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Thread: Charging system questions

  1. #1
    Boater Bill
    How many volts should a fully charged battery register? Is there a way to do a load test on the battery in my boat? Is there an easy way to check the alt output? I just have simple tools, ie multimeter. Thanks!

  2. #2
    12.6 is 100% charged batt / 12.4 is 75% / and 12.2 is 50 %
    Once the battery is determined to be in good working condition, connect the positive voltmeter lead to the starter where the battery cable is attached. Connect the negative voltmeter lead to the starter case making sure you have good contact with the metal. This will insure that the positive and negative circuits will be tested. Disable the ignition spark. This is important. On conventional ignition systems, this can be as simple as disconnecting the coil positive wires. It is not a simple thing on electronic ignitions. You must consult the manufacturer's specifications on methods to avoid damage to electronic ignition. Diesel engines may require that someone holds down the kill button on the engine to stop the flow of fuel so the engine will not start. Crank the engine for about 10 seconds while observing the voltage reading. Use of a meter with clip lead ends will aid in this measurement. A reading of at least 9.5 volts would indicate there is sufficient voltage being supplied to the starter to operate properly. If the starter does not function, as it should, chances are the starter is defective or the engine has a problem causing it to be difficult to turn. If you have a flywheel turning tool, you can attempt to turn the engine over by hand. A voltage reading below 9.5 volts would indicate a voltage loss between the battery and the starter. For example if we had a reading of 9.6 volts at the battery, and we had a reading of 8.0 volts at the starter terminal. This would be a 1.6-volt drop in the circuit. The maximum drop allowed in the starting circuit is 0.5 volts (0.25V per cable). Corroded battery cables, dirty connections, or insufficient wire size would be the problem here. This could be any connection in the positive or negative circuit. Even corrosion between the starter and the engine block can reduce the available voltage.

  3. #3
    LeE ss13
    If the battery is in good condition and fully charged, the Alternator should be putting out 14.6 volts at the battery. That is equal to 1/4 ohm of resistance from the battery. Sun Electric used to make a 1/4 ohm attachment and battery isolator so you could imitate a fully charged good condition battery to check the voltage of the charging system. Reading amperage is fine but Voltage tells you much more. jstwkd has a good test for the starter. Checking to see if you can manually turn the engine is the most over looked part of starting troubles.
    P.S. Just after charging a battery, it may read 13.5 volts. That's just a surface charge it has. By turning the head lights on and off, it will drop back to 12.6. fyi
    [ May 20, 2003, 09:09 AM: Message edited by: LeE ss13 ]

  4. #4
    Boater Bill
    Thanks guys!

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