Measure a LiPo battery voltage

[almeos]

Hello,
I have a lopy with an expansion board and a lipo battery connected on the jst plug.
I made some research but I do not find, for my configuration, a simple micropython command to read the battery voltage? (equivalent of read_battery_voltage() for pysense)
Could someone help me please?
Denis

[troy salt]

You can create a voltage divider and read the voltage with ADC read. Then do some simple math to back out the voltage.

[almeos]

@troy-salt Thank you for your answer.
A voltage divider is an additional equipment that I have to connect on the expansion board?

[robert-hh]

@almeos If you are using the pycom expansion board: that one gas already a voltage divider built-in, which allows to measure the battery voltage at P16. The Jumper 'Bat' has to be in place for that. The division factor is 2, so the voltage you get at P16 is up to 2.1V. You have to set the ADC to a 11dB attenuation. If you measure 1.8V, the battery voltage is 3.6V. Since the impedance of the voltage divider is rather high, the measurement will be noisy. Sampling several times and calculating the average gives better results.

Note: If you have an older expansion board, the attenuation factor may be 3.

[almeos]

@robert-hh Thanks for your answer.
I placed BAT jumper. My expansion board is V3.1.
I wrote this code:

>>> from machine import ADC
>>> adc = ADC()
>>> adc.channel(attn=ADC.ATTN_11DB, pin='P16').value()
2499
>>> adc.channel(attn=ADC.ATTN_11DB, pin='P16').value()
2492
>>> adc.channel(attn=ADC.ATTN_11DB, pin='P16').value()
2479
>>> adc.channel(attn=ADC.ATTN_11DB, pin='P16').value()
2515
>>> adc.channel(attn=ADC.ATTN_11DB, pin='P16').value()
2497

With an average result of 2500, the voltage should be 2500 x 2 = 5000 mV.
I measured my battery with a voltmeter: 3810mV.
Where is my mistake?

[Gijs]

You probably have USB connected, which would explain why its 5V :) To test the battery voltage, you should remove the USB cable and perhaps use a separate USB-Serial converter to get the data out, or a telnet connection

[almeos]

@Gijs
Yes you are right, it was battery and microUSB both powered.
I made the same test with the expansion board powered only by the battery (with wifi connection), the result is different (better) but it still does not correspond to the real battery voltage (3810mV):

>>> from machine import ADC
>>> adc = ADC()
>>> adc.channel(attn=ADC.ATTN_11DB, pin='P16').value()
2162
>>> adc.channel(attn=ADC.ATTN_11DB, pin='P16').value()
2174
>>> adc.channel(attn=ADC.ATTN_11DB, pin='P16').value()
2160
>>> adc.channel(attn=ADC.ATTN_11DB, pin='P16').value()
2155

[Gijs]

Not sure how to explain this, but it looks like the voltage declines from the charging voltage (around 4.2V) to the battery voltage you measured (3.8V). There could be a slight delay due to the battery chemistry. Also the ADC in the module is not 100% correct. By the way, I'd recommend to use the following instead: (as you keep re-instantiating the ADC internally)

from machine import ADC
adc = ADC()
bat_voltage = adc.channel(attn=ADC.ATTN_11DB, pin='P16')
bat_voltage.value()

#this should return the voltage with the correct attenuation set above, but without the attenuation from the resistor divider
#bat_voltage.voltage()

[jcaron]

@almeos you should use .voltage, not .value.

.value is the raw value of the ADC in its range which IIRC is 0-4095 (4095=1.1V at 0 dB).

If you use .value you need to divide by 4095 and multiply by the max value at the chosen attenuation to get the actual voltage.

[robert-hh]

@almeos The value returned by adc.value() is to be taken with respect to the full range value of 4096, which you read at a input voltage of about 3.5 V. It is NOT a mV value, and the full range Voltage varies from device to device.
So a reading of 2160 corresponds to 2160/4095* 3.5 *2 mV ~3.7 V.

Edit: In any case you have to make your own calibration.

[almeos]

@Gijs @robert-hh @jcaron
Yes it works! Thanks for your help:

>>> from machine import ADC
>>> adc = ADC()
>>> bat_voltage = adc.channel(attn=ADC.ATTN_11DB, pin='P16')
>>> bat_voltage.voltage()
1898
>>> bat_voltage.voltage()
1898
>>> bat_voltage.voltage()
1888
>>> bat_voltage.voltage()
1894
>>> bat_voltage.voltage()
1893