No problem, the bike is designed to overcharge.
The rectifier doesn't adjust voltage to suit the condition of the battery.
get a voltmeter and measure the voltage after a ride before you shut it off.
Mine charges up to 13.3vdc, higher would be good for a Li ion battery.

Thanks, fellas!

Turns out the guy at my local parts shop was wrong about fitment for that Yuasa YTX14L-BS. I went there with my old battery and did a side-by-side, and the one he pulled was smaller, so I ordered a Big Crank ETXL15 per the recommendations on the forum - it's half the price of the other one anyways.

So, Verslagen, when you say "no problem" - does that mean that I can still keep using my stock battery that was bone dry, or am I misunderstanding you? I did go out and buy some distilled water just-in-case, and if I could use the old one for a pinch - until the Big Crank AGM gets here, then that would be very helpful - but I don't want to cause any unintentional explosions (occasional backfires withstanding).

I will do the post-ride voltage check, thanks.

When I checked yesterday, it was 12.69V with ignition key off, 12.49 with key in on position, and then as the bike was starting the meter showed a lot of crazy numbers, but I did see 7V flash up there for a bit and I think it went lower. When I throttled, I saw numbers in the 13.5-14 range.

Thanks again, sirs!

I wouldn't use the battery even if it charges up as it will probably appear to be fine and then suddenly die, at a time that is bound to be the most annoying. The S40 is battery is such a PITA to check that it should have an AGM installed as stock, especially these days when they aren't that more expensive anyway.

You will be happy with the Big Crank. Quality product.

You may have to trim or flatten the end of the cable ends to lay on the battery termilal posts. Common with those AGM style batteries, so expect it and don’t think you got the wrong one. You will understand when you see it. Simple.