Simple and cheap Battery Pack temperature management/monitoring

mhoekstra

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Dec 24, 2018
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Hi!

This is my first post on this forum.
I'm open for constructive feedback regarding the way I post this (if necessary) ?

I'm looking for a way to monitor the temperature of a battery pack or the temperature of (a few) single cells.
I'm not a big fan of a standard (and often cheap) BMS but also avoiding a really expensive (but professional) BMS for now.

My first idea is to put 1 thermistor (thermal resistor) between every 4 cells in the pack. A pack with for example 40 cells would have 10 thermistors. The leads to these thermistors should go to 1 monitoring device that would simply cut of all power to and from the whole pack (with a relay), when 1 of thermistors reads a temperature that is to high. Or it could sound an alarm.
A temperature monitor on AliExpress is very cheap, but of course most of the time it only has one sensor/thermistor with relay. Is there a chance your could wire multiple thermistors to this one temperature sensor, so the the relay will engage when 1 of the thermistors reads a high temperature?

My second idea is 1 dedicated thermistor to each cell. Since a thermistor can limit current when it's getting hot, it could limit (or totally cut off) the current going to the cell, when it's connected between the charger and the battery. Thus possibly preventing it from getting worse. It sounds like a very cheap and reliable option to protect cell from overheating, whiteout a central monitoring device.
Is this a very stupid idea, is this a good idea and are there more things to take into account or is it a good idea but am I missing something why it wouldn't work in practice?

Marc
 

Chablism

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Jan 27, 2018
Messages
177
image_avbeca.jpg


This style tab spring holder already has a cutout

Could this simplify the Conversion?
 

mhoekstra

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Joined
Dec 24, 2018
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Chablis_m said:
image_avbeca.jpg


This style tab spring holder already has a cutout

Could this simplify the Conversion?
Thanks for your reply!

How could this cutout be helpful for temperature monitoring?
 

Riplash

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May 27, 2018
Messages
66
Hello Marc,

Are you familiar with Aurduinos or similiar? I have been saving a lot of the thermistors from the battery packs I take apart. Then I make a simple voltage divider using a 5kohm resistor and a 1k ohm resistor. I measure the voltage and hence the resistance of the thermistor through the analog port. The offbrand arduino nano costs about $5 usd and the two resistors cost maybe $0.10. The nano has 6 Analog ports, and I think you could add a multiplexer if you wanted to measure more.

Let me know and I will post the sketch, pictures, and stuff here. or try setting it up on my Github.

Cheers,
Rip
 

mhoekstra

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Joined
Dec 24, 2018
Messages
3
Riplash said:
Hello Marc,

Are you familiar with Aurduinos or similiar? I have been saving a lot of the thermistors from the battery packs I take apart. Then I make a simple voltage divider using a 5kohm resistor and a 1k ohm resistor. I measure the voltage and hence the resistance of the thermistor through the analog port. The offbrand arduino nano costs about $5 usd and the two resistors cost maybe $0.10. The nano has 6 Analog ports, and I think you could add a multiplexer if you wanted to measure more.

Let me know and I will post the sketch, pictures, and stuff here. or try setting it up on my Github.

Cheers,
Rip

Sounds pretty interesting!

I have an Arduino and i'm really interested how you have set this up. Do you think it's possible hooking up more then 1 thermistors in series parallel to one analog port, and just measure a specific range of voltage? When one of the thermistors opens due to high temperature, the Arduino recognizes this?

What are your thoughts about the second idea I have regarding hooking up the thermistor to the cell itself?
 

Charly144

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Nov 22, 2017
Messages
287
Use esp8266 with several da18b20, maybe together with espeasy as software...
 

Riplash

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May 27, 2018
Messages
66
Sounds pretty interesting!

I have an Arduino and i'm really interested how you have set this up. Do you think it's possible hooking up more then 1 thermistors in series parallel to one analog port, and just measure a specific range of voltage? When one of the thermistors opens due to high temperature, the Arduino recognizes this?

What are your thoughts about the second idea I have regarding hooking up the thermistor to the cell itself?

Hello Marc,

It sounds like you are thinking about using a thermal cutoff fuse. That opens the circuit when it gets too hot. I think it ispossible to run more than 1 thermal cutoff fuse to an analog port with a ladder of resistors. I think you would spend a lot of effort avoiding ground loops and short circuits, and might cook the analog port if it sees more than 5V. I would use an inexpensive 16 channelmultiplexer hooked up to an analog port, then sample channel 1 (one cell/ cell fuse), then sample channel 2 (the next cell/fuse) and so on.

The thermistor is a resistor thatchanges resistance with changes in temperature and is used to measure the temp.
-Rip
 
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