Not all rechargeables are alike. I would not recommend NiCads. They have too many problems, like memory. I would recommend Nickel Metal Hydride types. Each detector pulls a different rate of amps so life span varies.
I used exclusively the 9 volt Varta 150 mAh for about 8 years until they wore out. I got these for about $1 on ebay new. They work great and NiMH have no memory problems. You do not have to run them all the way down like Nicads. They do test at 9 volts fully charged. I have replaced these with 230 mAh ones and am going strong. NiMH come in slow and fast charge types. Match the charger to the type or else. NiMH chargers are not like NiCad chargers. They typically have a timer to shut them off. These "9 volt" batteries come in a range of voltages and are listed as 8.4 volt, 9 volt, 9.4 volt and even 9.6 volt. Like Nicads they are internally composed of 1.2 volt cells. So they really ought to be 8.4 v or 9.6 v. Some folks rant and rave about 9.6 volt being such a deal because they start out higher. I don't think this is such a big deal. Most electronic devices have internal voltage regulators and step down or restrict the maximum voltage anyway. I bet the Fisher CS does too. If we knew the precise cut off the CS voltage regulator uses then I would agree the voltage might make more of a difference. The real difference in duration is the mAh (milliamp hour) rating. Watts=Volts X Amps. It's all about Watts folks. Based on a well charged and properly burnt in battery, my 150 mAh ones run 5 to 13 hours on the CS using the speaker. If you use the light expect more like 5. I simply carry the spares in my pocket or switch them over at lunch when I have the time. I spent $4 for all the batteries I will need for the next 10+ years! NiMH batteries go up as high as 260 mAh. The greater the mAh the more the cost. Fast charge often costs more than slow charge too. Everyready & RayOvac make slow chargers that do 2 batteries for under $20. Chargers also come in car models or combo models. Remember you need a 2 battery charger. To see a listing of the very hottest 9 volt batteries on the planet click here. I'd imagine you could find these same batteries cheaper somewhere, but this is the best comparison I know of. They sell the matching chargers too. The Maha Powerex 300 mAh are the current champs.
Note: Make sure you burn them in charging and discharging them fully five times. If you get 2 sets of batteries then rotate the ones you start out with each time. Always do a fresh charge the day before use.
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