nRF51822 and battery low level detection

RE: nRF51822 and battery low level detection

MartinBL — Mon, 23 Apr 2018 07:46:26 GMT

At least that is the default setting at system startup:

RE: nRF51822 and battery low level detection

Alisha — Sun, 22 Apr 2018 17:42:10 GMT

It seems I found that, the default NRF_ADC_CONFIG_DEFAULT is using 1/3 scale. am I right?

RE: nRF51822 and battery low level detection

Alisha — Sun, 22 Apr 2018 17:35:29 GMT

Hi Martin,

Thanks for your sample code, but unfortunately I couldn't find where you set prescaler value to 1/3.

Would you please explain where the prescaler value has been set?

Thanks,

RE: nRF51822 and battery low level detection

Carlos Villavicencio — Fri, 17 Feb 2017 23:39:41 GMT

Thanks! Have you ported this sample code to SDK 11?

RE: nRF51822 and battery low level detection

Stefan Birnir Sverrisson — Wed, 20 Jan 2016 16:41:41 GMT

@Tizana The tps62736 DCDC converter chip that you point to seems to be very suitable for nRF51 and lithium battery. You get the best efficiency if you program the output voltage of the DCDC converter to be close to the minimum nRF51 supply voltage of 1.8V. 1.9V would be a good output voltage for the DCDC converter to have a safe margin from the 1.8V minimum.

Another thing. As Alex points out, you should have resistor values in the voltage divider optimally below 1kohm to prevent any ADC output error. A good method however to prevent error with higher resisitor values is to connect a capacitor between the ADC input and ground.

Third thing. The resistor values in your voltage divider are very high. This might be adequate in your situation but your ADC input signal will be very vulnerable to any external noise. This blog post suggests perhaps more suitable voltage divider setup for measuring lithium battery voltage.

RE: nRF51822 and battery low level detection

Tizana — Mon, 07 Sep 2015 14:38:30 GMT

thx alex, i find a good chip for low power energy the tps62736 : www.ti.com/.../slvsbo4c.pdf and it cost only 2.8$. and it can monitor the low voltage level of the battery, i'll keep you in touch.

RE: nRF51822 and battery low level detection

Alex — Fri, 04 Sep 2015 07:41:55 GMT

Look at this chip. Quiescent current and efficiency at your current range is better. I couldn't find better converter for myself, will be happy to hear about more efficient chip. As for divider, you need to remember that input impedance of ADC is not that high and the Reference manual states that the output impedance of voltage source must be 1kOhm or lower. Check page 166. If you don't need to report state of charge of the battery and need only to catch event when battery voltage dropped below let's say 2.8V then I would use voltage monitor chip. If you need to know battery state at any time then Gas Gauge chip can be a solution but it is more expensive solution (need to program it as well).

RE: nRF51822 and battery low level detection

Tizana — Fri, 04 Sep 2015 01:10:33 GMT

Thanks for the answer Alex and Martin, i appreciate that.

here's the schema that i made for the circuit:

I/ as i'm using a rechargeable coin cell battery so the voltage varies from 4.2v when its fully charged to 2.7v when its fully discharged. we note that the battery voltage when its fully charged is above the maximum voltage supported by the nRF51822.

So the only solution is to drop the voltage to 2.5V and to do that there are 2 slolution :

using a linear regulator. this is not a good idea because the efficiency is so bad. (4.2v-2.5v) * I = power dissipated in heat.
using a DC-DC converter. i find a good circuit the LM3671MF-2.5 , here's the datasheet : www.ti.com/.../lm3671.pdf

the input voltage range is between 2.7 V to 5.5 V which is OK for the battery and the output voltage is 2.5V which is OK for all circuit (nRF51822, accelerometer, temp.sensor). the DC-DC converters are know by their good efficiency. but when i took a look at the "Efficiency vs. Output Current" curves in page 9 of the datasheet i notice that the Efficiency are bad for a low current.

For example for Vin = 3.6v and current = 0.1mA the Efficiency = 74%. and as known the nRF51822 consume less current than 0.1mA. so the Efficiency will be lower than 74% (nRF51822 consume 2.6µA @ 3V ON mode, all blocks in idle mode) this is not good for the battery mainly when my objectif is to maintain the battery charge for a long time. this is a waste of energy. do you have another solution to drop the voltage with a good Efficiency and with a low current consumption ??

II/ for the battery low detection i'm gonna use an external voltage divider to measure the battery voltage because the maximum battery voltage can reach 4.2v which is above the ADC threshold even with using a 1/3 prescaler.

so the voltage divider consist of two resistor with high value (470M Ohm) to minimize at maximum the current consumed by the divider circuit. i (max) = 4.2v/ (470 M * 2 ) = 4.4 nA which is insignificant comparing to the battery capacity 120mAh . this circuit will divide the voltage to two. for example for 4.2V the divider will drop the voltage to 2.1V then i will use an internal prescaler of 1/2 to drop the voltage to 1.05V

is this external circuit OK or there are a better other solution ?

Thanks for help

RE: nRF51822 and battery low level detection

Alex — Wed, 02 Sep 2015 11:24:32 GMT

When battery is fully charged it will have 4.2V. Look at the datasheet at the end of page 4 to understand how voltage linked to state of charge. Also you will need to have power supply to reduce 4.2V to 3.6V or less to supply to nRF. It will affect the efficiency of the system. Upd: You probably think that while charging and applying 4.2V to the battery you can disconnect nRF from high voltage and when charger unplugged battery will have nominal voltage 3.6V. It is not correct. Battery will have 4.2V after charger unplugged

RE: nRF51822 and battery low level detection

Tizana — Wed, 02 Sep 2015 11:15:39 GMT

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,

RE: nRF51822 and battery low level detection

MartinBL — Wed, 02 Sep 2015 09:39:09 GMT

You are right, of course. I didn't notice the datasheet and assumed 3.6V would be the maximum.

RE: nRF51822 and battery low level detection

Alex — Wed, 02 Sep 2015 09:21:48 GMT

His battery is Li-ion rechargeable and when fully charged provides 4.2V.

RE: nRF51822 and battery low level detection

MartinBL — Wed, 02 Sep 2015 09:16:11 GMT

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.

RE: nRF51822 and battery low level detection

Alex — Wed, 02 Sep 2015 09:05:32 GMT

First of all you need to remember that maximum voltage on your battery will be 4.2V. As for measuring it, it is fully described in nRF51 Series Reference Manual page 165. You most likely need to use external voltage divider and not to worry about ADC reference voltage because it is lower(1.2V) than minimum battery voltage.