18v lith ion Battery charger 12v makita will not charge battery? why? why?

SOURCE: Battery Issues

When Li-Ion batteries are discharged to a too low voltage (below 2,75 V/cell, in this case 13,75V) the charger will assume that the battery is damaged, and will not charge the battery. First measure the battery voltage with a volt meter, if the voltage is significally below the 13 V the battery are no good. If you have a reading of lets say around 10 V, you can raise the voltage shortly on the battery, by jumping it with a fully charged battery. Connect the "-" poles on each battery. After this, shortly connect the "+" poles, for no more than 5 seconds,disconnect the wires and immediatly put the battery in question in the charger. If it starts charging, every thing if fine. If not, try the process again, you might wanna recheck the voltage if it rises.
Please be aware, that over discharging Li-Ion batteries, will lead to damage of the Li-Ion cells, which results in loss in capacity, therefore, if you are not to use your Li-Ion tools for a while, make sure the batteries are fully charged.

Hope you guys could have sucess with the above

Best regards
Soeren

SOURCE: problem charging makita batteries 18v

Unfortunately, Makita’s li-ion battery packs have a design flaw. After having the same problem with my two batteries, I took it apart and saw the problem immediately. You see each battery pack has ten li-ion battery cells and a circuit board with a memory chip witch holds the charging history of the battery pack. But that memory chip constantly draws power from 2 of the 10 batteries. The current it draws is very small but if you consider it over 8 month or more, the power drain becomes very significant. You end up with a battery pack with 8 still fully charged battery cells and 2 drained battery cells. When you put this battery pack in the charger, it detects weak battery cells, assumes they are defective and refuses to charge. To avoid this problem you should charge your battery pack often, even if you haven't used it, every two months should be ok.
I suspect that Makita doesn’t make these battery packs, they make power tools, good ones too. Buy Makita should definitely have a few words with their supplier before they become a liability!

SOURCE: 18v Lithuim Ion battery will not charge

If you have been a bad boy and managed to short the battery by using it for non-proscribed purposes, the fusible link will blow (kind of like a fuse) To fix it, open up the battery (use a Torx 10 security bit, or a small flathead in a pinch) On the battery connection nearest the spring-loaded white catch there is a small bridge of metal with a hole in the center. If this is melted you can solder it back together by sanding the two pieces and putting a glob of solder on them. This will void your warranty (duh!) and remove the battery's fuse protection, but it WILL work again. If the link is intact and nothing else is obviously wrong, you almost certainly have a bad cell. I recommend pulling out the bad cell ( it will be the one that does not read between 2.5 - 4 vdc) and replacing it with one from another dud battery- this requires some fudging and re-soldering. Or do what I did, pull the cell, toss the electronics and the short pink wire, add a cigarette lighter socket and voila! you have a portable power supply giving around 14 volts. Charge it up by wiring two cigarette lighter male ends together (check polarity , + to + (tip end) and - to -) and plug it into your car, but only while it is running or you will be charging your car battery with it! Hope that helps.
Matt Binns GiantGlobes.com

SOURCE: Makita 18v Lithium-Ion BL1830 Battery Won't Charge

If you have been a bad boy and managed to short the battery by using it for non-proscribed purposes, the fusible link will blow (kind of like a fuse) To fix it, open up the battery (use a Torx 10 security bit, or a small flathead in a pinch) On the battery connection nearest the spring-loaded white catch there is a small bridge of metal with a hole in the center. If this is melted you can solder it back together by sanding the two pieces and putting a glob of solder on them. This will void your warranty (duh!) and remove the battery's fuse protection, but it WILL work again. If the link is intact and nothing else is obviously wrong, you almost certainly have a bad cell. I recommend pulling out the bad cell ( it will be the one that does not read between 2.5 - 4 vdc) and replacing it with one from another dud battery- this requires some fudging and re-soldering. Or do what I did, pull the cell, toss the electronics and the short pink wire, add a cigarette lighter socket and voila! you have a portable power supply giving around 14 volts. Charge it up by wiring two cigarette lighter male ends together (check polarity , + to + and - to -) and plug it into your car, but only while it is running or you will be charging your car battery with it! Hope that helps.
Matt Binns GiantGlobes.com

SOURCE: MAKITA LXT BL1830 18V LITHIUM-ION 3.0AH BATTERY

If the charge count on the batteries is under 300, Makita will replace them. Makita has a known defect in this battery that causes the battery to shut down before it is dead. So as of now they are exchanging ones with low use.

If they are truly defective and makita won't replace, you have to open them, find the dead cell or blown fuse, and replace. Search youtube makita battery for a video on repairing this battery. But once the red and green lights on the charger flash, the the battery can't be charged on a Makita charger -- it's been locked. So use a third party charger.