I have been using a variety of batteries to keep my GPS gps'n and my lights shinin' with mixed results. I don't know if anyone else can verify my experience, but, all AA's are not the same. Regular Panasonic AA's will not stay in contact with my Garmin Etrex CS, so, every time you hit a bump, you risk shutting the unit down. The same is true of some Empire rechargeables.
I have found that the only reliable batteries (that seem to fit) are those made by Duracell.
To make a long story short, Duracell has introduced a new line of rechargeables and a slick 15-minute charger to go with them.
I picked up the charger and a couple sets of AA batteries. Now I am in battery heaven. These batteries fit snugly into my Garmin, and (as did all other batteries) work fine with all my head and tail lights, and what a pleasure to stick four AA's into a charger and have them fully charged in just fifteen minutes.
If you use AA's you should check into these. I think they are awesome.
I had the same problem (as have MANY others- Garmin has ignored the design issue) and solved it completely by stuffing a small piece of foam rubber in between the batteries above and below the central point where the mechanism for the back lock inserts. Doesn't take much- just two pieces about 1/2" long by 1/4" square. It may take a little experimentation to get it just right.
This is a simple, cheap and effective solution to an annoying problem.
On topic- I just bought some Duracell rechargeables, and they work just great with my Maha charger.
I find Duracell 2650's and Energizer 2500's work on some stuff. They have a thicker coating which doe not work with my SkullCandy speaker bag. They get stuck inside my Canon S2-IS digicam. They are hard to fit into 6xAA holders for battery packs.
I find the old Energizer 1850's fit well in my SkullCandy backpack. They go in and out smooth in the Canon S2-IS. They work well in battery packs.
Tho for the GPS all of the cells work fine. As long as it's not a tight spacing the newer cells are ok. I'm guessing the reason got the thicker plastic is to insolate them more? Probably does not make sense. Just a thought.
I like the Duracell rechargables, but the 2650s won't fit in my Cateye (EL-500) light. I use the 1850s that came with my charger. For other stuff, though, any of the Duracells fit great.
One thing about rechargables, though -- for tail lights where battery life seems to go on forever, most rechargables self-discharge over a couple months, so I've been recharging my tail light batteries approximately every two months to be on the safe side.
The solution to that is the new slow-discharge batteries like Rayovac Hybrids or the Sanyo version. Perfect for tail lights. I picked up some of the Rayovacs and will see how it goes. Supposedly they'll hold their charge for longer than 6 months without self-discharging. They could be used for remote controls too, which most normal rechargables aren't ideal for. But for things that suck down battery life really fast, like my Dinotte 200L, the Duracell 2650s are perfect.