John Faughnan wrote:
I have SONY SRS-T55 travel speakers; I use them with my iPod. They use
4 1.5V batteries, at low volume it will run for 24 hours.

Rather than investing in a power adapter, I'd like to use rechargeable
batteries.

Most of the newer devices on the market are designed for digital
cameras. I'm not sure what would work best for replacing AA batteries
in a device like this. I'd like to buy something that it's reasonably
easy to buy compatible batteries for; preferably an "intelligent"
charger that minimizes memory effects.

Any recommendations in terms of capacity, manufacturer, NiMH, NiCad?

Ray-O-Vac makes their Renewals. Rechargeable alkaline batteries.

Good for about 20 charges or so - full Alkaline performance. $6-$8
a package of four, last I checked. I used two pairs in my Palm Pilot
for two years. Pretty darn cheap solution.