I have been using Energizer rechargeable batteries in my radios for a few years now. They work fine, but they do have some draw backs.
First and foremost, they take about 18 hours to charge 4. Not that big of a deal as I just throw them on the charger every time I'm done using them.
Next, the price, they are about $11 for 2. I have bought them over time, first I got 4 for my flashlight, then another 4 to use in my Lasonic TRC-920 that uses 8 D cells, then I bought another 2 for my 777 which uses 10.
The ones I have are 2500 mah. They last about 4-5 hours if your really twisting the volume knob, longer if your just easy listening.
I plan on making the BIG rechargeable purchase for 10,000 mah batteries and a digital charger. The 10,000 mah batteries are $10 EACH! $100,,,,,plus the digital charger which is around $60. I have been told those should last 10-12 hours plus!
If it's true, that would be awesome. Plus the digital charger can diagnose your batteries and charge them allot faster.
I have never had any Boombox injuries to report so I don't think there is anything to worry about when using them.