nRF51822 and battery low level detection

[Tizana]

Hi,

I'm making my own prototype board using the nRF51822. this board is powered by a rechargeable battery lir2450 (3.6V): www.powerstream.com/.../Lir2450.pdf

i want to know how can i make the nRF51822 chip know when the battery is low. i want something indicating me that the battery is being emptied like, led, Buzzer... .

I was thinking about using one of the ADC input of the nRF51822 to measure the voltage battery and when the voltage drop to a certain threshold that's mean the battery level is low.

But this is won't work because the ADC reference voltage will drop also as its powered it by the same battery.

so i want to know if the nRF51822 have an internal low battery detection or something like that which let me know that the battery level is low ?

Best regards,

[MartinBL]

Hi

You can use the nRF51's 1.2V internal voltage reference, Vbg. This voltage reference will not change when the battery is drained. Then you can connect Vdd directly to one of the analog inputs and use 1/3 prescaling. The prescaling will then divide your input voltage at 3.6V down to a maximum of 1.2V which is within the range of Vbg. Refer to the Reference manual v3.0 Chapter 31 for details about voltage references and prescaling.

I have attached an example. It is based on the ble_app_hrs example in SDK 9. I have configured the ADC to use Vgb as reference voltage, measure voltage at AIN2 (P0.01) and to use 1/3 prescaler. You don't need any external components, just connect Vdd directly to AIN2. A timer updates the battery voltage value every second. To use the example extract the .zip-file to "your_SDK_9.0.0_folder\examples\ble_peripheral". Search for "//NORDIC" in main.c to find the changes I have made and further comments. Note that the ADC is sampling continuously and I have not cleaned up the code after making the changes to the battery service. So you need to do some cleanup on your own to optimize the example.

Example: adc-battery-measurement

Keep in mind that 3.6V Vdd is at the edge of what the nRF51 can handle. Refer to the Product specification v3.1, Chapter 7.

[Tizana]

THX for the answer,

actually i'm gonna choose this one :www.farnell.com/.../1475807.pdf the nominal voltage 3.6V and charging voltage 4.2. this mean the voltage not exceed 3.6v??

i'll seprate the charging battery circuit and the supply voltage for the circuit using a switch.

I'm worriying if the coin cell votage will exceed a little bit the 3.6V like 3.62V what gonna happing then ??

can i add an external circuit to protect the nRF51822 ?? and this is gonna affect the efficiency of the power meaning the power will discharge so fast using the protection external circuit

Regards,