Rebuilding Makita Power Tool Battery Instructions

Got Makita power tool at home and the cordless drill battery wouldn’t take a charge, no matter how hard I tried to “wake them up”.  Checked the prices of a replacement Makita power tool battery online,  they run around AU $70 each and more than I wanted to spend on a $37 cordless drill kit, so decided it was time to rebuild Makita cordless power tool battery.

Rebuild Instructions

First trick was to decide how to cut into the solvent welded case.  I decided to take a chisel tip on a light soldering iron and cut an outline just on the top of the base, that way after I glued it back together the latch on the Makita drill would still hold the cordless drill battery pack together. Turned out that it was a good decision.  The pack came apart pretty cleanly and even left some of the battery locators in place.

Pack looks like this with the top section removed.  The 14.4V power tool battery pack has 12 sub-C NiCads in series.  That’s nominally 1.2v per cell, and about what Ni-Cd MAKITA 1422 Power Tool Batteries are rated at.  The black insulator sheet you seen on top of the main battery pack is sticky on both sides, and takes some persuasion to allow the top to slide off. One way I’ve found to do this in a pack of series batteries is to first charge them up and start looking for weak sisters singly, then to do the same starting at one end and check the build up in series.

Patching in a new power tool battery. Charge them up, check the voltages, run them down check the voltages. By now, I’ve replaced four of the cells with better cells from my spare parts stash, but things aren’t getting much better!  I’m finally starting to realize that the Makita 9120 Power Tool Battery cell packs are really pretty well balanced and if it isn’t a premature failure, then they’re pretty well all gone at once!  It probably wasn’t helped by the previous owner riding them hard and putting them away dead.

A couple more cells, and I realized that the only way to rebuild this pack was to replace all the cells.  NiCad sub-C’s are about $5 each and NiMnH are about $6 each.  That means I could rebuild it with NiCads for about $60 – or about what a new battery pack would cost!  It might be reasonable to try to find a premature cell failure in an existing pack, but not rebuilding one completely.