Another hyper-technical FilmHelp article by Greg Pak
I recently replaced the battery on a first generation Macbook Air (1.8 GHz). The Macbook Air battery isn’t considered “user replaceable” by Apple — it’s locked inside the enclosure, held in place by nine screws. Apple charges $129 to replace the battery. But I wasn’t thrilled about wiping the drive (for security reasons) and giving up the computer to be serviced. Instead, I bought a new replacement battery for $70 on ebay and followed the incredibly helpful instructions at ifixit.com to open the case with a tiny Phillips head screwdriver and replace the battery.
I started the project with some trepidation because I’d generally seen laptop batteries peter out bit by bit — I’d never had a battery suddenly cease to hold a charge the way this one died. So I had my fingers crossed that this wasn’t part of a bigger problem involving the logic board. But since the computer works perfectly post-op, it’s pretty clear I just had a dud battery.
Since I couldn’t find an exact description of the symptoms I was seeing online, I’m posting what I experienced in hopes that it helps others.
Symptoms:

1. The battery suddenly stopped charging. The computer would work as long as it was plugged in, but the LED on the magsafe charger would stay green rather than turn to the amber charging color and the battery monitor would report that the battery was empty and wasn’t charging. This might have been shortly after the battery was totally drained. The battery had 227 cycles on it. I followed the instructions at Apple.com to reset the SMC. And I reset PRAM for good measure. Neither procedure helped.
2. In OS 10.5.8, under the “Power” tab in the system profiler, the battery showed up but was tagged with “Check Battery.” When I restarted using a Snow Leopard (OS 10.6.1) installation on an external USB drive, the battery icon in the menu gave the message “Replace Battery.”

If the power cable was disconnected after shutting down, upon restart, the computer would give me an alert that time and date were incorrect. On other computers, that would be an indication that the internal PRAM battery was dead. But now that I have a working battery in the laptop, there’s no problem with losing date and time settings. I’m deducing that the Macbook Air has no internal PRAM battery — so if the laptop battery is totally drained, the settings that the PRAM battery would normally maintain are lost.

3. Several times after the computer died because the the AC adapter was unplugged, it made a long “bong” sound upon restart. Not the normal startup chime, but a long, more alarm-like bong. That’s generally a sound associated with memory failure. But the computer started up normally after making the bong and the System Profile showed all memory intact. I ran the Apple Hardware Test, which also showed no problems with the memory.

Solution:
Replacing the battery fixed everything.
Conclusions:

1. A completely dead battery in a Macbook Air apparently causes Date & Time settings to be lost — presumably because the computer has no separate PRAM battery.
2. A Macbook Air battery might indeed just conk out suddenly rather than gradually lose its ability to recharge over time.
3. Still no idea where that long “bong” sound came from.