Rebuilding the Makita Battery Pack

I was really pleased to get a 14.4V Makita portable drill kit at a garage sale.  Got it home and the battery packs wouldn't take a charge, no matter how hard I tried to "wake them up".  Checked the prices of a battery pack, and they run around $60 each and more than I wanted to spend on a $35 drill kit, so I decided it was time to rebuild mine.
Checked the web for pages on rebuilding them, and couldn't find anything worthwhile so I decided to try it - what do I have to lose?

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 drill would still hold the 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 pack has 12 sub-C NiCads in series.  That's nominally 1.2v per cell, and about what NiCad 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.

I started hunting down the weak cells.  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.  For instance 1.3v, 2.6v 3.9v 5.2v 5.4v WHOOPS, there's a bad one mark it and replace it!

Patching in a "new" battery.

Charge them up, check the voltages, run them down check the voltages.  Find more weak sisters.

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 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.

Well, what a waste of TIME!  More searching of the net pointed to services that would "refresh" battery packs.  The description was a long charge that would help to break down the limits that NiCads seem to build up.  I'm assuming that they have chargers that are regulated differently than the Makita charger to "force" the charge onto the batteries.  What I had was a standard NiCad battery charger that could handle singles and doubles.  I tested a couple of the
pull-outs from the battery pack, and they seemed much more receptive to charging from the NiCad charger.

I already had the pack apart, so I figured out the functional series of the cells and labeled them A through L.  I then tapped out of the two cell section of
my NiCad charger and commenced charging them as pairs. A-B, C-D etc.  All the connections (except the two top connections) were assessable from
the bottom of the pack.  The batteries started out at about 2.4v a pair, which had given me about 14.4v fully charged and about 20 seconds of run time before the pack quit.  I found that about 8-10 hours on my NiCad charger would give me a reading right around 3v per pair.  Working on them a pair at a time meant that I also didn't have to disassemble the battery strapping!

It takes a couple of DAYS to slowly step the cells through the NiCad charger, and when I was finished they were actually holding an original charge around
17v, which is what I've read is the normal top for the packs when freshly charged.   After running them through the drill a few times, I found that the Makita charger was topping them up to just under 16v.  Not good as new, but probably half as good as new, and a lot better than dead.  I think I'll experiment with extended times on my NiCad charger and see if that doesn't improve their performance further.

I put the packs back together.  I found that I didn't even need to glue the top back on the battery pack since friction held the pack together well enough to handle out of the drill, and since the drill attached to the bottom of the pack it held the pack together while working.
The results make me think that if somebody that knew what they were doing could cobble up a good trickle charger that could "refresh" those batteries without disassembly (or blowing them up!)

9/15/02
A friend gave me his 9.6v Makita to fix.  The diagnosis was bad battery packs, these are the long, skinny packs instead of the fat based 14.4v units.
These battery packs had snap on bottoms, so the battery unit was easy to remove from the pack and examine.  They took a good "refresh" charge of
12v-2A for about 4 minutes and successfully charged to 11.4v - yet the drill would not work.  I decided to disassemble the drill.  Disassembly required
removing the chuck, which is held on by a left-handed thread screw inside the jaws, and a normal right hand thread on the chuck itself.  I couldn't get
the chuck off after removing the screw, so I "chucked up" a quarter inch bolt in the jaws and then spun it off quickly with a light application of a 3/8" impact driver.

What I found was that the contacts for the battery connection were badly oxidized, and were preventing sufficient current getting to the drill.  With the long 9v handle, the contacts were not available for examination without disassembly.

So I  got out the Dremmel tool and wire brushed them off, then lightly coated everything with a corrosion preventer.  On reassembly, the drill is running close to new.

Know more about fixing battery packs and Makitas?  email me and I'll try to add the info to the page.

The Makita site: http://www.makita.com/html/Service_Centers.asp

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