when to get a new battery?
 

how do i know when my battery is wearing out? i bought a used car recently so i don't know the history, there isn't a volt meter on the dash... just curious

Your battery should have a date code sticker on the side. If the last digits are "02" or earlier, do yourself a favor and replace the battery before the cold weather sets in. Regardless of the length of warranty offered on a battery, 4 years is as far as you should trust, especially since you don't know the history.
Part of your winter prep should include a check of the entire charging system. Lots of parts stores offer the service for free. Some will even give you an honest appraisal, and not just try to sell you some parts.
Hope you got a good deal on the car. Enjoy!

If your car will not crank, then it is time to replace the battery.... OK...OK... just being a smart alec!...lol. Seriously though, Grumpy is right, MOST batteries don't last more than 4 years in Northern states, the clod weather tends to kill a battery pretty easily if it is not maintained properly. If you live up North, it is not a bad idea to invest in a battery blanket, or even a battery warmer in extremely cold regions. But as he said above, most all parts places will test your starting, charging system for free, just go to a reputable one.

^ true dat

Don't wait for that snowy, icey morning in Baltimore and finding yourself not being able to start your car.

A rare example of super long life battery, and I've heard similar reports from other owners of these Japanese OEM batteries, the original factory Denso battery that came in my '95 Toyota finally showed signs of weakness just a few months ago and I replaced it with an Interstate.

I agree mostly with what was said above. And, fwiw, the voltmeter or idiot liot for the battery more often than not won't be of much help until the battery is dead. If the voltmeter reads low, or the battery light comes on, it is usually an alternator, at least in my experience.

Usually I carry my portable jump start tool in my trunk, and when I have signs of it not cranking over like it should, I then check the alternator, if that is good, then I replace the battery.

I agree with the previous posters, regarding age, in general, especially if you live in a cold climate. Better safe than sorry.
I was working on a detailed post about the types of battery failures, when I ran across This site. It is a commercial site, but it has some good information.

"6. Battery Testing can be done in more than one way. The most popular is measurement of specific gravity and battery voltage. To measure specific gravity buy a temperature compensating hydrometer and measure voltage, use a digital D.C. Voltmeter. A good digital load tester may be a good purchase if you need to test batteries sealed batteries.

You must first have the battery fully charged. The surface charge must be removed before testing. If the battery has been sitting at least several hours (I prefer at least 12 hours) you may begin testing. To remove surface charge the battery must experience a load of 20 amps for 3 plus minutes. Turning on the headlights (high beam) will do the trick. After turning off the lights you are ready to test the battery.........*Sulfation of Batteries starts when specific gravity falls below 1.225 or voltage measures less than 12.4 (12v Battery) or 6.2 (6 volt battery). Sulfation hardens the battery plates reducing and eventually destroying the ability of the battery to generate Volts and Amps.

Load testing is yet another way of testing a battery. Load test removes amps from a battery much like starting an engine would. A load tester can be purchased at most auto parts stores. Some battery companies label their battery with the amp load for testing. This number is usually 1/2 of the CCA rating. For instance, a 500CCA battery would load test at 250 amps for 15 seconds. A load test can only be performed if the battery is near or at full charge.

The results of your testing should be as follows: (Table Omitted)

Hydrometer readings should not vary more than .05 differences between cells.

Digital Voltmeters should read as the voltage is shown in this document. The sealed AGM and Gel-Cell battery voltage (full charged) will be slightly higher in the 12.8 to 12.9 ranges. If you have voltage readings in the 10.5 volts range on a charged battery, that indicates a shorted cell.

If you have a maintenance free wet cell, the only ways to test are voltmeter and load test. Most of the maintenance free batteries have a built in hydrometer that tells you the condition of 1 cell of 6. You may get a good reading from 1 cell but have a problem with other cells in the battery.

When in doubt about battery testing, call the battery manufacturer. Many batteries sold today have a toll free number to call for help.

Hope this helps :)