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where to find temp sensor for 18 volt batteries? Answered

I  work  for a  company that  has  a large  amount  of   cordless drills  adn saw alls  and power  tools. I  have been ask to   see what  it takes to repair the    battery  packs .  I have  a few  questions  i would like to ask if  someone  can help me with  ?

1.  Where  can   one  find  the  temp  sensor  switches   that are  on  the    milwauki red lithium  and   dewalt  18 volt  nicad  battery  packs.

2. does anyone  have  a better way  of    building them other than  soder  each  battery.? I found  a battery spot   welder  but it is  like  $5,000 bucks.  Take  a long time   to pay  for that.   I  soder  my  rc  battery  packs   but  just looking for a better  more cost effective way.

Thnaks for any help anyone  can  give.



They're just thermistors: The trick is to find ones of the correct range. Try and measure one at room temperature, about 25 C. Thermistors are specified by their resistance at 25C.

Also, there are DIY tab welders that cost considerably less than 5K.
Soldering batteries directly is EXTREMELY bad for the,

I forgot to mention:  Not all battery packs use solder or spot-welding to connect the cells together.

There exists a thing called a "battery holder",

wherein the cells are held in place by contact forces. You know, friction and little metal springs, and you can take the cells out, and put them back in again, by hand.

You have probably seen such "battery holders",  but never inside the battery pack for a power tool.

I'm just saying there might exist a way to build a battery pack with battery holders, or some way of connecting the batteries electrically with just contact forces, little compressed springs, or something...

1 They are thermisters and hardly ever go wrong. It's a common component.

2 Welding or soldering are the options I am afraid. The most common issue is failed NiMh batteries. often only 1 or 2 cells but that effects the performance.

The component you refer to as "temp sensor switches" is usually a thermistor; that is a resistor whose resistance changes with temperature.

You can see a picture of one here:

Thermistor Specification

Like the more common, non-temperature dependent, resistor, this is a two terminal device, and if you have an ohmmeter, or a multimeter with resistance measurement  setting, then you can actually measure the resistance offered by this component.

If you're like most people who write to this forum, you will find the idea of actually making  measurements to be repulsive.  It's like you think the people who answer the questions possess godlike omnipotence, or maybe we've got an informant working inside of the engineering departments of Dewalt(r) or Milwaukee(r).

The sad truth of the matter is the people answering this forum are not vastly more enlightened than you are.  I guess good help is hard to find.

Back to the subject of thermistors, the way these things are specified are using two numbers:  R(25C),the resistance at 25C, and the temperature coefficient α, so that the actual resistance as a function of temperature is approximately:

R(T) = R(25C) + α*(T - 25)

From the equation you can see that is just a straight line with slope equal to α.   I won't promise that that's the curve you'd actually get, if you plotted R, at several different temperatures, but it is approximately correct, approximately a straight line.

More importantly, those are the two numbers the electronic parts mongers use to sell thermistors.  So if you go looking for thermistors at some place like Digikey, or Jameco, or some similar partsmonger, they should mention those numbers (room temperature resistance and temperature coefficient) in their catalog, in the section where they are selling thermistors.

Because otherwise, you wouldn't know what they were selling, or how to order the right one.

Also I'm hoping that now you at least have a few clues about how to test the thermistors you've got.

Connecting Battery Cells
The manufacturers of battery packs connect one cell to the next using metal tabs, and these tabs are spot welded to the terminals on each cell. 

There are two reasons why spot welding is used rather than soldering: (1) Because it is faster.  (2) Because the amount of heat delivered to the cell is much less.

It seems that getting the cells too warm is BAD.  I mean that's why they put a thermistor on them for temperature sensing.  Right?   So you kind of have to wonder about soldering to the cells, and about how much heat is being delivered to the cell by putting a soldering iron on it?  Maybe you just have to do it fast, make the solder joints so quickly that a minimum of heat is delivered to the cell.

You'll find lots of chatter on this subject.  I mean there is anecdotal evidence that soldering is harmless, if you do it right, people on forums who will write about how they rebuilt a battery pack, soldering each cell, and how their battery pack worked great after this.  In fact it looks like there's one in the related panel ------------->

I don't know much about spot welding battery tabs myself, but I am guessing this is the superior way to attach cells together, if you can find a way to make it affordable.

Also there are plans for homemade capacitive discharge spot welders out there.