We had a unusually warm and dry day recently, so I decided to give the K1100LT some exercise (and also give myself a mental health break).  Rolled the ol' girl out into the sunlight, put on my gear and then found the bike wouldn't start.  The battery was flat.  I generally keep it on a C-tech charger.  But, it had sat without that for only 2-3 weeks.  The MotoBat is quite new - less than one year old, IIRC.  Did I buy a defective battery? How much current could a digital clock draw?!
I hate electrical mysteries! 

Uncle Karl has explains battery failure and testing here, and covers more battery subjects  here. Charge the battery, breakout the multimeter and have fun!

Fast test: charge the battery until it's full (charger reading between 13.8 volts and 14.2 volts - or the green light comes on ), then leave it overnight. Put a multimeter on it the following day. Anything below about 12.8 volts indicates a probably fooked battery. I have three MotoBatts here, one of which is over a decade old and still in use with no problems.

brickrider2 wrote:We had a unusually warm and dry day recently, so I decided to give the K1100LT some exercise (and also give myself a mental health break).  Rolled the ol' girl out into the sunlight, put on my gear and then found the bike wouldn't start.  The battery was flat.  I generally keep it on a C-tech charger.  But, it had sat without that for only 2-3 weeks.  The MotoBat is quite new - less than one year old, IIRC.  Did I buy a defective battery? How much current could a digital clock draw?!
I hate electrical mysteries! 

The clock will draw 1-2mA typically. that should not deplete your battery in 2-3 weeks.

What's going on is that a part in the electrical circuit has gone bad. I helped a guy debug this same problem on a late model K100 once and it ended up being a defective FI relay on that bike. Replacing the relay fixed the problem in his particular case.

Head on down to the ChiCom hardware store and get one of these circuit ammeters. You plug it into the fuse block on the side of the relay box and it lets you see how much current that circuit is drawing. It's a good way to find parasitic current draw as well as finding out why a fuse may be blowing.

In your case, it will point you toward the circuit that is killing your battery which is at least 50% of the troubleshooting effort.

https://www.harborfreight.com/automotive/battery-tools-accessories/testers-tools-accessories/30-amp-automotive-fuse-circuit-tester-67724.html

Another quick check. With ignition off, remove fuse 3 (power to the clock) Remove the battery negative lead from the battery, and place your multimeter between them, set to 20Volts DC. You should read close to 0 Volts ( I got 0.02V) If you read about 12V, you have a circuit draining the battery. If removing fuse 1 doesn't drop it to 0V, replace it and try the other fuses until you get to 0V, then look at the circuits related to that fuse. Finally, replace fuse 3.

Great suggestions, all!  They are just what I need and had hoped for.  Thanks!  

If you have ABS leaking transistors can discharge the battery I have had both the K1100 and K75 do it. It showed up as an internal ABS fault code 7 and was very difficult to reset. I eventually sent both the units to Tosi Tabuki and both are well again.

I think I've solved the mystery  -- pretty simple really.  It appears I bought a dodgy battery.
I took Point Seven Five's suggestion to buy the HF tester.  Nice tool!  I failed to find any parasitic current draw.  Over time, I now see that the nearly new battery rapidly loses it's charge when taken off "life support." The battery is from a well-respected name.  However, I've come to understand that buying a battery is something of a roll of the dice. "You get what you pay for" sounds nice, but isn't necessarily true.  Unfortunately, I've verified that several times. 
What now?  It's cold and getting colder here, but if history can inform me at all it suggests that there will be the odd day during winter when a "mental health ride" may be possible. Here in the valley, we enjoy pretty fair weather.  It's possible to ride if one avoids higher elevations.  My short term solution is to pull out an old battery from the workshop that I'd changed out due to age and install it in the big red K1100LT.  It's a veteran of nearly six years of age, but a load test says it's still serviceable.  (Actually, it's rated at an impressive 400 CCAs, so a bit of degradation won't be fatal.) 
I'll deal with rolling the dice on a new battery in the New Year.  

I believe it's best to buy batteries from a shop that has a high turnover, some places have batteries on sale their shelf for yonks.

daveyson wrote:I believe it's best to buy batteries from a shop that has a high turnover, some places have batteries on sale their shelf for yonks.

When we get a new supply of batteries into the shop we stamp the date received on the box. After a spell we charge, using a low amp 'tender type' of charger, a select amount of them based upon a quirky mix of seat o' the pants, actual use/sales history and anticipated sales in-season. The first-charged-by-us-date gets stamped on the newly charged or freshly charged battery. You can see when it arrived here and when we charged it first. What the supplier and manufacturer did to maintain it on their storage shelves prior to us receiving it is the gamble you take. To back that up, BMWNA offer a two year/unlimited mileage warranty whether we install it or if you buy it over the counter and take it home yourownbadself. Just save your receipt. We will charge and load test a suspect battery. What the Midtronics brand load tester spits out is what BMW says is proof enough and they pay the warranty claim. You pay more for a BMW battery from a dealership, no doubt, and you get the requisite warranty coverage.

We tend to warranty a bucket load more of sad, tired batteries come the Springtime riding season, simply because folks haven't bothered to keep 'em up, and at that time BMW is a bit more dubious about paying any old claim, and require us to show charge figures and load test figures.

Sorry to hear about your battery.  Is there any warranty on it?  Did you purchase it locally?  Is there a sticker on the side(usually the end) with some numbers or a letter and number separated by a slash?  Those are usually manufacture date stickers.  Can't speak to battery retailers, but the boatyard I work in has a dealer who goes by those stickers if a warranty claim is made without a receipt. Most batteries are sold within a month or three of when they were made.  It's also a good idea to put the date you bought it on the case with a Sharpie pen and stick the receipt for the battery in the tail cowl or the relay box.  Either place will keep it dry and readable for as long as the warranty is in effect.

The failure of a one year old battery is usually evidence of a bad charging system.  

Most alternators and electrical systems are designed to recharge the battery for the current used in starting in about the time it takes to warm up the engine, usually less.  From there, the alternator will take care of all the loads on the bike with power to spare, even at idle.  

If the system is working properly, the battery will always be fully charged as long as the engine starts readily and there are no major power loads that have been added.  A fully charged battery on a bike with just the clock running should hold it's charge for at least a couple months, and then fully recharge in about 10 minutes of idling.  

On the other hand, if the bike is shut down with a partially charged battery, there are chemical processes in that battery that will destroy the plates in it, reducing the amount of charge it can store, and the amount of time it will hold that charge.  The plates in a partially charged battery will develop coatings that reduce not only the charge it will hold but will discharge the battery when the bike is not being used.

Bottom line, check the charging system.  It might be doing the job when you are using the bike often in warm weather, but not keeping the state of charge high enough to prevent damage that will kill it over the winter.  I know I have a voltage regulator that needs to be changed on one of my bikes this winter.

Okay, I had time to dig into the K1100LT's battery issue; here's what I found.  First, the installed battery is not as young as I had imagined.  I was able to see the date I installed it:  3.17.2019. A dealer offering a two-year warranty would find the proration on this one painless.  Second, when I got under the seat I realized the battery I was to pull from the shelf and install is from a BMW airhead -- much too large for the K-bike.  Third, I don't expect to find fault with the charging system, but I'll check it out later.  I have a volt meter on the bike and, when underway, there is consistently at least 14.2 V. available to keep the battery happy. I didn't find any loose connections or anything otherwise obviously amiss.  
I'll now stop grousing and buy a new battery as my Christmas present. 

Okay, I had time to dig into the K1100LT's battery issue; here's what I found.  First, the installed battery is not as young as I had imagined.  I was able to see the date I installed it:  3.17.2019. A dealer offering a two-year warranty would find the proration on this one painless.  Second, when I got under the seat I realized the battery I was to pull from the shelf and install is from a BMW airhead -- much too large for the K-bike.  Third, I don't expect to find fault with the charging system, but I'll check it out later.  I have a volt meter on the bike and, when underway, there is consistently at least 14.2 V. available to keep the battery happy. I didn't find any loose connections or anything otherwise obviously amiss.  
I'll now stop grousing and buy a new battery as my Christmas present.