This is a known problem with BL18XX batteries. Makita designed the battery supervisory circuit to run off only one of the 10 cells, so leaving your battery uncharged for a long time eventually discharges one cell much more than the others. When you put this battery in the charger, it reports that the battery is defective. Another Makita fault is that the intelligent (DC 18XX) chargers will only attempt to charge a 'faulty' battery 3 times, then they will re program the micro in the battery to lock out charging for good. The only solution is to buy 3rd party replacement battereis that don't have the faulty Makita supervisory circuit.
Maybe there is something wrong with the charger itself. If you have a friend that has the same drill try to charge the battery on his charger.
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Not a problem with the charger. I actually have 3 chargers and 7 batteries (3 of which are currently showing as defective on each charger) My remaining 4 batteries charge fine on all of my chargers. Thank you for the suggestion though, tvenetis.
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