Multiple batteries

[rlogiacco]

On behalf of a user:

I have a challenging circuit.

From a battery point of view it looks like

Battery A
+---
     !
     1000 ohm
     !
     .-------   Arduino A2
     !
     200 ohm
- ---

And

Battery B
+---
     !
     1000 ohm
     !
     .-------   Arduino A3
     !
     100 ohm
- ---
     -----  Gnd

Both batteries are 12 Volt.

The problem (or challenge) is caused by the fact that the minus of the top battery A is the same as the plus of the bottom battery B.

To measure the voltage for the bottom battery is easy and straight forward ratio n = 1100 / 100.
And the battery definition is: Battery battery1 (9000, 12000, A3);

But what are the parameters to define the Top Battery ? You can not put a ground to the minus of the top battery. Because that would cause that both ends of the bottom battery are grounded. In that case I do not anything to measure the voltage. It will always be zero.

Another setup I tried is

Battery A
+---
     !
     1000 ohm
     !
     .-------   Arduino A2
     !
     200 ohm
- ---   Gnd

And

Battery B
+---
     !
     100 ohm
     !
     .-------   Arduino A3
     !
     1000 ohm
- ---

Now the ground is to the top battery. And I switched the resistors on the bottom battery.
Now for the top battery the parameters are easy. But how the deal with the bottom one?
The same parameters as before, does not work. Maybe because the voltages are from ground to A3 are now negative?

Any ideas?

[rlogiacco]

First of all, Arduino cannot read negative voltages, so your Arduino GND should always go to the negative side of the B battery.

Second point, what you are trying to do is to measure the voltage of a 2S battery pack (2 in series) composed of 12V cells.
Measuring each cell separately (this is what it looks like from your description) can get quite complicated as you'll have to consider the top cell as a 24V battery and the bottom cell as a 12V battery, but the top cell lower voltage will vary, following the bottom cell discharge curve... It can be done, but you'll have to work out some math.

Before going on, unless you have a particular need I can't see, if the two batteries in the pack are used evenly and the two cells have comparable age and manufacturing process, measuring the cells separately might be overkill...

If possible, simply consider the two batteries as one 24V battery and use a voltage divider with a ratio not less than 5 (like 47k and 10k) or your Arduino will blow up as soon as you start your first measure...

[rlogiacco]

Starting with version 1.1.1 you can also provide a mapping function of your choice: in your case the mapping function for battery A will have to take into account the voltage level measured on battery B, like:

// lead acid 12V batteries range between 16V and 11V
// battery A is top cell, battery B is bottom cell
Battery battA = Battery(11000, 16000, A0);
Battery battB = Battery(11000, 16000, A1);

uint8_t multiCell(uint16_t voltage, uint16_t minVoltage, uint16_t maxVoltage) {
  return linear(voltage - battB.voltage(), minVoltage, maxVoltage);
}

#define RA1 47.0
#define RA2 6.8
#define RB1 25.0
#define RB2 10.0

void setup() {
  Serial.setup(9600);
  battA.begin(5000, (RA1 + RA2) / RA2, &multiCell);
  battB.begin(5000, (RB1 + RB2) / RB2);
}

void loop() {
  Serial.print("Battery pack voltage is ");
  Serial.println(battA.voltage()); // battA voltage is the pack voltage
  Serial.print("Battery A charge ");
  Serial.print(battA.level());
  Serial.print("% with voltage ");
  Serial.println(battA.voltage() - battB.voltage()); // the voltage of battery A is the difference
  Serial.print("Battery B charge ");
  Serial.print(battB.level());
  Serial.print("% with voltage ");
  Serial.println(battB.voltage());
  delay(1000);
}

You can find a simulation of the circuit I'm figuring at this link.