Dead battery - 8/2018 bought. Positive cell totally dried. Looks like tender has nothing to do with it. Water levels must be checked once a year. Problem is - it so hard to reach.
Update: take battery to the store, they recharged it, put to test, said it is in great condition. Bike starts - no problem, but still a mystery why it dries up quickly and only one cell totally. Had to check it at least once a year.
Electrolite evaporates from battery fast. Looks like battery too close to hot engine. Will buy sealed one next time. There is not a drain, but some little opening in the battery for gases to come, maybe plug it with a cotton?