Monitoring 2 x 18650 batteries
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@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...
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@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.
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@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.
@nca78 Correct, easily verified and corrected for however...
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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.
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You mentioned your two resistors, but how they are placed? So which one is connected to gnd and which one to the batteries?
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@mfalkvidd been there already but thanks.
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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.
@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.
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What is the value you read from the analog port, when the battery is 8.4v? So the value before calculating the voltage.
@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.
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@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.
@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. -
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.
