13.8 to 14.5V or more while idling. If your alternator gives a output reading of less than 13.8V, it is considered weak and may not be able to properly charge your car battery. This may result in the dreaded “car can’t start” situation and you should visit your car workshop as soon as possible to service/replace your alternator. If you replace your car battery with FastFix, we will test your alternator and will advise you if servicing/replacement is recommended. We know of workshops who specialize in alternators and can provide recommendations – available on request.