Hi,
I have the latest 12speed 105 di2.
It's less than a year old and battery states it was manufactured early 2025.
I had been happily using the bike since summer (june / july time) with no problems, only recharging once every 2 months or before a large event.

It has since been put in the garage for two months with only a few uses each month, however the derailleur shows green for 5 or 6 days after charging then suddenly you get no battery light or shifting, I have tried re-charging several times, each time after a few days it is suddenly dead! The garage is fairly well insulated and is not much below room temperature all year around.
I've taken out the battery from the seatpost and checked all connections but there doesn't seem an obvious issue.
Is it the battery? the derailleur? or something else all together?
Any ideas or assistance would be greatly appreciated.

Batteries go bad. Take you receipt and the battery to a bike shop and have them check it and then arrange for warranty.

Are both the shifters not working - as in, neither derailer works ?. That would indicate bad main battery, in which case Kontact has this, sounds like a bad main battery. Either pull it or brig the bike to a shop, they can use the shop lever diagnostics software to look at the battery. Should be a warranty replacement at this early stage.

Or just Amazon a replacement to have as backup, then deal with the bad battery whenever. I keep a 2nd battery as just in case,

Di2 historically has not been happy when charged from a 1A (amp) charger.

If you are using a freebee charger that came with an iPhone or similar, try a charger rated above 1A.

Barry

I had an experience where my battery would go flat soon after charging. After trying several things I switched the charger and the problem went away. The one I was using had worked fine for over a year so it must have gone bad somehow.

I'd try swapping chargers before spending any money.