Quote:
Originally Posted by dieselworks
My 1985 300sd began having starting problems. My battery was 6 years old and I didn't drive it every day. Checked voltage and it was a little over 12 volts.
But a fresh battery will charge to around 14.6 volts.
When my battery got tired I started to put it on a charger and had good results but knew it was time to get a new battery. I did and when I put it on the charger it had 14.6 volts.
My experience is when the voltage is around 12 volts it's time to get a new battery.
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While on a charger be it a external one or the cars alternator. Unless the battery has a shorted cell the reading is not too signifigant. A normal battery will read about 12.6-12.7 after sitting for some hours. This is just a superficial test though. The ability to deliver current is far more important.
This is why a load test is so important. Any car this old or at any age can develop bad circuit connections that can mimic a bad battery.
It varies a little with temperature but in general about 12.7 is considered a fully charged battery. It just may have had the storage ability reduced by age or has too much internal resistance when asked to deliver a lot of current.
This is what determines how good the battery really is. Always check the internal fluid level before rushing to judgement. An alternator that is over charging will boil off the fluid. Use distilled water to top it back up.
I have had very poor overall usable battery lifespans on cars not used frequently enough. I have never really looked for information. If batteries last longer on average in one temperate area of the country versus another.
I have heard that batteries do not like a lot of heat. True or not I do not know.
There is not much sense taking a reading right after a battery charge either by an alternator or external charge. You will only read what is described as a surface charge that will bleed off after sitting awhile.