What is an allowable voltage drop across the battery while cranking say for 10 seconds .
Have done a search but not found the answer
many thanks
Depends on the cca of the battery, the temperature and a few other anomolies.
e.g.
Cold Cranking Amps (CCA)
The most important consideration is sizing the battery's CCA rating to meet or exceed, depending on the climate, is the car's OEM cranking requirements. CCA's are defined as the discharge loads measured in amps that a fully charged battery at 0 degrees F (-17.8 degrees C) can deliver for 30 seconds while maintaining the voltage above 7.2 volts.
Batteries are sometimes advertised by their Cranking Performance Amps (CA) or Marine Cranking Amps (MCA), which is measured at 32 degrees F (0 degrees C) or Hot Cranking Amps.
HCA is measured at 80 degrees F (26.7 degrees C), which is not the same as CCA. Do not be misled by CA, MCA or HCA ratings. Multiply the CAs by 8 to convert Cas to CCAs. To convert HCAs to CCAs, multiply HCAs by 0.69.
Buying batteries with double or triple CCA ratings that exceed the OEM requirement are a waste of money for hot climates. However, in colder climates the higher the CCA rating the better, due to increased power required to crank a sluggish engine and the inefficiency of the cold battery. As batteries age, they are less capable of producing CCAs.
One of the major battery manufacturers, Exide, publishes the following table:
Available Power from battery Temperature Degrees F Power Required to crank engine
100% 80 100%
65% 32 155%
40% 0 210%
25% -32 350%
If more CCA capacity is required, two (or more) 12 volt batteries can be connected in parallel. Within a BCI group size, generally the battery with more CCA will have more plates because a larger surface area is required to produce the higher current.
