Car starts and runs fine now. BUT... Last week left the key "on" while washing car for ~1hr with all accessories off, dead as a door nail. No click or anything. Now i'm eyeing the voltage and notice it drops to 9.5 when cranking, then right back to just over 14 when running. I'm thinking it will be gone soon and don't want to be left stranded.

 

If the engine cranks strongly now, and the battery isn't very old, I'd say don't bother. If the engine doesn't crank strongly now, and/or the battery already has a lot of years on it, go ahead and swap it out for the peace of mind.

Be well,

SJW

 

Your measurements show the battery is probably good. When cranking the battery terminal voltage should not fall below 9.0 volts. Be sure to charge your battery up with a plug in battery charger, do not use your alternator to charge your battery!

 

I did charge it for most of the day and trickled it too. I thought it was odd running down in one hour to nothing. Seemes it was cranking at 10.x last week, then this week 9.x. I can almost hear a difference in the starter, but I am probably just imagining it. I'm trying to figure if I am making someting out of nothing...

 

What's the voltage with the key on and the engine not running? Or just check the voltage at the battery terminals. If you show less than 12v, the chances are that the battery is going south and probably not going to hold a charge.

Have the battery charged by a shop and then have a load test done on it. That will indicate whether or not the battery can hold a charge.

The alternator is not designed to re-charge a battery. 14-15 volts may be OK at initial start-up but the alterantor can really only sustain that for just a short period to charge the battery from that current draw from the starter. A voltage display of 14+ volts for long periods will ruin the alternator after awhile.

 

There are a number of ways a battery can fail. Your battery should not discharge in one hour if you have normal leakage current. Remove the negative cable and place an ammeter between the battery terminal and the cable. After the courtesy lights time out you should measure less than 50 milliamps, my 87 measures 27 ma. High leakage current of course can discharge your battery. Plate material flakes off and builds up on the bottom of a battery and it eventually touches the bottom of the plates and causes current to flow and discharges that cell. Three days ago my 6 yr Delco battery wouldn't start my 87 vette and it had 10.4 volts, so one cell was dead and I now have a new battery installed. Also, as batteries age their amp-hour capacity (reserve time) decreases and you may have normal leakage current, but a very low reserve time. Reserve time is the time a battery can supply 20 amps until the terminal voltage is 12.0 volts, and usually it is around 90 minutes.
You can tell the state of charge by measuring the battery terminal voltage after it has not been charged or discharged for several hours (battery companies use 24 hours). 12.0 volts and below, discharged. 12.9 volts and above, fully charged and linear in between. Example, 12.5 volts, 50% charged.
Alternators are designed to have a constant output voltage of 14.7 volts. C4 alternators output voltage is 14.7 cold and they drop to 13.7 volts when they get to their operating temperature. 14.7 volts is safe to charge a battery with because as the battery is charged the charging current drops exponentially and this is the reason why it takes 24 hours of charging to fully charge a lead acid battery.