12v battery

yes, even though it's working fine, t could potentially leave you stranded at any time.
what do you mean, 'already', is it not 8 years old?

 

There's a natural voltage drop with load.

To get your unloaded voltage, After your last drive of the day, park the car and pop the hood.

In the morning, or about 12 hours later, check the voltage at the jump start connection points under the hood. 12.4V is about 70%. 12.65V or higher is fully charged. If it's more than a few years old and not showing fully charged, my inclination is to replace it.

Note that the only 100% sure fire way of confirming the health of a battery is with a load tester. Most auto parts stores will do this for free if you take it in.

 

if you test it as s keith mentions above, and find it's lower than 12.4, try charging it. if it won't hold 12.4 or above, i would plan on replacing it. what kind is it? have you discharged it at all, or left it sitting without driving much? 3 years isn't very old.