Makita 14.4 battery pack

[moc]

I love my Makita 14.4v impact driver and drill but the batteries are shot.

I just can't justify throwing them out to "upgrade" to the 18.8, so I decided to rebuild the battery packs.

I bought some sub-C NiMH batteries off Amazon for a decent price and set to soldering. Soldering was a bit of a challenge for a beginner because of the two-layer pattern (two batteries are in the connecting shaft of the pack and 10 are below). I had to strip the label off the batteries to fit in the shaft (which I didn't realize until after I was done).

Regardless, I got everything back in and put it in the charger (which I put on an extension cord outside so as not to blow up my garage).

I had 30 seconds of joy as the charger went to red (charging) but then the red started flashing. That means the battery is too hot to charge. Clearly, the battery is not hot, so I assume that means that I did something bad to the thermistor during this process.

I can't seem to find any information about what specific thermistor to buy, though. I see websites with various rebuild parts and even some thermistors for other brands, but I don't know how generic such parts are.

Makita's site is no help.

Or maybe I screwed something else up. I don't know enough to know what damage I may have done.

I would appreciate any help from those who have been through this or have a better understanding of electronics than I (which is pretty much everyone).

Still, I didn't blow up my garage, so that's a bit of a victory.

Thanks.

[LCHIEN]

not all battery chargers work the same way regarding the lights...
considering the battery pack was just rewired there could be a miswire contributing to the error more likely than the thermistor went bad.
Thermistors come in all variety of resistance values plus they come in negative and positive resistance changes vs temperature. Which kind they used can vary as well as the value depending upon the circuit that measures it.

One thing you can do is to check the thermistor. Either remove it from the circuit or observe which two contacts on the battery pack it uses. connect an ohmeter to the two contacts or two leads of the thermistor and observe the resistance at room temperature.

Mkae sure its not open (infinity or greater than 10 Megaohms) or shorted (less than 1 or 2 ohms). Note the value if its not short or open.

Then use a hair dryer carefully to heat the thermistor. I say careful because a hair dryer can probably reach a few hundred degrees quickly near the output nozzle. wave it back and forth with generously large arcs across the thermistor to heat it to maybe 100- 150 degrees.

The initial resistance may be something like 10K ohms to 100K ohms. But can be more or less. When heated from 70 degrees to about 150 it should change noticably (more than say 10%) and like I said before, may go up or down. Make sure you get the same results measuring at the battery pack terminals as you do at the thermistor itself.

If OK, then you may have a wiring error. Some battey chargers use flashign red for battery failure alone or in combo with other LEDs. So You may also have wired some batteries in backwards. Or possibly shorted some cells out.

[toolguy1000]

or just send them to these guys:

http://www.voltmanbatteries.com/servlet/StoreFront