Definitely not a scientific test but...

[rpcrowe]

I normally use Eneloop batteries in my 430EX flash. However, the other day, I visited my daughter's house about an hour away and forgot to load the Eneloops in my flash. I left them in the charger. This was really stupid since I have several extra sets which I could have inserted when I removed the four Eneloops for charging.

In lieu of Eneloops, I inserted four fresh AA Energizer Alkaline batteries at my daughter's house. The Energzers had a noticeably shorter life span. It wasn't very long before I noticed that the recharge time was growing longer and longer. That is really a PITA when shooting dogs.

Like I said, this is not a scientific test. In fact, I am not sure how many exposures I made. Most of the time I shot in ETTL fairly close to my subjects and at around f/4 or f/5.6 so this should not have resulted in excessive drain from the batteries.

However, my conclusion is that I am happy that I chose Eneloops and will continue using them in lieu of throw-away batteries. They are good for the environment and my wallet also!

[Brocken]

I use industrial NiMH batteries in my speedlights - they have almost twice the capacity of branded supermarket rechargeable cells.

As for alkaline 'throw away' batteries, their internal resistance increases as they are used up, so their cycling time before the flash 'ready' light comes on also increases. They do recover a bit, if left to one side, more so if they are put on a warm radiator, but I only use alkaline cells as an expensive standby, as you do.

[FrankMi]

With throw away batteries now only 17 cents each I've almost given up on replacing expensive rechargable betteries when they go bad. But I must admit the this approach works better for some applications than others, such as in my wireless mouse and keyboard.

[Manfred M]

I use PowerEx rechargables. They have slightly higher power densities than the Eneloops, so they last longer. They are a bit harder to find. They are made by a company called Maha.

They are sold by battery speciality stores, rather than by normal electronic retailers.

The regular Eneloops AA cells are rated at 2000 mAh while the PowerEx are 2700 mAh.

Both brands are NiMH, so they don't have the battery memory problem, but can be more easily damaged by deep discharge cycles.

[rpcrowe]

I use industrial NiMH batteries in my speedlights - they have almost twice the capacity of branded supermarket rechargeable cells.
As for alkaline 'throw away' batteries, their internal resistance increases as they are used up, so their cycling time before the flash 'ready' light comes on also increases. They do recover a bit, if left to one side, more so if they are put on a warm radiator, but I only use alkaline cells as an expensive standby, as you do.

I am not sure of the the following because I will probably change my Eneloop batteries more often than I change Alkaline throw-aways (simply due to the fact that it costs virtually nothing to replace a set of Eneloops with a charged set) but, it seems that what you mentoned is true. The recharge time of the throw-away batteries grows longer and longer fairly early in their life cycle as I am shooting the flash. The Eneloops seem to keep approximately the same recharge time and then just quit when they are expended. The Eneloops also appear to provide more flashes per recharged set than do throwaway alkalines...

Are my observations in line with your observations?