iStat Menu

offers to display both percentage and Time. http://bjango.com/mac/istatmenus/

When running SL, you could get the old free version. However, it does not contain the battery menulet: http://mac.brothersoft.com/istat-menus-2.0.html

Does the real-world battery life scale linearly with it's capacity?

No, because it also depends on current draw. If you have a 10 watt hour battery, and you pull 10 watts from it, you will likely get less than 1 hour of time due to losses inside the battery.

If you instead pull 1 watt from it, you'll get closer to 10 hours of usage, because the losses are not as good at lower current draws.

As batteries age the losses become worse. The internal damage that occurs over time due to the charge/discharge cycle causes additional internal cell resistance. So a new battery will give you more of its rated capacity (and use less internally during use) than an old battery. The old battery will give you less of its rated capacity (and consume more internally than the new battery) than a new battery would.

Is this number a reliable indicator of present and future battery life?

It's an estimate, and now that the battery has higher internal losses it will depend much more greatly on how much power you're drawing. If you're using the processor and graphics chipset at capacity, then add wireless, bright screen, etc on top of that you probably won't be able to use the entire measured capacity.

Therefore it's not reliable, as it depends on your actual usage scenario.

If, instead, you've got it playing itunes with the lid closed, it will use a larger percentage of that capacity before it indicates depletion.

What kind of realistic usage am I likely to get from a new battery under Lion?

Unfortunately you're trying to compare Lion's consumption with your previous OS consumption. I can't answer that question, and even if I had the same model and recently changed the battery I have a different use case, so my experience wouldn't necessarily apply to use.

I can tell you that often one battery will degenerate more than the others in multi cell pack, and if discharged under controlled conditions you can get the full capacity of each individual cell, but under heavy discharge the bad cell is a weak link, and the whole battery pack will shut down prior to depleting the other good cells.

This means that there are several good reasons to replace your battery, and the most likely outcome is significantly increased battery life.

However there's no way to guarantee that due to your individual circumstances and usage. The tools you're using are ok, but without removing and testing each cell individually under a few different discharge conditions you can't know fully how the battery will compare to new batteries. So the tools can't be counted on to give you more than a rough estimate.

If I were in your situation, though, I'd expect to get approximately the same life out of a new battery under Lion as I got out of the new battery under the previous OS. The difference might be slightly greater, but still minimal.