The 12" PowerBook battery is a pack made using 6 Lithium ion cells with the 16850 form factor. The 6 cells are arranged in 3 pairs (each pair in parallel), so using 2100mAh cells yields a pack with 4200mAh capacity and about 12v. Typical Li-ion batteries are charged to 4.2v and should not be discharged below 2.8v. There is circuitry in the battery pack that monitors each battery pair and makes sure these voltage limits are met when charging and discharging the pack. The battery status can be seen using the System Profiler, under the Power tab. The capacity, current draw/charge, and pack voltage are reported there. Each pack keeps track of its capacity as it's used and I've seen new OEM packs start with 4400mAh and 4600mAh capacities. This number is used to calculate the percent remaining, or time remaining, as the pack monitors the energy extracted. If the pack has not been fully discharged in a long time, the Full Charge number may be overly optimistic, and the computer may shut down with battery capacity still showing. The pack can be calibrated by running the computer on the battery until it shuts down; the pack will update the "Full Charge Capacity" with the amount of energy extracted in the process. The capacity will also depend somewhat on how the pack is used. With the processor at half speed, the display backlight off, wireless off, playing music I get an indicated current draw of 0.7 amps. Playing a video I can get up to 1.8A and with everything maxxed I can get over 3. At a higher current draw the internal resistance of the battery drops the output voltage more. Li-ion batteries have pretty similar amp-hour capacities at higher currents, but when the output voltage is lower the computer will draw more current to get the same power. Because of the cell arrangement and protection circuitry, the pack will shut down if any pair of cells gets down to 2.8v. This means that a single bad cell will cut the useable capacity of the whole pack more than half, and the cell paired with the bad cell will be working harder because it gets cycled deeper and at higher current. One indication of this condition is the pack voltage when the computer shuts down. If all 3 pairs are matched in capacity, their voltages should be similar when they're nearly dead. For example, three nearly dead but matched pairs: 3v + 3v + 3v would give a 9v pack. If two pairs are still going strong while one has already reached the cutoff voltage you'll see a higher pack voltage, like: 3.8v + 3.8v + 2.9v = 10.5v. So a "bad" pack that shuts down at a higher voltage, may have a single bad cell. I have a few OEM battery packs that are showing their age. Only one of them reports over 4000mAh capacity. I've not had much luck with the aftermarket packs either. They're typically cheaper construction (all plastic instead of aluminum), but mostly they're no better than the OEM packs in use. Two packs I bought were advertised with a 5200mAh capacity, but were actually less than 3000 (less than a 15 year old OEM pack). After returning them, I decided to try my hand at putting new cells in the old OEM packs. My plan was to open up two packs in poor condition, possibly piece together a decent pack with the cells from both, and install new cells in the other. When it comes to 18650 lithium cells, there are only a few actual manufacturers of good cells, and a lot of re-wrapped cells and a lot of lying. The current state of the art for these cells is 3400mAh in the low power spec. There are some cells in this form factor that are made to deliver high current, but these have less capacity and laptops do not need high current. So, anyone advertising a 16850 cell with significantly more than 3400mAh is simply lying. For example, here is a magic "9800mAh" cell: http://www.ebay.com/itm/351498875684 And here's a test of a "6000mAh" cell that yielded 1000: http://lygte-info.dk/review/batteries2012/UltraFire SJ18650 6000mAh (Black) 2015 UK.html So basically, if you want to re-cell a pack, mind where you get the cells. Most of the latest and greatest cells go straight to car and portable manufacturers, so individual cells are less available and somewhat pricey. However there are good cells available that are recent manufacture and affordable because they're last year's model. Another good source for cells is to open up packs. Laptops go obsolete very quickly (except for PowerBooks), so you can often find cheap OEM laptop packs that will have relatively new, good quality cells. I went with Sanyo NCR18650BF cells, rated at 3300mAh at around $5 each.