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NIMH is old technology, especially the older generation NIMH cells: they have the highest natural discharge rate of any rechargeable battery on the market. Newer NIMH batteries with the marketed "low natural discharge" retain 85% charge after one year of non use. However, they don't have as high of a capacity as the older NIMH batteries.

A123 batteries, li-po or li-ion would be the way to go as far as current technology, A123 preferred. They have higher charge/discharge capabilities and are MUCH more stable.
Great Post!
ALL current NiMH technologies gradually lose capacity due to various electro-chemical degradation (eg. dendrite formation, increasing plate resistance etc.) as they experience normal "life" cycles (varying charge/discharge rates, temperatures etc). Some battery designs demonstrate better resistance to this phenomena than others but eventually they WILL need to be replaced (10 years?... we shall see)

Assuming the OP's original link was referring to the voluntary recall issued for the GM BAS hybrids, those were for Cobasys sourced NiMH batteries that actually developed electrolyte leakage that created voltage imbalances between the NiMH cells/ strings. When detected by the energy managment system this imbalance would trigger specific DTCs that result in the disabling of the auto stop>start feature (since the BAS hybrids are a "mild" hybrid that's THE primary fuel-saving, emissions reducing feature)

The entire 36V battery pack for nearly the entire range of BAS hybrids (Vue, Aura, Malibu) produced before ~spring 2008 are being replaced with an improved version regardless if they are currently experiencing any issues in hybrid battery performance.
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