Because that's what a single cell NiMH can output in the way of voltage. A 12V car battery is actually 8 batteries in series. 1.5V x 8 = 12V. A 9 Volt battery is actually 6 cells. Chemical makeup determines how much voltage you can get from a single cell. Usually no more than 2 volts.

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Current Stable:
01 ML55 AMG
92 500E (a few mods)
87 300E (lots of mods)
00 Chevy 3500HD Diesel Box Truck
68 18' Donzi Marine
06 GT i-Drive7 1.0 Mountain Bike (with GPS!)

PREVIOUSLY OWNED:83 300SD, 87 420SEL, 88 420SEL, 90 420SEL, 86 560SEL, 86 190E 2.3-16V AMG, 94 E320