This morning car cranks but doesn't start. Starter runs a few sec doesn't start. Repeat a few times. I open the hood. Check voltage (12v) screw around a little. Do nothing that should fix anything. Tried starting again in desperation and starts like nothing was wrong.
Battery is < 1 yr old

Starter didn't sound as expected. Just didn't sound like it was working hard but not free spin noise.

 

Does your MINI sit a lot? A typical reading would be in the 12.6-volt range when the battery is fully charged. If the reading is 12 volts or less, then the battery needs charging or needs to be replaced with a new one. Good DIY here with lots of information.

 

I want to say it was 12.4v to be exact. Stopped by AutoZone and battery tested "good".
It's my daily driver, it doesn't sit. I did just replace the alternator two weeks earlier but plenty of problem free driving during those two weeks.

 

I'm sure you have, but just want to make sure everything is covered before suggesting to start replacing things. Make sure to check the ground strap and make sure that the terminals aren't loose. I would test the starter as well (not that this is your issue, but want to rule it out).

 

Thanks once more pelican
I didn't think of the ground straps. I did have the battery and the ground strap next to the upper engine mount off while replacing the alternator. Both seem on tight now but cleaning connections couldn't hurt.
Still a one off problem so hopefully just grounding.

 

+1 for double checking the battery connections, I'd assume you disconnected the battery when you were working on the alternator. I had a similar problem with my car for a while, and it turned out that my negative battery terminal was vibrating loose just enough to not make good enough contact. It still looked fine, but I could push it down a little bit and the bolt was just a little loose. Do be careful with the bolts connecting to the battery terminals, I tried to tighten one too much and it snapped. I think it cost me like $10 at the dealer for a new one.

 

You're welcome. It's always an important item to check on your car. The engine is electrically isolated from the chassis by rubber motor mounts. If the engine ground strap is missing or disconnected, then you might have a whole bunch of problems, including electrical system malfunctions and difficultly turning over the starter. The ground strap is located just in front of the passenger side engine mount. It is important to be sure your battery is good (which you've done) and in good condition and that the battery terminals, positive connections and ground cables are tight and corrosion-free before condemning your starter. Check over everything and make sure nothing is pinched or not connected. It could be very simple so just make sure you have checked over everything that you can before replacing things. Here is some info on the starter and a person tested it, but while doing so found the clutch safety switch disconnected at post #8. (Not sure what your MINI is or if it's auto/manual). Hope some of this helps with your diagnosis and let us know what you do or don't find.