I'm not sure how to respond here, other than suggesting you read up on how batteries work. They aren't like fuel tanks in that you can have a gauge that tells their remaining capacity.
An automotive battery will do it's best to hold "charge", and keep it's output constant at 12 volts. As it reaches the end of it's "charge", that output voltage will drop quickly. Batteries are tested on their ability to hold a constant voltage output over a certain period of time. They are also tested on their ability to hold this charge while not producing output. When a battery cannot hold a steady output voltage, or cannot store a "charge"; then it's time to replace it. Using a voltmeter gives you the ability to see the current output voltage of a battery, and how long it can hold that steady voltage. My meter shows around 14 volts as the engine charges the battery; then it's around 13-12 volts when running just off the battery. After a period of time, that will drop to around 10 volts, and it's time to start the engine. The amount of time that my battery can produce 12-13 volts determines the "health" of my battery. After a few years, that time will decrease as the battery ages. If I notice that my output voltage drops quickly to around 10-11 after I turn the engine off, then it's time for a new one.
I use a graduated sweeping gauge because I want a good visualization of my battery's output voltage. I want to know if it's producing 13 volts or 12.5 volts. You can't get this kind granularity on a 10 segment LED display. I'm not saying the LED display isn't a good gauge, but it's just not accurate enough if you want something to replace the stock voltmeter in your gauge cluster. There are some computerized LED displays that will actually monitor the battery over time, and can give good indications of it's projected capacity, but you won't find those in the $30-$40 price range. Most are for aviation or commercial applications, and they aren't cheap.
Last edited by SilverTank; 06-12-2006 at 08:16 AM.