Quick Answer
A flooded lead-acid battery may not charge due to a faulty or disconnected terminal, excessive internal resistance, or a sulfated battery that requires equalization. Check for loose connections and clean or replace the terminals if necessary. Perform a load test to determine internal resistance.
Inspect Terminal and Connections
Inspect the battery terminals for any signs of corrosion or damage. Clean the terminals with a wire brush or replace them if necessary. Make sure all connections to the battery are secure and not loose. Use a multimeter to check the voltage drop across the connections. If the voltage drop is high, it may indicate a faulty or corroded connection.
Check Internal Resistance
Perform a load test on the battery to determine its internal resistance. Connect a load tester between the battery and a load, such as a light bulb or a small motor. Measure the battery voltage with a multimeter before and after applying the load. If the voltage drop is excessive, it may indicate high internal resistance, which can prevent the battery from charging. A typical flooded lead-acid battery should have an internal resistance below 10 milliohms, but this can vary depending on the battery type and age.
Equalize the Battery
If the battery is sulfated, it may require equalization to restore its capacity. Sulfation occurs when the battery is left in a state of partial charge for an extended period, causing the lead plates to form a hard, brittle sulfate layer. To equalize the battery, connect it to a charge source and set the charge controller to a high voltage (around 2.45 volts per cell) for several hours. Monitor the battery temperature and voltage during the equalization process, and adjust the charge controller as necessary to prevent overheating.
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