Battery Testing

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.
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.


read it at: http://www.aa1car.com/library/2003/us20310.htm