Two standard ratings are used to measure a battery's storage capacity:

Amp Hours

The Amp Hour rating tells you how much amperage is available when discharged evenly over a 20 hour period.

Reserve Minutes

Reserve minutes is the number of minutes a battery will carry a 25 amp load before dropping to a fully discharged level (10.5 volts).

The OEM battery is rated at 55 AH with a reserve capacity of 90 minutes.

In looking at a Optima Red Top battery to replace my OEM battery, they surpass the OEM in cranking power, match or exceed the OEM in reserve capacity but have lower Amp Hour ratings (34R = 50 AH and 35 = 44 AH).

Are these lower Amp Hour ratings a concern?

 

It shouldn't be a problem. If you drain a regular battery to 10.5V, there's a good chance you'll never get it charged back up again anyway (and if you did, its life would be severely compromised). Those ratings are a good indication of how long you can leave the battery without charging it up (ie. you go on vacation for a month). Otherwise, cranking amps are much more important. There are several owners who go with small lightweight batteries that have very little reserve and have to worry about leaving the car for a week without driving. Some aftermarket stereos and alarms are the main two problem causers. I have an Optima Red Top 35 and have no problems with leaving my MINI unattended for 2 weeks.