Does anyone know if the K100LT battery can be charged through the cigarette lighter socket or the auxiliary plug socket on the upper dash?

Yes, you can and I’m doing that too.
You shoud NOT try to jumpstart through this plug.
The cables are NOT made for this!

Cheers

Yes you can.

Here's an inexpensive way to get the plug.  Buy one of these. You can unscrew the cover on the plug part and then wire it up to the battery charger:

https://www.aliexpress.com/item/DC12V-24V-EU-Plug-For-BMW-Hella-Motorcycle-Charger-Car-Cigarette-Lighter-Adapter/33012049614.html

Thanks for the info

@moenv wrote:Does anyone know if the K100LT battery can be charged through the cigarette lighter socket or the auxiliary plug socket on the upper dash?

Yes you can.  BMW sells a charger, Part number 71 60 7 688 865 charger that is intended to do this.  You plug it into the mains and plug the jack into the socket, and done.  The K-100 has no "smarts" controlling the jack so if you get the plug just about any charger will work, but this charger will communicate with the CAN bus controller on later machines, like my R-1200 RT and R-1200GS.

This link will take you to the plug:

https://www.advdesigns.com/stgepopl.html?gclid=CjwKCAjw5pPnBRBJEiwAULZKvslZkMcQcMYJKE7yxT-uzvqdzKtQ1gfPYgZP800kWRme8cMwqF6PMxoCNvwQAvD_BwE

BB

No need to get a BMW charger and our old Ks are not CANbus.  

Electrons don't care about brand names.

Battery Tender Junior is a great product.  I have several of them.

The only reservation I have with the Battery Tender Junior (new version will do lithium too) or other plug and leave chargers is the potential for overcharging if the microprocessor fails to go from charger to maintainer when a battery is left for months on the BTJ.

I've been using BTJs for well over a decade and never had any issues with them. (Except for the one that I left out in the rain that got really soaked but I consider that one to be my fault. DOH!)

The circuitry in them is seems to be very good.  DAMHIK but they even detect and avoid issues if you accidentally reverse the polarity.

Any decent charger will do, you just need to wire up the plug.