The low self discharge AA batteries I installed in a String Trimmer seem to have lost their charge after just under a year. This may be because they were left plugged into a dumb charger (the batteries are supposed to be disconnected from the cgarged when the are charged and then hold their charge). Or perhaps a year is all that can be expected in a heavy duty application from these batteries. Sub C cells, which the trimmer originally had, should offer more power.
Tenergy are offering Pre-charged Nimh C Cells on Amazon.com. The catch is that they are only 3000mAh capacity. It is likely these are actually AA cells fitted into a larger empty metal cylinder to make them up to C cell size. Other C cells are usually 5,000 to 10,000 mHa.