I looked for a good answer to some of your questions and got a host of replies. Some like the BCR's some don't, some will tell you only use the MTH batteries. Some will tell you any decent battery from Nicads to nimh to l ion. First a 5 volt ps2 and a ps1 unit use an 8.4 volt (looks like a 9 volt) these have no charging port or a round one on the bottom of the unit. The ps2 3 volt first used a double pack AA sized then a double packed AAA size. the ps2 3 volt units have a square charging port.
Based on the answers I received on my thread. I'll stop short of advice except to say a good fresh battery is never a bad idea in older equipment especially if it's 15 years old or older. I also have found exact replacements for the 8.4 volt units on the bay, many 5 times or more less expensive than a bcr. I went that route as all the experts say the 5 volt boards will fail anyway and are no longer available. (so why pay so much for a bcr you wont need much longer for a board you can't replace?) The batteries I use work just fine and except for a sound amp, I've never had a 5 volt ps2 unit totally fail. I have never had to replace batteries in a 3 volt board unit.
