If A pack has different quality cells, some will drain faster than others. This is likely due large difference of internal resistance between cells. This will eventually lead to some sets of cells being less than 4vdc while others at 4.2vdc. Since an M18 charger only charges a battery as a whole, it allows charge until any one cell sets reports that it is at full capacity. To fix issue, we need eat set back above 4vdc. 

1. Disassemble the pack with the T10 and T9 Safety Torx. Be cautious around the exposed cells! (You only need the T9 for the 12Ah)
2. You will notice that the 8Ah battery pack has 5 sets of 2 18650 cells while the 12Ah has 5 sets of 3 cells.
3. Measure each set of cells and make note of which ones are below a 4VDC threshold.
4. Connect your charger with polarity in mind and let it sit for a while. If you trust your charger, you can leave it alone. Though, I recommend that you monitor the voltage of each cell set you are charging to help ensure over-voltage does not occur. FAILURE TO DO SO MAY RESULT IN A THERMAL EVENT!
5. Re-assemble the pack and use it down to one blinking red bar.
6. Charge it back up using the M18 charger and verify that each cell is withing 0.1 VDC