My makita 18v 2.0 battery won't keep a charge

Return it to the place of purchase if under a year or send it to Makita UK in Milton Keynes

Your problem states you have a Makita tool and your question is about a Dewalt lithium battery. In either case if the battery won't take a charge or if the charger is showing the battery is defective the only fix is to replace the battery. Some causes for defective batteries or batteries that won't take a charge are bad cells in the battery, bad or shorted IC board in hte battery, broken or burnt connections between the many cells or between the cells and IC board or even bad or old cells that will no longer charge up. The time and effort needed to find out what the problem is then finding another battery to scavenge parts from (individual battery parts are not available) is never worth the cost of just buying another battery and going back to work with the tool(s).

according to the schematic I saw yes you can and I think that you will be pleased with its performance, their operator manual even lists the weight of tool with this battery. Makita has a 1 year limited warranty on the Li-Ion battery that you might want to consider if you have the receipt.

This battery has a known design defect that causes premature failure. The battery is still probably good, but the safety circuit n the battery spuriously locks it, due to a bug. Once locked, it won't charge in the Makita charger (flashing red and green lights). Makita knows about this problem but so far is not actively recalling the batteries. Makita will replace the pack if the charge count is under 150 (some people say 300?) regardless of warranty status, but you have to take or mail it to a Makita Service Center -- the dealers won't handle this unless it's in warranty.
http://www.makita.com/en-us/Modules/Service/ServiceCenters.aspx

If Makita won't fix it, you can try to repair it. If the red and green lights are flashing, the battery is probably locked and cannot be repaired in a way that will let it be charged in the Makita charger (but it can be charged in off brand chargers). Buy a clone charger on eBay for $60 and try it. If it charges, just use it, no repair needed.

Be aware that lithium batteries do lose capacity permanently at a rate of about 20% per year. So, 3-6 years is about the best battery life you can expect - from date of manufacture, so if it sat in the store a while that adds. Makita derates the cells to allow for some shelf life (down from 2400 Mah to 1500 Mah rating).

If the battery is weak, and only gives brief usage, you can replace some of or all of the cells in the battery. These are expensive and worth repairing. See this video:
http://www.youtube.com/watch?v=Z5taguEdlkY

If just 1 or 2 cells are weak, you can replace them with the same Sony 18650 size 2400 Mah capacity battery, green sleeve. The Sony's are not sold retail, and are hard to find other than from another Makita pack or from some laptop packs. If more are weak, you're better off just replacing all of them with new 18650 2400 Mah (or higher) batteries with tabs. Look for Tenergy or other quality brand.

Hope that helps.

This isn't true because I have a second charger and it works fine

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.

Sorry Mate your battery is dead...
The makita lithium batterys dont like getting worked hard quickly.
There is a chip in them that recognice each battery to each specific charger. Try another charger. This has not worked for me. Try anyway

It appears that your battery is worn out. They will charge, but miminally. If it is a nickle cad battery, and relatively new, you may have a memory problem. Try draining the battery completely several times and recharging each time. Be sure the battery is completely dead before recharging.

looks to me like a defective batterie

make sure the charger is cold or keep it in a cold place, when makitas charges get hot, they will not charge.