Hey everyone. I have 3 batteries in my system (Optima yellowtop, 2 xs d3100s) with a dcpower 270 xp alt powering a sundown 4500 at .5 ohms with 1000+ watts of mids/high power. Full tilt @ 2k rpm, i drop to arond 12.4-12.8 volts depending on the music. Well, i just got my car serviced at a dealership last week and they said that my optima battery was dead, but all 3 batteries were wired together, so I was wondering how they came to that conclusion. A buddy mechanic of mine decided we should test all the batteries with a device that measures cca. After doing the test, the optima battery had 32 cca (its over 3 years old), and the car wouldnt even start so it was clearly dead, but the xs batteries (about 8 months old) showed around 370 cca on one and 380 cca on the other when it should be around 1300. He claims that the device used may not be accurate to test the d3100 and to get it carbon load tested, but nowhere within 80 miles from where i live will do that.
My question is, why would the voltage be good on 1 dead battery and 2 bad batteries, assuming the device is correct. Any input would be great