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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