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Hi Robert....if you've UNLOCKED your OBC...you can do TEST #9 to check either the battery (ignition in position 1 or 2 engine NOT running) or the alternator (engine running) if you don't have any other tool to measure voltage. To unlock your OBC you have to use the two buttons on the instrument cluster PLUS know the sum of the last 5 digits of your car's VIN...this sum will be the number you enter when you go to TEST #19 which is the test to lock or unlock the OBC.
You could also drive your car to a place like AutoZone, Advance Auto, Pep Boys, Batteries Plus etc...where they have machines they can hook up to your car and check you car's alternator & battery to see if it can hold a charge. The battery should consistenly be between 11.8volt - 12.5 volts...because at 11.89 volts the battery is at 0% state of charge. This means that the car may be dipping into RC (reserve capacity)...and if the battery is below that, it may not even start the car...especially if the battery dips below 11.2 volts.
Again, if you OBC was unlocked, you can quickly do TEST #9 before you attempt to start the car to see what the battery voltage is or even if you get into a no start situation again.
My battery recommendation is any reputable dealer that sells a class size 49 or 95 battery. If you don't have any large electrical loads for added equipment like subwoofers and amps...or DVD players and extra monitors...you could save some money and get something like a Duralast 49DL from AutoZone (if they are in your area and you're in the USA). If you require extra electrical uumph...then go for a 95 class size battery which you can find from vendors like Battery Plus...or your BMW dealership (these batteries cost more money).