Windows Mobile Team Blog : Why Persistent Storage Is A Good Thing

Mike Calligaro has posted a very interesting discussion about ROM/RAM battery usage in PDAs.

The requirement was that, at the point where we decided the batteries were “critically low,” they had to still have enough power to keep the RAM charged for 72 hours. The idea there was that you could discover that you were out of power on Friday on the way home and you’d still have your data on Monday when you got back to your charger.

A typical battery holds 1000mAh of charge. 128M of RAM takes about 500mAh to stay resident for 72 hours. 64M takes about 250. This is why you never saw a 256M WM 2003 device. It would have run for a minute then decided its batteries were critically low.

This is why switching to Persistent Storage can radically improve your battery life. With PS, we removed the 72 hour requirement.


  1. Richard Parry Said,

    July 21, 2005 @ 3:01 pm

    This is funny, my PDA came with 128MB on board but because it was so useless, I got myself a flash card for it, which is where all my data lives all the time. I don’t really notice it being “slow” because it’s always been like that, I got bored of losing data in RAM (and because it’s old and USB 1.1, it’s very slow to restore).

    I’m all for persistant storage, and if they release a flash-based device in future I might actually use that as an excuse to upgrade. I suspect however they’ll use this as a good opportunity to core you further with high speed CPUs, returning the same real-world battery life with more fruit.

  2. Somewhere out there! » Windows Mobile 5.0 Upgrade Said,

    October 1, 2005 @ 10:04 am

    […] Reviews look good. Especially the comments about the persistent storage feature and battery life, 10-30% more battery life. […]

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