Does a li-po/li-ion battery that has a pcm and ntc thermistor need 3 wires?

Thread Starter

WLT

Joined Apr 10, 2016
10
I want a battery with a thermistor for safety but I wanted to know if there are batteries with 2 wires and thermistor internal, instead of three wires connecting to a circuit. I know little about thermistor and cant find anything explaining this. I have searched and seen thermistors as a third wire on the battery but does this mean all 2 wire batteries don't have a thermistor?
 

Dodgydave

Joined Jun 22, 2012
9,205
Only batteries with three terminals or more will have thermistors inside, two terminals means no thermistor, however you can use a battery protection pcb.
 

paulktreg

Joined Jun 2, 2008
784
Could you use a thermal fuse (like this)? This would only require two wires. You would of course have to select a temperature suitable for your application.
 

Thread Starter

WLT

Joined Apr 10, 2016
10
Could you use a thermal fuse (like this)? This would only require two wires. You would of course have to select a temperature suitable for your application.
Thanks for the suggestion. If I were to hook up the 2 wire battery to a charger board, and then from the charger board to my circuit. Assuming the board has a thermistor, I wouldn't need the battery to have one right?
 

Thread Starter

WLT

Joined Apr 10, 2016
10
Could you use a thermal fuse (like this)? This would only require two wires. You would of course have to select a temperature suitable for your application.
Also, I'm not knowledgeable on thermistors, but do the 3 wire batteries, does the thermistor on it need to be connected to a microcontroller? If not I would rather have one like you suggested.
 

Dodgydave

Joined Jun 22, 2012
9,205
Thr thermistor is connected to a microcontroller in the charger, it monitors the temperature of the cells, and if it gets too hot stops the charger.
 

Thread Starter

WLT

Joined Apr 10, 2016
10
Thr thermistor is connected to a microcontroller in the charger, it monitors the temperature of the cells, and if it gets too hot stops the charger.
which do you mean? the thermistor that comes with the charger board? or connecting the thermistor from the 3 wire battery?
 

crutschow

Joined Mar 14, 2008
25,118
A Li battery can explode and cause a fire (as many have) if they are overcharged and overheat.
So the battery charger monitors the thermistor in the battery and terminates the charge when the temperature goes above the battery limit.
 

recklessrog

Joined May 23, 2013
985
The point that is being missed is that the Thermistors resistance changes with temperature, so needs to be tight against the cells, the charge controller connected to the 3rd wire monitors the resistance so that if the temperature rises too much, the charge current is reduced or cut off for safety. Also,that and the charge characteristics of the cell are programmed into a micro processor which is sometimes built into the battery casing so you only get two wires. I recently opened up a failed camcorder battery pack which had the charge and monitoring circuit board all included in the pack but it still had three connections, one common, one for raw charge input to the controller, and the other was the voltage out from the pack which was monitored for overload to shut down the output in case of a short circuit. All clever stuff!!!
 

Tonyr1084

Joined Sep 24, 2015
4,705
If you look close at the picture of the battery and charger you posted you should be able to see under the kapton tape a circuit board attached directly to the battery itself. Though there are only two wires delivering a charge to the battery, the on-battery circuit board monitors the condition of the charge and discharge rates as well as the temperature of the battery.
 

PAFMC01

Joined Mar 20, 2013
11
Information about replacing a 3 wire LIPO into a device that previously used a 2 wire battery is very scarce. I have seen posts that variously say the (usually white) thermistor wire should be connected to ground, to ground via a resistance, left unconnected...pretty much everything except connected to positive.

By measurement and varying the temperature of my test 3 wire battery, it seems that the white wire is simply one end of a 10K thermistor that connects to ground/black/common at the other. 10K seems the most usual thermistor rating, but be aware manufacturers can choose any thermistor. Hence the advice to measure the resistance between white and black on the outgoing battery. The thermistor is located on the battery because that's the temperature one is monitoring.

The thermistor has no control function, but is simply a sensor that when connected to an appropriate lipo charger gives a temperature measure that is used to shut down charging should the battery overheat. It could in principle also be used to vary the charging characteristics to optimize charging at any given normal temperature. In any case it does not seem to have any control function within the battery itself, but is only used by an external charging circuit.

The posts about terminating the connection with a resistance equal to the thermistor resistance make sense, but only when the terminating resistance is connected between the thermistor terminal of the charger and ground. Connecting a resistance between white and black on a battery is meaningless.

The white wire on a 3 wire battery should be left unconnected when that battery is used with a 2 wire charging circuit, because that kind of charger doesn't look at battery temperature.

However a 2 wire battery connected to a three wire charger should have the thermistor connection on the charger terminated, usually with 10Kohm, to ground. That simulates a three wire battery, and fools the charger into passing current by signalling the temperature is "normal". Without such a termination the charger will think the temperature is out of bounds, and stop charging.

In summary:
Connecting a 2 wire battery to a charger with a 3 pin connector.... terminate the third wire with 10K to ground
Connecting a 3 wire battery to a 2 wire charging circuit...... leave the white wire unconnected
The other 2 cases (2 wire to 2 wire and 3 wire to 3 wire) are trivial and obvious

Now for the caveat: this is based on a lot of reading, some measurement, some deduction and a little experimentation.
Your results may vary depending on battery construction and on-board circuitry, and charger design.
And remember LIPO batteries can be dangerous if mistreated so proceed with caution, a current meter and an eye to the temperature.

I welcome anyone responding with better conclusions than mine.
I want a battery with a thermistor for safety but I wanted to know if there are batteries with 2 wires and thermistor internal, instead of three wires connecting to a circuit. I know little about thermistor and cant find anything explaining this. I have searched and seen thermistors as a third wire on the battery but does this mean all 2 wire batteries don't have a thermistor?
 

PAFMC01

Joined Mar 20, 2013
11
Information about replacing a 3 wire LIPO into a device that previously used a 2 wire battery is very scarce. I have seen posts that variously say the (usually white) thermistor wire should be connected to ground, to ground via a resistance, left unconnected...pretty much everything except connected to positive.

By measurement and varying the temperature of my test 3 wire battery, it seems that the white wire is simply one end of a 10K thermistor that connects to ground/black/common at the other. 10K seems the most usual thermistor rating, but be aware manufacturers can choose any thermistor. Hence the advice to measure the resistance between white and black on the outgoing battery. The thermistor is located on the battery because that's the temperature one is monitoring.

The thermistor seems to have no control function, but is simply a sensor that when connected to an appropriate lipo charger gives a temperature measure that is used to shut down charging should the battery overheat. It could in principle also be used to vary the charging characteristics to optimize charging at any given normal temperature. In any case it does not seem to have any control function within the battery itself, but is only used by an external charging circuit.

Having said that, it seems unlikely that the on-board lipo circuitry would have no temperature monitoring capability . I suspect that if it exists, it is independent of the thermistor on the white wire.

The posts about terminating the connection with a resistance equal to the thermistor resistance make sense, but only when the terminating resistance is connected between the thermistor terminal of the charger and ground. Connecting a resistance between white and black on a battery is meaningless.

The white wire on a 3 wire battery should be left unconnected when that battery is used with a 2 wire charging circuit, because that kind of charger doesn't look at battery temperature.

However a 2 wire battery connected to a three wire charger should have the thermistor connection on the charger terminated, usually with 10Kohm, to ground. That simulates a three wire battery, and fools the charger into passing current by signalling the temperature is "normal". Without such a termination the charger will think the temperature is out of bounds, and stop charging.

In summary:
Connecting a 2 wire battery to a charger with a 3 pin connector.... terminate the third wire with 10K to ground
Connecting a 3 wire battery to a 2 wire charging circuit...... leave the white wire unconnected
The other 2 cases (2 wire to 2 wire and 3 wire to 3 wire) are trivial and obvious

Now for the caveats: this is based on a lot of reading, some measurement, some deduction, a little experimentation and no circuit diagrams.
Your results may vary depending on battery construction and on-board circuitry, and charger design.
And remember LIPO batteries can be dangerous if mistreated so proceed with caution, a current meter and an eye to the temperature.

I welcome anyone responding with better conclusions than mine.
 
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