I have a li-ion Makita BL1830 18V 3.0Ah battery that can't be charged...when on the charger the bottom two lights flash red..it will charge my other battery, so it doesn't seem to be the charger

Sometimes, this is a result of a battery that is too cold (below freezing-warm it up). Sometimes it can be too hot (discharged under heavy stress-let it cool down). Sometimes there may be insufficient voltage, I have had problems in older houses, get closer to the electrical panel to plug in, or you may have to wait until you get home. Red flashing may mean https://www.youtube.com/watch?v=8rZqYmPsZycthe charger or battery is to hot and must cool down before it will charge. The charger will not charge a battery if the lights are flashing. Solid Red light means it is charging and solid green light means it is charged. Plug the charger in first before putting the battery on the charger. If the charger has been dropped or hit the floor the coil inside could have broke loose and damaged the charger requiring it to be replaced. Below is an unconventional way to charge the battery if the charger is no good. BE CAREFUL
how to charge broken makita BL1830 Li ion battery

You can try to get a universal charger for Makita:
Makita Universal Charger for 7 2V 18V Ni MH Ni Cd Li ion Batteries...

I live by hitachi, one of the good things about it is lowes will exchange them as long as you have a receipt and it's under 30 days. If not chargers are not very expensive, or you can send it back for repairs. I have 10 of those combo packs and never had a problem with a single one, for you to get 2 is a freak accident.

check that you are trying to charge the correct batteries. . The come in 2 types ni- cad and li ion. Most battery chargers will not charge both types because of the different charging requirements.

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.

on my charger (DC18SC) alternating red and green dual flashes mean a defective battery.
http://ubermanuals.com/product/makita-dc18sc-8866 also same for the DC24SA

Have you tried just waiting a few minutes, it might start to charge anyway. But if not, is there some reason you would assume that you don't have a bad battery? There are lots of bad Li-Ion batteries.

your batteries are toast;must replace

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 6 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. But Makita should definitely have a few words with their supplier before they become a liability!

I think your tryiong to charge NIMH batteries oon a charger designed for Lithium Ion batteries. With all the new style batteries such as NICAD, NIMH and LI-ION, charging becomes difficult. Most chargers pre NIMH and LI-ion, were designed for NICAD's. The best advise, match your batteries with the correct charger