Re: don't use rechargeable AAA on bike light?
The only devices I've encountered which warn you not to use 1.2V rechargables instead of 1.5V primaries are ones with no current regulation in the circuit at all.
For example, very cheap LED lights where the LED is directly connected across the battery, and the internal resistance of the battery itself provides current limiting.
Also, those micro radio controlled cars costing £15 or so that were all the rage a few years back. The car (about an inch long) had a minature rechargable battery in it. You charge it by plugging it into the control, which runs off three AA batteries. Presumably it just directly connects this little RC across the AA's, and the charge cycle (which takes only a minute or two) is current limited by the internal resistance of the AA cells.
The problem is NiMH batteries have much lower internal resistance even than Alkaline cells, and NiCads are capable of more current still.
I doubt the voltage is an issue, in fact rechargables and non-rechargables are a lot closer than you think.
Brand new, fresh Alkaline batteries read 1.55V per cell under no load.
Hook them up to a Halogen bulb (the sort that flattens them after 4 to 5 hours operation, typical on a bike light) and they immediately sag to not much over 1.4 for the first 30 minutes of runtime.
For the remainder of the lifespan it's more like 1.2V, and they are considered dead at 0.9V to 1.1V depending on the application.
NiMH rechargables are more consistent.
They start at 1.4 something when fully charged, and sag much less under load because of the lower internal resistance. They maintain a voltage over 1.2 for 80% of their life.
For this reason, most flashlight bulbs are designed for 1.2 V per cell. Eg. a 4 battery bike light typically runs a 4.8 Volt bulb, even though it was not supplied with rechargable batteries or a charger.
So anyway, i'm not convinced this Cateye light is quite as cool as the guy in the shop reckons it is. In fact, a bike light that cannot use rechargables does not sound fit for purpose. Like many i suppose - back in the 90s i ended up building a couple of my own custom lighting systems.