New battery or not?
 

3 + year old battery. Overnight when I meter the battery, I get anywhere from 11.95 to 12.3 volts. If I turn the headlights on for a couple of minutes, the voltage goes up a little. After a prolonged drive the battery is usually 12.3 to 12.4. Every so often, after the car sits after driving 45 minutes to 1 hour it takes a few seconds to start. If it only sits 5 - 15 minutes, it usually starts right away or maybe an extra second. Compression is good. I' m thinking the battery is on its way out.

Clean it up real good and see what happens, as dirt on the battery can sometimes become conductive. Also check for any parasitic loads on the battery when the car is off.

I did clean it. There was no evidence of corrosion on terminals. How do I check for a parasitic draw?

Where are you located? I don't know about other countries, but auto parts stores in the USA will test your battery for you for free.

Disconnect the negative post and use a multimeter to bridge battery negative to the negative cable, and measure the current (amps). Normally the car draws 0.01 amp or less with everything off and doors closed. If you see more than that, something is drawing current that should not be. Sometimes the trunk light stays on with the lid closed. In my case the headlight washer circuit draws 0.05 amp at all times for some reason (so I disconnected it).

Use rubber gloves and don't complete the circuit with your body ;)

To check which specific system is drawing the current, connect the battery as normal, but then test each fuse position with the multimeter and see where you find current flowing despite the system being off.

Thanks

In the USA. I used a multimeter. Wouldnt they do the same thing?

Battery voltage is not the same as its ability to produce current and store energy. A battery load test will determine its health, you can't do that with a multimeter. Well, I guess you can, but you have to crank the engine while measuring the battery voltage. It shouldn't drop below about 10V while cranking. It's probably easier to just go to AutoZone.

I tried that and it did drop below 10v. Once it was 8v and it also dropped to 5 v although my lead slipped off the terminal for a fraction of a second. I should do it again to be sure. So if it is under 10 v, should I replace the battery?

What Loki said.

A battery's voltage is not a good measure of its ability to deliver current.

The standard test for a starter battery is to put it under a specific load at a certain temperature for a given period of time and monitor the voltage. If the voltage stays above a certain threshold during the test, the battery meets the requirements for that load. The SAE version of the test for a 600 CCA battery (for instance) will chill the battery to 0°F for 24 hours. The battery will then have a 600 amp load placed on it for 30 seconds. If the voltage stays above 7.2V, then it passes the test.

Most parts stores have a machine that does something similar. They won't chill the battery and it won't be a pass/fail result but it will put a big load on the battery and monitor the voltage. What the machine will do is give you a guess as to how much life is left in the battery.

You should use the machine at the auto parts store since it has the proper load/resistance to generate the 0.5 CCA rated current.

How Do You Perform A Load Test?

To pass a load test, the battery must maintain 9.6 volts at 15 seconds when tested at one-half the CCA rating and 70°F (or above). This test must be done with a true load (carbon pile) and not one of the hand-held testers that work off a conductance algorithm. The test must be run with the battery in a high state of charge. Be sure to read and follow all safety and handling instructions on the battery, this website and your battery tester. If you would like your battery tested, use our Find a Retailer for a location near you.

same with charging - good chargers will load current based upon read voltage (Battery Tender) and fancy cars like Steve Dallas' BMW will read temperature/voltage/draw and charge accordingly.
https://cimg3.ibsrv.net/gimg/www.rx8...cb1eaf90b5.jpg

Thanks guys.

You're welcome