this is alternator (70% chance) and/or battery (remaining 30% chance). the faulty alternator (regulator within) can kill an otherwise good battery. electrical fluctuations will create a serious havoc on the car's modules, especially the spikes... 16V is deadly for most modules... you are at 15V, so the modules survive but not for long... it might be easier to replace the whole alternator as opposed to just the voltage regulator...

do a search on the forum, there is wealth of info on this subject...