Anyone buying a new Aluminium MacBook and hoping to extend the lifespan of the battery by removing it when using the AC adaptor may want to think again. According to testing done by Gearlog, doing so results in a performance drop of some 37 per cent.
This isn't an 'issue' as such, but rather a feature. According to Apple's support section on its site, system performance is dropped when the battery is removed to stop the machine from shutting down if it tries to draw more power than the outlet can provide.
That seems a little strange, really. The ability of the battery to deliver power drops off over time, but MacBooks almost certainly won't detect that change and drop performance in order to prevent potential shutdowns in that situation.
Of course if you're one of what I expect are very few people actually noticing this as an issue, then the simple solution is simply not to remove your battery. Otherwise chalk this up as interesting, but irellevent.