What is the relationship between car battery charge and cranking amps?

New car 12v Battery is marked 95Ahr. When parked, there is a constant drain due to car's security system %26amp; computer. Is a 40ma drain a reasonable estimate?


How low can the charge drop (in Ahr) with the battery still able to start my V8?|||The battery capacity in Ah gives an idea of how much cureent (A) it can deliver. The 95Ah one is capable of delivering a constant 5A current for 19 hours (5 * 19 = 95) or 10A for 9.5 hours or any other combination.


That's theory, because any real-life battery (even the new one) can deliver just a fraction of its nominal Ah rating - in a new battery it will be around 95%, in an old one about 40% at mild temperatures. Those percentages may be halved when it is very cold.


Cold cranking power (CCP) is another way of providing a similar capacity - it states the A value at a given (low) temperature for a given duration (seconds); the values are in the order of several 100s, like 320A. It is more realistic than the Ah rating.


Your 40 mA estimate seems valid - I remember that car manufacturers limit the total drain with engine off to max. 50mA (that's for factory-installed equipment, of course).


Assume your battery at this point has only 40 Ah (or some 42% of its nominal capacity).


Therefore, your 40mA drain leaves you 40/0.04 = 1,000 hours (or almost 42 days) to run the battery flat.


Even that's unrealistic because most electrical/electronic equipment will operate normally within a certain voltage range, typically 9...16Vdc. Draining the battery further will bring the voltage below 9V where most equipment will switch itself off or start malfunctioning. So the practical threshold will be much shorter, say about 1/2 or 500 hrs (=21 days).


To start a V8 will take well over a kA (1,000 A); luckily, it is a short burst and a healthy battery, even if not fully charged, will start a tuned-up engine in warm weather - it's a different story for below-freezing temperatures and/or longer cranking, especially after a cold night outside.


To be sure, next time you have to buy a battery, have the strongest one (in terms of Ah or CCP A) that fits in. This will provide ample margins of safety even in adverse conditions at little extra cost.