Battery Tid-Bits

So, I spent some of my lunch hour with a few friends over at the Apple Store today.  Turns out that my friend Evan was having some battery issues with his shiny MacBook C2D.  It would randomly shutdown, even though the power indicator showed around 30% full.  While trying to look into things, he noticed that that in System Profiler, the battery info seemed a bit odd:

MacBook battery

Notice the ‘Amperage (mA)’ field — it’s negative.  Now, our first thought was, “that’s weird — you can’t have negative amps, can you?”  And, when presented with the same info, even the Geniuses at the Apple Store were perplexed.  But, as we looked at the floor models, all of the MacBooks and MacBook Pros showed an amperage of 0.

That is, until we unplugged them.  At that point, the amperage jumped down to a negative number.  It would fluctuate based on the CPU activity, but it always stayed negative.  Then we plugged them back in — and then it went positive.  That’s when it dawned on me — the amperage is how much power is flowing to or from the battery.  So, having a positive amperage indicates that the battery is charging.  A negative amperage means that the battery is being used.  And 0, of course, means that the battery is completely charged, and is running on wall power.

Now, I realize that for some people, this is going to seem painfully obvious.  And in retrospect, it is.  But, I’m also pretty sure that there are plenty of other people out on the web that may have come across this negative amperage and wondered what does it mean — especially in the course of troubleshooting other battery issues.

BTW — can anyone tell me from looking at the pic how long I can expect that battery to last?  Post in the comments and be eligible for a pile of geek points that you can use to impress your friends and co-workers 🙂

EDIT — Evan pointed out my complete inability to tell the difference between positive and negative.  I had everything swapped, and now it is fixed.  How embarrassing!

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