Originally Posted by JohnJ80

You know, these are specious arguments. If you use an all in one sort of light (i.e. L&M Taz 1200) and if you have the presence of mind (a low bar) to just put it on the charger when you are done, it's much simpler. You don't forget to put your bike away when done, right? I mean, this argument is valid for the sort of person that has difficulty in remembering to come in out of the rain and it's all solved by having an extra battery and having one on the charger.

In addition, you don't have any power loss in propulsion and, while I haven't checked, you don't have the added weight.

As for the arguments about failures of batteries, lights, etc... - those are no different than failures in the rest of the dynamo light or in the light itself. Given that the dynamo is more complex, it's likely to have an MTBF (mean time between failure) that is worse. But for purposes of discussion, I think it could generously be stipulated to be the same and in practice would be found to be pretty similar. Either way, one would be foolish to ride without a backup light given the safety consequences of not having light when or if one or the other system fails.

Cold weather riding - again, no big deal. A little heat pack in the battery pack (those iron based air ones) work very well for long rides, or having a cord and keeping the battery in your jacket. Even then, batteries are amazingly resilient in cold weather and I have some considerable experience with this as a ski patroller - lights and radios as well as cordless drills (hazard marking) will hold up very well down to very low temps. True for chemistries like NiMH (drills and radios), radios and lights (li-ion or NiMH). Yes, there is a loss, but it's easily compensated by an extra battery carried in your jacket. Most of the bike lights have very small batteries where this is just not a problem.

Then the dynamo system fails badly when it comes to cost - custom built wheel etc...

So, again, I don't see the attractiveness of a dynamo system unless one is someplace or has some application where recharging is not an option. It's driven by a time when batteries were not available and battery driven lights were not as bright.

J.