Battery Bank State-of-Charge Monitor

LiFePO4 voltage stays almost flat between about 20% and 90% charge, so a voltmeter can't tell you much. Coulomb counting adds up current over time — the same method used by commercial battery monitors.

Mission steps

1. Place the sensor

   Put the INA219 in series with the battery's negative or positive lead so every load and charge current passes through it.

2. Mind the limits

   The common INA219 board with a 0.1 Ω shunt reads up to about 3.2 A and 26 V, which suits small 12 V banks; bigger systems need a higher-current shunt monitor.

3. Count amp-hours

   Every second, add charge current and subtract load current from the stored amp-hours.

4. Re-sync at full

   When voltage reaches the charger's full level and current tapers off, reset to 100% to cancel accumulated error.

5. Show it

   Print to a terminal, an OLED or a small Flask page with percentage, watts and estimated hours left.

Code

import time, board, adafruit_ina219

ina = adafruit_ina219.INA219(board.I2C())
CAPACITY_AH = 6.0
ah = CAPACITY_AH            # assume full at start
FULL_V, TAPER_A = 14.2, 0.1 # adjust to your charger
last = time.monotonic()

while True:
    time.sleep(1)
    now = time.monotonic(); dt = now - last; last = now
    v, a = ina.bus_voltage, ina.current / 1000   # amps; + = charging (depends on wiring)
    ah = max(0, min(CAPACITY_AH, ah + a * dt / 3600))
    if v >= FULL_V and 0 <= a < TAPER_A:
        ah = CAPACITY_AH
    soc = 100 * ah / CAPACITY_AH
    hours = ah / -a if a < -0.01 else float('inf')
    print('%.2f V  %+.2f A  %5.1f %%  %.1f h left' % (v, a, soc, hours))

Tip: test every script on the bench before installing it in a moving machine.

Questions