BMW Motorrad charger

I've bought the BMW Motorrad charger for my 2018 1200GS while I'm not using it on salty roads. It's plugged directly into the aux socket and seemed to be doing the job. However, I've noticed sometimes when I go into the garage that it is charging and it's giving me a really low reading on the battery charge status. I've seen it down at 38% once. I thought it would fully charge the battery and then just give it small top ups every so often.

Does it sound like my charger is somehow abnormally discharging my battery?

Cheers

Normal mate, charger conditions battery as it goes, also only puts in what it has too

Actually had the same on my 2017 GS. I bought the BMW charger with the bike and use it via the dash socket. Left the bike over winter 2017-18 and regularly checked to see if it was charging. Regretfully when it came to seeing if the bike would start after the winter the battery was dead. Maybe I had a duff battery, so I bought a new one. Now I check it more regularly and it still seems to be doing the same. Only thing I do is unplug and replug it, then it seems to spring into life and starts charging the battery. Again I have no clue how these things work but time will tell. If in the spring my battery is dead again then the charger is going in the bin.
Ah, yes, my bike is kept in a centrally heated garage so it is never left outside or gets cold.