I've dealt with this subject extensively and here are a few things I know firsthand.
The battery left alone loses somewhere around 1-2% of its charge every day. After 10 days, the battery lost somewhere between 10% - 20% of its charge. If you have any alarm installed, this also drains the battery. For example, a famous Scorpio alarm draws at least 5ma (0.005A) without perimeter sensor activated, and about 8ma with perimeter sensor activated. To put it in perspective, 0.005A * 24 hours * 10 days = 1.2Ah, so for a 10Ah battery that's an additional 12% loss.
Now, when the battery is being charged by the bike's charging system, the charging current changes, but it could be anywhere from 0.2A - 1.0A. Let's assume the average of approximately 0.5A.
If you didn't ride the bike for a week, and depending on the presence of the alarm, you lost somewhere between 0.5Ah - 1.5Ah. In order to recover that loss (even assuming 100% efficiency of the charging process, and assuming the 0.5Ah average charging) you would need to ride somewhere between 1 hour - 1.5 hours. If you ride less than that (which is often the case, especially during cold season), you only replenished some of the loss. So, week after week your batter gets weaker and weaker, and many riders mistakenly blame it on the battery and go out an buy a new one.
Now that it's clear, it's not your battery, there are a few ways to go about it. First of all, if you don't know the actual state of your battery, it's like guessing in the dark. I recommend to everyone to buy
this V-meter from RadioShack
and install it on your bike and connect directly to the battery. It draws less than 1mA and therefore can be connected to the battery all the time. I used my digital V-meter and calibrated it carefully so I know the exact marks for 12.6, 12.0, etc.
I observe it all the time, how the battery Voltage goes down starting from 12.6, and a day later 12.5, and by the end of the week 12.3. After a short ride it goes back to 12.5, and then continues the slide so a week later it's 12.1, and so on, and so forth.
Back to the main subject. Either take your bike home after the ride and charge it for a few hours with a simplest AC adaptor style charger to make sure the battery is 100% full when you put the bike back in storage.
I had a spare battery (which I though was bad for the same reasons I explained - LOL). So, to provide some light in the shed outside, I permanently installed it there together with one of those 12V emergency lights. I connected battery with one of those cheapo solar chargers, and set up the solar panel at the optimal angle. I've had it for months and the cheapo solar panel does the trick. The battery is always 100% charged. The few seconds a week I occasionally use the light in the shed is not important - too short time to put a drain on the battery. What I also determined by measuring the voltage and current of this solar panel is that it only produces meaningful current to charge the battery (more than 5 mA) when the entire surface of the panel is under direct sunlight (maximum I observed was 30+mA). So far over a period of a few months, the battery is 100% charged most of the time. Another words, a couple of hours of direct sunlight and not even every day is enough to maintain the battery level.
This would be your second option.
