Monitoring 2 x 18650 batteries

[Crumpy10]

Hi Guys,

I have been playing around with the Battery Powered Sensor sketch, trying to get it to report correctly the level of 2x 18650 batteries. I am not a programmer and am getting a bit lost with the sketch. I have a voltage divider of 2.2Meg and 330K, So in my calculations 2 x 18650 batteries give me a max voltage of 8.4vdc fully charged, therefore with that divider Vout should be 1.096vdc or around that. So I have calculated that my Vmax is 1.100007332 and my Volts per bit for the sketch should be 0.001075275.
But when I run the sketch I am getting around 1vdc on the serial monitor, when the voltmeter is reading 7.90vdc

Heres the Loop part of my sketch, can some one point out my mistakes please?

void loop()
{
	// get the battery Voltage
	int sensorValue = analogRead(BATTERY_SENSE_PIN);
#ifdef MY_DEBUG
	Serial.println(sensorValue);
#endif

	// 1M, 470K divider across battery and using internal ADC ref of 1.1V
	// Sense point is bypassed with 0.1 uF cap to reduce noise at that point
	// ((1e6+470e3)/470e3)*1.1 = Vmax = 3.44 Volts
	// 3.44/1023 = Volts per bit = 0.003363075

	int batteryPcnt = sensorValue / 10;

#ifdef MY_DEBUG
	float batteryV  = sensorValue * 0.001075275;
 
	Serial.print("Battery Voltage: ");
	Serial.print(batteryV);
	Serial.println(" V");

	Serial.print("Battery percent: ");
	Serial.print(batteryPcnt);
	Serial.println(" %");
#endif

	if (oldBatteryPcnt != batteryPcnt) {
		// Power up radio after sleep
		sendBatteryLevel(batteryPcnt);
		oldBatteryPcnt = batteryPcnt;
	}
	sleep(SLEEP_TIME);
}

Thanks

[mfalkvidd]

@crumpy10 do you have analogReference(INTERNAL); in setup()?
Are you using a 3.3 or 5V device?

[Crumpy10]

@mfalkvidd I have left the analogReference as it was in the original like this-:

void setup()
{
	// use the 1.1 V internal reference
#if defined(__AVR_ATmega2560__)
	analogReference(INTERNAL1V1);
#else
	analogReference(INTERNAL);
#endif
}

void presentation()
{
	// Send the sketch version information to the gateway and Controller
	sendSketchInfo("Battery Meter", "1.0");
}

Just for learning about battery monitoring, I am using a Nano with two 18650's in series to power it. in my final project I will be using a Pro Mini. Unfortunately the ultrasonic sensor I need to use is a 5v sensor and needs a good supply. Hence learning about monitoring the 18650's.
Cheers.

Thanks for the reply.

[mfalkvidd]

I did the calculations and got the same result as you, so that part seems ok.
No sure what else to check :(

[Crumpy10]

@mfalkvidd Well thanks for looking at it, at least my maths is still working ok! Perhaps someone else may know whats going on...

The strange thing is if I use this line-:

float batteryV = sensorValue * 8.6 / 1023;

I can get close. I found another thread where @TimO was using 7.2v with this in his sketch-:

float batteryV = sensorValue * 6.1 / 1023;

So I just increased the 6.1 slowly until my read value was close to my multimeter voltage. But I dont understand what the 6.1 in this line of code is doing, or where it comes from...

[mfalkvidd]

@crumpy10 maybe double-check that the resistors have the values you think they have?

[Nca78]

@mfalkvidd said in Monitoring 2 x 18650 batteries:

@crumpy10 maybe double-check that the resistors have the values you think they have?

That + imprecision of the voltage reference. If I remember well, it's stable over time, but not precisely at 1.1V.

[electrik]

You mentioned your two resistors, but how they are placed? So which one is connected to gnd and which one to the batteries?

[zboblamont]

@nca78 Correct, easily verified and corrected for however...

[kimot]

My way for monitoring battery.
Usually I have not got precise resistor with tolerance +-0.5% etc. for correct calculation, so I build resistor divider with what I approximately found.
Load my device with simple sketch, which reads data from ADC and write this raw ADC data to serial port.
I make notice, that for example 1009 corresponds to 6041mV with fresh battery measured with multimeter.
In my final sketch then using "magic" map Arduino function:

int raw_volt = analogRead(A1);

int volt = map(raw_volt, 0, 1009, 0, 6041);

Not very useful for "mass production", but for my prototyping it is ok.

[Crumpy10]

@electrik Hi, the 2.2Meg is connected to the battery and the 330k is connected to GND.

Cheers

[Crumpy10]

@mfalkvidd been there already but thanks.

[electrik]

What is the value you read from the analog port, when the battery is 8.4v? So the value before calculating the voltage.

[Nca78]

@kimot said in Monitoring 2 x 18650 batteries:

My way for monitoring battery.
Usually I have not got precise resistor with tolerance +-0.5% etc. for correct calculation, so I build resistor divider with what I approximately found.
Load my device with simple sketch, which reads data from ADC and write this raw ADC data to serial port.
I make notice, that for example 1009 corresponds to 6041mV with fresh battery measured with multimeter.
In my final sketch then using "magic" map Arduino function:

int raw_volt = analogRead(A1);

int volt = map(raw_volt, 0, 1009, 0, 6041);

Not very useful for "mass production", but for my prototyping it is ok.

For "mass production", you would run a config script, with known vcc applied to the board, and save measured value in EEPROM. Then you just have to reload this value to pass to the map function.

[Crumpy10]

@electrik Not sure what it would be at 8.4v because that would be two fully charged 18650's and the two I have on test are currently giving 7.80v on the multimeter and returning about 870 on the serial monitor, reporting battery at 0.94v and 86%

I would need to fully recharge two batteries later to tell what the max would be.

[zboblamont]

@crumpy10 Am a bit rusty on this, but would an ADC reading 870 on a 7.8v supply not translate to an ADC reading of 1023 and 9.17v.?
Replacing the 6.1 factor in "float batteryV = sensorValue * 6.1 / 1023;" with 9.17 will derive the true voltage.
As to the weird results, it suggests the INTERNAL reference is not latching, perhaps remark out the #if define... etc lines to leave "analogReference(INTERNAL);" entirely on it's own.

[electrik]

You should know at what voltage level the ADC tells you 1023. It can be 1,1 but if it is at 1,12 you're already far off with your calculation.
So probably calibrating like mentioned above would be the best.

[tochinet]

A few "reality checks" with ADCs :

• The precision of the ADC is limited to 0.1% at full scale (1024). This means that when measuring smaller values, it's less precise. For example, when measuring 1/10 of the reference, your precision is down to 1% already.
• Analog noise is usually in the range of 10mV is your circuit is carefully designed. Similar to above, it has a bigger effect on small values, but also on smaller references (1.1V is 3x more sensible to noise than 3.3V). Add a 10nF capacity in parallel to your 330k for better resilience.
• ADC is slow and needs time to calculate. Wait enough time between changing parameters and reading value (the Ardunio routines don't).

I'd suggest you do a for() loop of 20x reading your ADC, and see the variation : mean(), std() and trend (for example last 5 - first 5). It could learn you a few things.