I recently purchased a 2012 1.6 EcoBoost Focus. I noticed that the start/stop function doesn't work and as the car hasn't been driven much the past couple of months I thought it was due to a low battery (shows 11.7V in test mode). However, I decided to check under the hood and to my surprise discovered a standard (not EFB) Bosch S4 battery. Is this bad only for the battery or for the car as well? I will put it on my smart charger to see if I can recharge it but I've read that normal batteries don't last very long on start/stop vehicles. Should I replace it right away or wait until it dies?  

I would replace it. Then reset the battery monitoring .

I did about 500km of highway driving over the weekend and the Start/Stop still isn't activating. I guess a new battery is in place. My issue is that I can't find an aftermarket EFB 60Ah/590A one. The closest one I can find is 60Ah/560A. Would the slightly lower CCA be an issue? There is also a 70Ah/660A but I'm not sure whether the car will be able to charge it and whether it'll fit as it's a bit larger. Also not really keen on spending the extra cash to be honest. There is also an AGM 60Ah/680A but that one is an AGM so maybe not suitable for the Focus? I've read AGM batteries don't like heat and are usually in the boot.

Re the CCA.

I do not know where you are living. What the makers specify would take into account worse case weather conditions in the region that the car was built for.

eg. I live in south of England. Weather conditions here never get as bad as some other parts of UK so if it was mine I would not worry about the lower CCA.

But if you live in an area with severe winter conditions maybe you do need to take the CCA into account more.