# Monitor a Lithium-thionyl Chloride battery charge level

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#### priestnot

##### Member level 5
Hello once again...

I have a problem with making a battery level monitoring.

I am building a RF low power module that is powered by a Lithium-thionyl Chloride battery https://www.farnell.com/datasheets/1445890.pdf

The problem is that the discharge curve is to abrupt.

The system has the following power cycles:

Trun = 6ms; Irun = 36mA

Tidle = 18mA; Iidle = 18mA

Tsleep = 2000ms; Idle = 1uA

So the average current is of aprox = 400uA

I am using the CC1110 SoC that has a internal VCC monitor so I can measure the voltage of the power source.
The problem is as the discharge curve is abrupt it goes from 3.6 to 0 very fast.

Is there a way to monitor this battery using the VCC monitor? Or any other method?

#### mtwieg

For lithium chemistry, measuring terminal voltage is not enough to judge capacity, you need to actually measure charge with a so-called "gas gauge."

#### Sunny55

##### Full Member level 3
For lithium chemistry, measuring terminal voltage is not enough to judge capacity, you need to actually measure charge with a so-called "gas gauge."

Yes but this way it can give you a basic idea of battery amount left although its not very accurate. What about going for simple battery level indicator circuits?

#### tpetar

Hello once again...

I have a problem with making a battery level monitoring.

I am building a RF low power module that is powered by a Lithium-thionyl Chloride battery https://www.farnell.com/datasheets/1445890.pdf

The problem is that the discharge curve is to abrupt.

The system has the following power cycles:

Trun = 6ms; Irun = 36mA

Tidle = 18mA; Iidle = 18mA

Tsleep = 2000ms; Idle = 1uA

So the average current is of aprox = 400uA

I am using the CC1110 SoC that has a internal VCC monitor so I can measure the voltage of the power source.
The problem is as the discharge curve is abrupt it goes from 3.6 to 0 very fast.

Is there a way to monitor this battery using the VCC monitor? Or any other method?

This discharge way is normal for rechargable Lithium based batteries, just see graph in datasheet what you attach.

Best regards,
Peter

:wink:

#### priestnot

##### Member level 5

For lithium chemistry, measuring terminal voltage is not enough to judge capacity, you need to actually measure charge with a so-called "gas gauge."
Do you know any that is cheap and simple to use? My micro controller does not have SPI, I2C and the serial port is already in use.

Yes but this way it can give you a basic idea of battery amount left although its not very accurate. What about going for simple battery level indicator circuits?
Simple battery level circuits like what?

This discharge way is normal for rechargeable Lithium based batteries, just see graph in datasheet what you attach.
This one is not rechargeable the discharge curve is more like this: **broken link removed** and it make very difficult to detect low battery based on voltage it goes from 3.6V to 0V in a very abrupt way.

#### tpetar

This one is not rechargeable the discharge curve is more like this: **broken link removed** and it make very difficult to detect low battery based on voltage it goes from 3.6V to 0V in a very abrupt way.

Its the same curve.

Best regards,
Peter

;-)

#### priestnot

##### Member level 5
They do not seam the same mine it goes from (at the end of its life) 3.7V to 3.6V and the to 0V almost instantly in your diagram at 30% it as 2.5V and at 10% it as something like 3.38V.

In your plot if I am looking for 3.4 V to issue a low battery warning I am at 10% of its load. In my case if I look for 3.4 it is already depleted.

I hope I am not making any mistake.

#### mtwieg

Yes but this way it can give you a basic idea of battery amount left although its not very accurate. What about going for simple battery level indicator circuits?

Only if you compensate for the battery's internal temperature, which has a much greater effect on voltage than charge. ESR also comes into play if current draw is significant.

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