Today I learned that I've been working from an obsolete understanding of how to keep my cellphone and laptop batteries from losing gobs of capacity over time. A simplistic summary follows for your benefit.

The batteries in my phone and my work laptop are lithium-ion batteries. Check yours -- the "Li-Ion" abbreviation means it's lithium-ion. As detailed sources explain, charging/discharging battery care for lithium ion batteries is the opposite of the conventional wisdom I had in my head, left over from the old days of nickel-based rechargeable batteries.

It used to be that you'd want to run batteries all the way down before starting to charge them again, because otherwise the capacity might get messed up. That's not true with lithium-ion batteries; it's recommended that you only rarely let an Li-Ion battery run down below 10% of its charge.

Lithium-ion batteries lose capacity, in the long run, if they sit overcharging a lot, or if they run hot a lot. So don't let them sit plugged into a charger all the time, and if you usually run your laptop plugged into AC power, think about removing the battery and setting it someplace cooler.

The moment a lithium-ion battery gets manufactured, it slowly starts losing capacity. So buying a primary battery + a spare battery simultaneously might be a worse decision than using a primary battery, then getting the spare battery years later, when your capacity has substantially degraded.

This came up because I assumed I should let my new N900 run down completely (on the partial battery charge from the factory) before plugging it in, and I was annoyed that plugging in the USB-to-microUSB cable to transfer files meant it was getting juice while the battery hadn't totally discharged. But I was wrong to worry! Thanks for straightening me out, Sjoerd.