Here are a few articles we've prepared on technical topics which other people may find helpful.
The Apple Newton MessagePad 2000 was originally released in 1997. You may come across one of these devices at a garage sale or a friend's house, and want to try to power the device on, or attempt to charge the rechargeable battery pack.
Recently when I found one of these devices, it would not power on, and did not come with an AC power adapter. The battery pack included was the Apple rechargeable NiMH battery pack, but it was completely drained, and had not been charged in many years.
The official Apple Newton FAQ found in many places online indicated that a power adapter with 5v / 400 mA would work to power the Apple Newton, and that the connector of the power adapter should have the tip (or inside) be positive, and the outside of the connector be negative. I created a power adapter matching these specifications, but the Newton would not power on, and did not appear to be charging the battery, even after several days had passed.
The original Apple power adapter for the MessagePad 2000 had higher power output, specifically 7.5v / 1200 mA, with the same connector polarity (the tip (or inside) was positive, and the outside of the connector was negative). This is 50% or higher than the 5v adapter I had been trying based on the FAQ's information, so this may have explained why I got no results.
I subsequently read several reports online describing power adapters between 6v and 7.5v working to power a MessagePad 2000. The amperage can be anywhere between 1A and 2A, as the Newton will draw the amperage it requires from the adapter. I obtained a compatible power adapter matching these specifications (6v / 2A), with the correct polarity described above, and the Newton immediately powered on!
So, it appears the anecdotal information in that FAQ about using a 5v / 400mA power adapter was INCORRECT. It simply does not provide enough power to turn on or charge the battery pack.
I can also confirm 100% that you can power and use a Newton MessagePad 2000 using the AC adapter alone, if batteries are not installed or available. There was little information available during my research as to whether the device could be powered through AC alone, or whether the AC adapter was only working to charge the battery when installed.
The original Apple battery enclosure for the MessagePad 2000 allowing use of your own AA batteries is now hard to find, and goes for very high prices online when they are available. The rechargeable battery packs are far more common, but will be less useful as time goes on as the original rechargeable batteries continue to degrade. That original AA battery enclosure would be a really good candidate for someone to create a facsimilie 3d model of, so that replacements could be 3d printed by users who wish to use and enjoy these devices into the future.