First of all, this isn't a problem that I am having. My father knows someone that goes to his church (drives a Mini Cooper, 2004 [iirc]) and is having a confusing issue:

Apparently, the battery randomly cuts power, then randomly comes back on. But when it comes back on, the power steering does not work. He said that the green light on the battery does not change. He took it in to the only local Mini specialist and the guy said that he couldn't find anything wrong. Sounds fishy to me, but I wasn't there.

To me, it sounds like a simple connection is being lost. The battery is most likely in good shape, and the mechanic said the Alternator was fine. So... Any suggestion?

 

New cars do strange things when the batteries begin to die. 4 years on a battery is a good life, he should consider replacing it. The problem is likely his power steering pump - a known week point with these cars.

Batteries don't randomly cut power, they are either good or not good.
There has to be more to this story.

https://www.northamericanmotoring.co...power+steering
https://www.northamericanmotoring.co...power+steering

 

The power steering pump will shut off if the power (voltage) available to it falls below a certain level. This does not happen when the electrical system and battery are up to snuff. However, the original batteries that come with MINIs seem to have a very limited life span compared to other brands (I got less than three years out of mine). The power steering pump has a power control feature that reduces the power needed when little or no steering boost in required. The motor driving the pump will be supplied with more power by it's internal controller when it is sensed that more power is needed. This causes a current surge that can be more than the alternator can supply, especially when the engine is at idle or at otherwise very low rpm. A weak battery will not have enough energy to make up for this current draw (which can exceed more than 100 amps). This causes an overall drop in total system voltage. This is not enough to affect most other subsystems, but the pump itself is adjusted to be the first thing to shut down. These voltage drops can be interpreted by the pump's circuitry to be an internal fault as well, and in this case, the pump will remain off. Before writing off the pump, have the battery tested first. Being a 2004 with the original battery it's most likely in need of replacement anyway. If it checks good or if the battery is replaced and the problem persists, then it's most likely the fault of the pump itself.

 

/\ What he said. Great info, great advice. Very common problem. The "green light" on the battery just indicates that that particular cell is full. 4 years on the battery is pretty average. Get it load tested and look for a replacement

 

Is there any size of the Optima gel type battery that fits S model? I noticed the stock batt was clamped down by the tie-down that looks very specific for that size.

 

like others have mentioned, get a battery load test if the mechanic didn't already. Check all connections (in case of corrosion or looseness could lead to intermittent)
I didn't know the power steering was electrical dependent.

 

The power steering pump is run by an electric motor. It is very, very dependent. Trust me on this one.