To test the condition of my original battery, I thought I would start and drive the car with the internal voltage meter displayed just to watch what it does. I have never done that before, so I have no baseline to compare.

Stone cold after sitting overnight it's at 12.1 volts. When starting cold, it drops for an instant to like 9.8, but on subsequent starts stays in the 10s. What i conclude from that is that my 8 year old battery is at least ok, but probably not as good as it was when new. But what I'm most curious about is what the voltage does after starting.

It then climbs to a steady 14.2 volts. But it's not instantaneous. Takes maybe 7-10 seconds as it climbs through the 13s with the car still in park. Normal? Once at 14.2, it makes no difference what accessories I turn on or off - AC, fan, seat heaters, lights, etc - it stays at 14.2.

Only one thing causes it to change. As soon as I touch the throttle to start from a dead stop, it drops to 13.4 volts and stays there for maybe half a block through a couple tranny shifts, then goes right back to a steady 14.2. It never drops to 13.3 or 13.5 - always 13.4. And, if I press the throttle while the car is in park or neutral it does not change from 14.2 - it only changes when I am actually driving, but only from a complete stop. Normal? I'm guessing there is some cutout for the alternator so you don't have to overcome the drag from a loaded alternator during initial acceleration, but that's just my hunch.

 

The delay in reaching 14v is normal. Fluctuations when driving are also normal. A healthy, properly charged lead acid battery at rest will give 12.6v. This is a function of the chemistry of the battery. AGM batteries can be slightly higher at 12.8v. Anything less than this that isn't attributed to current draw (radio and lights on) means the battery is bad. 12.1v is really too low. And getting down below 10v when cranking can cause issues with the car's electronics. I'd get a new battery. I like the Advance Auto platinum - it's an AGM with a full 3-year warranty, $190 (even less with a coupon).

 

Yeah, I figured I'd finally spring for a new one instead of trying to set a world record for age. Is the sudden drop on touching the throttle, but at no other time, by design?

 

I'm not sure, but it's certainly possible. I've never really paid that close attention to it, but have noticed voltage fluctuations when casually monitoring it.