Litmux Studio

ProjectProjects · 30 min

Project: a solar battery monitor

A solar home system's 12 V lead-acid battery must be watched by an Arduino Nano, whose analog input A0 reads 0 - 5 V (0 - 1023). The battery is 12.6 V when full, 14.4 V while charging and should never be run below about 11.9 V. The Nano's sketch (double-click the board) is already written for a 22 kΩ over 10 kΩ divider and prints the battery voltage on the Serial Monitor - but the circuit on the bench was built with 10 kΩ over 10 kΩ. Find the problem, fix it, and test the monitor across the battery's whole range.

The circuit for: Project: a solar battery monitor
The circuit you start with

The steps

  1. Press RUN, then double-click the Nano and choose Open the editor: its Serial Monitor tab shows what the sketch prints - "A0 = 1023, battery = 16.00 V". Impossible for a 12.6 V battery.1023 is the largest number the 10-bit ADC can give: the input is at or above 5 V.
  2. Find out why: measure the voltage at A0 - the multimeter on V⎓ across R2. It reads 6.3 V: half of 12.6 V, more than the 5 V the input can read (and more than it is designed to take). The ADC is stuck at full scale.A0 is wired to the middle of the divider: R2's top end.
  3. Fix the hardware to match the design: change R1 to 22k. A0 now sees 12.6 × 10 / 32 = 3.94 V (reading about 806), and the Serial Monitor shows the battery at about 12.6 V.Double-click R1 and type 22k. The sketch's (R1 + R2) / R2 = 3.2 now matches the circuit.
  4. Test the top of the range - the battery charging at 14.4 V (double-click B1). A0 gets 4.5 V (about 921): still safely under 5 V, and the monitor reads 14.4 V.Set B1's voltage to 14.4.
  5. And the bottom: a flat battery at 11.0 V. A0 gets 3.44 V (about 703) and the monitor reads 11.0 V - time for the controller to disconnect the loads.Set B1's voltage to 11.

What it shows