Understanding Battery Capacity

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As you cannot see electricity, it is hard to confirm the exact amount of energy available in a battery bank. You can use a simple hydrometer to measure how much energy is stored in a battery bank, but you will need to be careful when handling the corrosive acid - a multifunction meter is a better alternative.

These meters function like an automobile's gas gauge: they indicate the volume in the tank. They have many features but the most important is the measurement of battery voltage, energy capacity (Ah) and current flow in (charge) or out (discharge) of the battery. They can be placed far away from the battery bank for easy reading.

Meters function through a shunt connection which is a high-capacity resistor used to bypass small current flows in and out of the battery. The circuit has a DC rating that allows easy recording of current direction. The energy shunted is a ratio that is measured based on the total energy flowing into the battery circuit. An increased current flow in or out of the battery circuit will calibrate the current before flowing into the meter.

The meter constantly monitors current flow in and out of the battery to record the amount of energy in amp-hours and its percentage, but it is not 100% accurate. The conversion of electricity into chemical storage in the battery's electrolyte is also not 100% efficient - some energy may be lost during the battery's cycling (charge and discharge) process. Moreover, this inefficiency is inconsistent during a cell's lifetime. Consequently, regular recalibration of the energy meter is required and should be performed when the battery is fully charged (use a hydrometer to determine its status).

Battery voltage and electrolyte specific gravity are directly related:

Battery voltage = Electrolyte specific gravity + 0.84

For this equation and the reading of the hydrometer to be accurate, the batteries must have a light or no load and the electrolyte must be at room temperature. The measurement should be performed about two hours after the batteries are charged to allow dispersion of gas bubbles that affect the gravity reading.

The ideal way to confirm the energy level accurately and the battery's general "health" is a regular measurement of the specific gravity through a hydrometer. A thermometer can be used, as recommended by the manufacturer's specifications, to check the specific gravity of the electrolyte, which can be very cold or very hot.

Batteries can be recharged several times a year through an equalization charge process that uses renewable energy or the grid. This process keeps the batteries fully charged and also resets the meter's calibration of the battery level so that it gives more accurate readings for a few months.

Besides the described "main" features above, there are also some "secondary" indicators from the metering process:

  • Length of days after batteries are fully charged
  • Total amp-hours of energy charged or discharged after installation
  • Battery recharge time
  • Excessively low voltage
  • Charging the battery
  • Fully-charged battery