But understanding RC may do more than prevent a customer from being stranded somewhere. It also may help techs solve mysterious cases where batteries go dead for no obvious reason.
The industry definition of RC is the number of minutes a battery can provide 25 amperes of current while maintaining a minimum voltage of 10.5 volts. Amperes (amps) represent electrical volume; voltage (volts) is electrical pressure.
To give you some perspective, I have seen RC ratings ranging from approximately 70 to 130 minutes. That's a big range of values.
Rest assured that delivering, say, 120 minutes of reserve capacity is no small feat for a battery.
Some service personnel focus solely on a battery's cranking performance, which is measured in cold-cranking amps (CCAs). On the one hand, a battery must provide adequate oomph to crank an engine under harsh weather conditions.
On the other hand, a seemingly hardy CCA rating cannot and does not ensure an equally robust RC.
For instance, I was doing homework on a familiar as well as very popular car. The label on the car's OEM battery showed 582 CCAs and a whopping 125 minutes of reserve capacity. The tech working with me checked on a new battery just to see what his vendor offered.
A supposedly correct replacement battery offered 650 CCAs but only 90 minutes reserve capacity. Battery-wise, an RC of 125 minutes is a long way from 90 minutes.
Engineers have told me that installing thicker plates boosts a battery's RC rating. Of course, thicker plates increase both the weight and cost of a battery.
Another potential factor is vehicle design because it influences the allowable, overall size of the battery case. The smaller the case, the less material an engineer can stuff inside it.
Let me share some observations that may help you.
First of all, the OEM rating for CCAs and RC usually appear in the electrical specifications section of the auto maker's manual or service literature.
Second, some battery catalogs — paper or electronic — provide RC ratings but others do not. Some battery labels always show a battery's RC rating but others do not.
Third, the greater the "key-off" electrical demand, the greater the RC rating may be. You see, certain electrical components normally remain on after the driver shuts off the ignition switch. After a certain period of time, these components shut off.
Now suppose that an uninformed person replaced a battery with one having inadequate reserve capacity for the application.
The car owner may never notice a symptom as long as he or she drives the vehicle fairly frequently.