Bypass the thermistor in a drill switch

Have you got a healthy battery you can measure? I'd measure the voltage and resistance of the white wire relative to the negative terminal. Then try to recreate that: either a fixed resistor, or a regulated voltage. I'm assuming it's a logic signal rather than delivering any current.
 
not sure exactly what you mean but unfortunately I dont have any batteries with the drills to check what they are reading to try and re-create.
 
Based on a little research it seems most Li-Ion batteries for quite a while have had intelligent circuitry. So that wire could be doing digital comms.
If it's a thermistor, you can experiment like this: air/open circuit is "infinite" resistance. You could apply a 1meg resistor. Then try 560kΩ, then 220kΩ etc. Right down to 4.7kΩ.

If that doesn't work, you almost certainly have a logic wire on your hands which won't be easy to bypass.
.are the motors simple coils, or do they have some logic/control circuitry?
 
does anyone know where to just buy knackered old batteries for this drill, as I can make this work but need the battery casing? There is nothing on ebay as I guess people dont think they have a value and bin them!
 
I have just popped open a 5 Cell 18v Li-Ion battery I have here on my desk (From a TITAN 18W31), working on the assumption that there is very little variance in the design of these things.

Looking from above, with the open end of the battery contacts facing away, I have from left to right B+ | T | BS | B-

B+ is the +18v feed from the back, B- is the "ground" at 0v

T looks to be (and guessing from it being called T) a data/reference voltage used by the charger to balance the pack (it has an onboard cell balancing IC). This one is not connected to the drill anyway.

BS (Presumably Battery Sense) is connected to the drill (although mine operates with it disconnected also). It comes in to the battery and immediately into a temperature probe wedged between the cells. The other side of that probe is connected directly to B- ("Ground"). When the drill fires up, it sends 5v back down that line into the battery, and by extension through the thermistor to ground. The thermistor (at my current room temperature measures 11kohms. Its resistance reduces at it heats up. This explains why my drill still works without it, it is looking for a resistance to "Pull" the 5v it sends to the battery down toward ground as the battery gets dangerously hot. Since not being connected doesn't pull the value down, it still works.

My suggestion to you would be,

1) Connect power to your switch. Check if some voltage appears on the mystery pin.
2) Buy or steal a 0-10k or 0-100kohm variable potentiometer, set it to 11k (using a multi-meter) then connect it between the mystery pin and ground. Twiddle it, see if it fires up.
3) you could also use a "variety pack" of fixed resistors from ebay, but it involves more twisting stuff together.

Lastly, power electronics for Lithium batteries are made to handle quite a bit of power. That means if you connect stuff wrong, the resultant bang is quite big. The above is a guess only, make sure you are confident and competent when dealing with these things, because I am just some guy on the internet - and worst case a decent short could hurt you or start a fire.

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:D moved over to using a mobility scooter as a base, had all the controls, and they worked without me having to do any electricity bits!
 
TBirdUK said:
:D moved over to using a mobility scooter as a base, had all the controls, and they worked without me having to do any electricity bits!
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I know this is an old thread - but I am trying to do something very similar. I want to use a Decker 20v battery in a Hoover 20V Vacuum. They are both made by the same parent company - but my adapter gas two wires and vacuum has four. Any advice on how to connect it up? Battery adapter has the big red and black wires - but the vacuum has an extra two thin wires, white & yellow.
 
HarveyCat said:
I know this is an old thread - but I am trying to do something very similar. I want to use a Decker 20v battery in a Hoover 20V Vacuum. They are both made by the same parent company - but my adapter gas two wires and vacuum has four. Any advice on how to connect it up? Battery adapter has the big red and black wires - but the vacuum has an extra two thin wires, white & yellow.
Click to expand...
Put a multi meter on the two thin wires, set to its resistance setting. Get a stable reading, put the battery in the freezer for an hour, measure again. If the resistance is higher when you check it after it is removed from the freezer the wires are most likely to a thermistor in the battery pack, to show if it's overheating.
 
TBirdUK said:
:D moved over to using a mobility scooter as a base, had all the controls, and they worked without me having to do any electricity bits!
Click to expand...
I know this is an old thread - but I am trying to do something very similar. I want to use a Decker 20v battery in a Hoover 20V Vacuum. They are both made by the same parent company - but my adapter gas two wires and vacuum has four. Any advice on how to connect it up? Battery adapter has the big red and black wires - but the vacuum has an extra two thin wireswhite & yellow.
Chris Wilson said:
Put a multi meter on the two thin wires, set to its resistance setting. Get a stable reading, put the battery in the freezer for an hour, measure again. If the resistance is higher when you check it after it is removed from the freezer the wires are most likely to a thermistor in the battery pack, to show if it's overheating.
Click to expand...
Thanks for your reply! Sadly I do not have a battery to the Hoover Vaccuum - I just have 20V Black & Decker battery that I am trying to adapt. Elsewhere online I saw some suggest using a 10K Ohm Resister to bypass this - but that was for a different combo...
 
HarveyCat said:
I know this is an old thread - but I am trying to do something very similar. I want to use a Decker 20v battery in a Hoover 20V Vacuum. They are both made by the same parent company - but my adapter gas two wires and vacuum has four. Any advice on how to connect it up? Battery adapter has the big red and black wires - but the vacuum has an extra two thin wireswhite & yellow.

Thanks for your reply! Sadly I do not have a battery to the Hoover Vaccuum - I just have 20V Black & Decker battery that I am trying to adapt. Elsewhere online I saw some suggest using a 10K Ohm Resister to bypass this - but that was for a different combo...
Click to expand...
 
Use the centre and one outer terminal of a 50k Ohm variable resistor and play with the setting until it works? Then measure the resistance across the terminals (with it disconnected from the devices) and replace with the nearest value fixed resistor if you're fussy.
 
Chris Wilson said:
Use the centre and one outer terminal of a 50k Ohm variable resistor and play with the setting until it works? Then measure the resistance across the terminals (with it disconnected from the devices) and replace with the nearest value fixed resistor if you're fussy.
Click to expand...
Sounds like a plan!
 
Hi again, I am stuck - the two middle terminals read as if they are also negative terminals - but I am still mot able to get the vac to go. Here is the inside of these batteries.

i did get an assortment of resistors and tried a 10k Ohm on each of the center terminal wires run to the marked end negative terminal.

any thoughts?
 
Are these two thin wires from the battery pack or the vacuum cleaner? I was thinking they were from the battery pack, but maybe I am wrong....? It (the vacuum cleaner's two wires) might be expecting a simple thermal switch for over temperature protection, and not a thermistor. If it's a switch it could be normally closed when the temperature is safe, or it could be open and closes when the pack is too hot. If you feel brave try shorting the two wires. I presume you have tried it with the wires disconnected?
 
Chris Wilson said:
Are these two thin wires from the battery pack or the vacuum cleaner? I was thinking they were from the battery pack, but maybe I am wrong....? It (the vacuum cleaner's two wires) might be expecting a simple thermal switch for over temperature protection, and not a thermistor. If it's a switch it could be normally closed when the temperature is safe, or it could be open and closes when the pack is too hot. If you feel brave try shorting the two wires. I presume you have tried it with the wires disconnected?
Click to expand...
Thanks! The thin yellow and white wires are two of the four inside the vacuum. I did try not touching them together - and also touching them together - I also tried touch each one and both to the negative black wires - as well as tried both together and separately to a 10k ohm resistor. I think it will try again shorting them to each other and make sure that does not work! I also tried both to the two middle battery terminals (each way) - can hear the motor click but nothing more.
 
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