Three makita batteries wont charge 18v li-on - Makita Tools 18V 1/2" Lxt Lithium - Ion Hammer Driver - Drill Kit - BHP451

SOURCE: MAKITA LI-ION 18V BATTERIES WILL NOT CHARGE

Try charging the battery with a car charger set on low (2 amps max) or a wall transformer from a phone or some such with a 12v output. (make sure it is 12vdc not 12vac)   You will need to know which of the charger wires is pos and neg, be careful not to short the battery as this will blow the fusible link. Connect + to + using jumper clips or stiff wire jammed into the battery contacts. Once you get the battery over 10-12 volts the charger will normally accept the battery. You can jump one good battery to the bad to do this, but in all cases watch out that the wires do not get hot, as this is a BAD sign, often a minute or two will be enough. Of course you may have a dodgy charger, they are not robust at all.  The fans also ****, noisy and sometimes they quit, causing the charger to quit too
Matt  Giantglobes.com
For other battery issues see below
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: problem charging makita batteries 18v

What is the battery or charger not doing correctly?

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: Makita BR1815 li-ion batteries won't charge

your batteries are toast;must replace

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