I'm not that educated about it either, but I might have some answers. I mostly looked into using lithium batteries because my hobby is solar power. I currently have enough lead-acid batteries and solar panels to run my washing machine twice a week off solar, which is putting a serious dent in my electric bill. And the setup I have didn't cost that much! But I had to figure out basic electronics as I went along. Now I'm trying to get some engineers to walk me through putting my product together and getting it on the market:
http://wat.asiaeast.org
Lead-acid batteries work really well because they've been around forever and everyone has most of the problems worked out. But like you know, they are heavy and take up room and have other problems. It hasn't happened yet, but I'm pretty sure everyone will be using lithium batteries some day in the future, just like you are suggesting. Imagine plugging something the size of the backup battery on your laptop into your motorcycle and then recharging it again overnight. That approach might work, but I don't think you can recharge a lithium battery from the motorcycle engine directly.
The problem with lithium on a motorcycle is this:
Lithium batteries charge in two stages. First, a microprocesor applies a predetermined current and then raise the voltage until the maximum voltage required is reached. At this stage, if you disconnect your battery, it will appear to be full. But in fact, the battery inside really isn’t.
During the second stage, current continues to flow into the lithium battery until it drops to 0. This usually means there’s no more room in your battery for electricity. Microprocessors for lithium batteries are programmed to assume you are going to leave the battery charging until it is full.
But, if you are recharging the battery on a motorcycle, the flow of current could stop simply because you switched the engine off. This would lead the microprocessor to think that the battery was full, when in fact it wasn't. Because it tracks where the battery is at in the charging stage, if you unplug it before you finish charging, the chip can lose track of where in the process you were.
At this point, you will need to reestablish where in the charging process you were. That's why I suggest unplugging the battery from the bike every time you are done and recharging it from the wall. This will reset the parameters inside the microprocessor again. After that, things should remain stable.
With my ipod, for example, I plug it into a solar panel. When the sun goes behind a cloud, the flow of energy drops to 0 and the ipod thinks the battery is full. It will even say "Charged" on the display. When I unplug it from the solar panel, the lithium battery is still empty! That's why I have to recharge my lithium battery from lead-acid batteries or recharge on a day when there's no clouds in the sky.
So on your motorcycle, unless you can reprogram the chip somehow, everytime the battery looks low, the alternator will start to pump out juice, right? But if you turned the engine off while it was recharging, the microprocessor would think the battery was full. Then, when you turned it on again, the microprocessor would never send out the single to the alternator to start pumping again. Eventually you'd have a dead battery.
Like I said, I'm not educated about this. However, my experiments with lithium tells me I'm pretty close to correct.
Probably best to recharge the lithium independently of the engine. Also, can you run a lithium battery for lights and radio, etc. and a smaller lead-acid battery for starting?
