Now that we're heading into hard nipple season, it's a good idea to check your battery with a multimeter. Some good info that I found for reference:
A good battery is essential for reliable starting, especially during cold weather. Low temperatures increase the strain on the battery, starting and charging system and tends to bring out any weaknesses in these components.
Cold weather thickens the oil and makes the engine harder to crank. Normal cranking loads can require 125-200 amps or more from the battery depending on engine displacement, compression and temperature. At 0 degrees F, that number can increase 200-250% depending on the viscosity of the oil in the crankcase.
At the same time, freezing temperatures also sap the battery's ability to supply amps. At 0 degrees F, most batteries can only deliver about 65% of their normal cranking amps. At -20 degrees, battery power is cut in half!
A battery can't deliver maximum cranking power if it is not maintained at or near full charge - especially when outside temperatures drop and reduce the battery's amp output. So reliable starting also requires a good charging system that can keep the battery fully charged and also supply enough amps to meet all of the vehicle's other electrical needs.
test battery
If the battery is low or is getting old, the starter is weak or there is too much resistance in the starting circuit, the combination of increased cranking load and reduced battery capacity may prove to be too much when temperatures drop. The engine may not crank fast enough to start or it may not crank at all. Either way, the vehicle is not going to start.
Test Battery
Test the battery to determine its condition. If a battery is dead or low, the first thing you have to figure out is whether the battery is still good or has failed and needs to be replaced. A good battery is one that can accept and hold a charge, and deliver the rated number of amps on demand. A bad battery is one that won't accept a charge or can't supply its normal dosage of amps due to cell damage or deterioration. Good batteries can be recharged and returned to service but bad batteries need to be replaced.
Even if a battery has a built-in charge indicator, use a voltmeter to check the battery's charge. The reason? Built-in charge indicators only read one cell, not all six cells. If another cell is bad, you would not know it by looking at the charge indicator.
A fully charged battery should read 12.6 volts. A reading of 12.4 volts equals about a 75% charge and is good enough for further testing. But anything less means the battery is low and needs to be recharged.
Battery Voltage and State of Charge:
12.68v . . . . . . . . . . 100%
12.45v . . . . . . . . . . 75%
12.24v . . . . . . . . . . 50%
12.06v . . . . . . . . . . 25%
11.89v . . . . . . . . . . 0%
(NOTE: these readings are at 80 degrees F. Battery voltage readings will drop with temperature roughly 0.01 volts for every 10 degrees F.)
(At 30 degrees F. a fully charged battery will measure about 12.588 volts, and at zero degrees F it will measure about 12.516 volts.
Subject: Checking your battery 
October 17th 2009, 1:50 pm
