As Apple has introduced new PowerBooks, they have also been increasing the charge capacity of their accompanying NiCad batteries, due to the higher power requirements of the newest models. These batteries are interchangeable, but using the old 2.5Ah battery in a 165c or 180c will give you a very short battery life.
The original Duos shipped with what is now known as the Type I Duo battery. At the time they didn't know that there were going to be more, so it isn't labeled this way. The Type II and Type III batteries that followed have a label in the upper right corner identifying them, which is good, because they share the same Apple model number.
Anecdotal measurements indicate that the Type II and Type III batteries are quite close to each other in capacity - 1.35Ah and 1.45Ah, respectively.
Battery model Apple part #
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Duo Type I M7782
Duo Type II M1499 (Type II and III share same model)
Duo Type III M1499
PowerBook 500 batteries
The PowerBook 500's shipped with a special, "Intelligent" battery. Unfortunately, this battery can get easily confused, and requires special utilities from Apple or other companies to straighten it out.
Battery model Apple part #
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PowerBook Intelligent Battery M1906
PowerBook 100-180c AC adapters
Apple has also been busy upgrading the battery chargers. Note that while all chargers work with all PowerBooks, charging a newer PowerBook on an older charger may take longer. Note that the M5140 model has a design flaw - a plastic ring around the tip of the plug inserted into the PowerBook can crack, and will blow the PowerBook's fuse if it shorts out. Note that Apple warns against using adapters that produce more than 19 watts with the PowerBook 100 and PowerBook 150.
The adapter for the PowerBook 500's is unique, due to the PowerBook 500's ability to hold two batteries. The adapter actually contains two separate power supplies. If the PowerBook is running, one power supply runs the PowerBook, while the other supply recharges the batteries in sequence. If the PowerBook is asleep or shut down, both batteries are recharged simultaneously.