaving it on the charger, the battery level is now 97%. Taking it off the charger, the battery level drops quickly, and less than an hour later it's at 1%. But it can last about an hour or so at 1%.

This is driving me nuts. I don't really know what battery life I'm going to get with a given charge. It's the second battery from iFixit now. Both have the similar issue. Has anyone seen such a problem? Anyone got a replacement battery working properly?

Thanks,

Joshua