Battery Replacement | Lenovo ThinkPad R61

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Laptop ThinkPad R61 needed to replace battery. However, the operation hours not like Lenovo said "Four-cell: Up to 3.6 Hours". Around 1 hour, similar as the bad one in fact. Just not sudden from 30% to 10%. Because of not an original battery? Not sure.
Li-Ion battery pack:
    Three-cell
    Four-cell
    Six-cell
    Nine-cell 

Approximate operation hours are:
    Battery operation for 14.1 in widescreen models:
        Four-cell: Up to 3.6 hours
        Six-cell: Up to 5.7 hours
    Battery operation for 15.0 in models:
        Six-cell: Up to 5.0 hours
        Nine-cell: Up to 7.0 hours
    Battery operation for 15.4 in widescreen models:
        Six-cell: Up to 3.5 hours
        Nine-cell: Up to 4.75 hours 


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