Pushed one of my M12 batteries a bit too far with my tire inflator and got the dreaded red/green blinking light of death when I stuck it on the charger. No chargy-chargy.
These batteries are made up of three sets of li-on cells in series to get to 12v nominal. There are sensing leads and contacts so the charger can measure the voltage of each cell, and if they get too imbalanced, shuts down charging and you get the red/green blinking light of death. There should have been a cell balancing system, but this is just a monitoring and "error - go buy a new battery" system. Note below, voltages are all different - the 4.002v cell was after charging. Resistors in the first pic were just convenient conductive objects of the right size to stick in the battery slots and get measurements.
Would be convenient if you could charge through these monitoring contacts, right? Too convenient - there's a high ohm resistor inline, likely to prevent short circuit. Could still be done, but would take weeks.
Disassemble the pack, hook the benchtop power supply directly to the cells, and let the electrons start flowing. To keep charging amps low, I kept charge voltage only slightly above cell voltage, bumping it up here and there as the cells charged. Have to be careful at this stage, with these connections I've bypassed the battery monitoring system and all the circuit protection.
Charged all the cells to 4v, then reassembled and slapped it on the Milwaukee charger. Success, no red/green blinking light of death, and it took a full charge.
M18 batteries don't seem to suffer from this problem, I think they have an actual cell balancing system, not just monitoring.
Lessons learned: Already knew this, but it really underlined that M12 isn't meant for high drain devices. When using high drain devices, make sure you start with full batteries, and don't push them until the tool shuts itself down. Soon as you hear the tool starting to struggle or slow down, stop and swap to a fresh battery. High drain devices really should be M18 wherever possible, and M12 should only be used in low draw devices where compact size and light weight are paramount. I have a ratchet and under hood light, perfect uses for M12. My shop vac and tire inflator, not so perfect.
These batteries are made up of three sets of li-on cells in series to get to 12v nominal. There are sensing leads and contacts so the charger can measure the voltage of each cell, and if they get too imbalanced, shuts down charging and you get the red/green blinking light of death. There should have been a cell balancing system, but this is just a monitoring and "error - go buy a new battery" system. Note below, voltages are all different - the 4.002v cell was after charging. Resistors in the first pic were just convenient conductive objects of the right size to stick in the battery slots and get measurements.
Would be convenient if you could charge through these monitoring contacts, right? Too convenient - there's a high ohm resistor inline, likely to prevent short circuit. Could still be done, but would take weeks.
Disassemble the pack, hook the benchtop power supply directly to the cells, and let the electrons start flowing. To keep charging amps low, I kept charge voltage only slightly above cell voltage, bumping it up here and there as the cells charged. Have to be careful at this stage, with these connections I've bypassed the battery monitoring system and all the circuit protection.
Charged all the cells to 4v, then reassembled and slapped it on the Milwaukee charger. Success, no red/green blinking light of death, and it took a full charge.
M18 batteries don't seem to suffer from this problem, I think they have an actual cell balancing system, not just monitoring.
Lessons learned: Already knew this, but it really underlined that M12 isn't meant for high drain devices. When using high drain devices, make sure you start with full batteries, and don't push them until the tool shuts itself down. Soon as you hear the tool starting to struggle or slow down, stop and swap to a fresh battery. High drain devices really should be M18 wherever possible, and M12 should only be used in low draw devices where compact size and light weight are paramount. I have a ratchet and under hood light, perfect uses for M12. My shop vac and tire inflator, not so perfect.
