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High battery resting voltage - sign of imminent demise?

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Darrell_Walker

Darrell_Walker

Jedi Knight
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I'm trouble shooting a slow starting condition on my TR8, and have discovered that my battery is showing around 13.25 volts, even after resting overnight. I don't think it is my meter, as my other batteries are showing the more expected 12.6-12.7 volts. Also, my battery tender is saying that the battery isn't achieving full charge (which is odd since it is showing such high voltage).

I'm not sure how old the battery is, I can't find any date markings. It was in the car when I got it almost two years ago. I haven't had anyone around to help measure voltage while starting, my theory is that the system voltage is dropping low when cranking the engine over, which is causing the fuel pressure to be slow to come up (fuel injected car).

I may swap the battery over from the 4A, just to see what happens.
 
A wise old man once said "If you don't know how old the battery is, it's probably time to get a new one". Beep the horn and see if the headlights get dim; quick test for battery strength.

That said, my three year old WalMart battery shows 12.5v every morning, without a maintainer, if it's been driven the day before.

When you say "slow to start" - you mean it turns over slowly? Or it turns over fine, but takes forever to fire?

Tom
 
NutmegCT said:
When you say "slow to start" - you mean it turns over slowly? Or it turns over fine, but takes forever to fire?
Click to expand...

A little of both. Last weekend I had it on the charger for a few days, and the first couple of start it turned faster (and started faster). Not a huge difference in cranking speed, but enough to notice. Now it is back to turning over slightly slower, but takes forever to fire (beyond a few pops).
 
if it were mine ... I'd swap with a known strong battery, and also swap coil and condenser as a test.

The car started fast and and ran strong until just recently, right?

T.
 
NutmegCT said:
if it were mine ... I'd swap with a known strong battery, and also swap coil and condenser as a test.

The car started fast and and ran strong until just recently, right?
Click to expand...

Thanks. I has been doing this for a while, just got to the point that it annoyed me enough to start looking into it. Once it starts, it runs fine, and restarts are OK, but degrade the longer you wait (a few hours just fine, 8 hours a little slow, overnight or over the weekend slower still).
 
Thanks for the details. In that case I'd just replace the battery, and check that all connections (battery, ground straps, coil, etc) are tight.
Edit: also be sure the lead from the coil to the *side* of the distributor is tight.
T.
 
My spit started doing the same thing. In my case the battery had frozen during a bad spell here and is now damaged internally. Car starts off a charger and will keep starting the car on restarts but will not start the car after a day or two. Time for a battery.
 
Andrew Mace said:
What sort of voltage do you see from the alternator when the engine is running? I wonder if the regulator is off, causing some overcharging of the battery?
Click to expand...

14.6 volts or so at idle.

I swapped over the battery from the TR4A, and will test start it once everyone is awake (bedrooms over the garage).
 
Well, the battery swap was inconclusive. The battery I swapped in, while working fine in my TR4A, is almost 6 years old, and has been abused (total discharge 2 or 3 times before I invested in a battery tender and started disconnecting it over long down times). It worked about the same as the battery in the TR8. I did pull all of the major electrical connections apart and clean them. While a few were dodgy, nothing seemed to really change.

Still confused by the high voltage reading on the battery, I would like to understand what could make that happen.

Next step I think is to see if the FLAPS can do a load test.
 
load test = good idea.

What is the voltage from the "replacement" battery when the engine is off?

What was the result of the horn/headlights test?
 
That's interesting. In theory a lead/acid cell can only produce 2.1 volts. That's really it, unless you used a different electrolyte. To get more, there has to be chunks of lead broken off inside the battery!

Oh, and yes, a new battery is in order...
 
Last edited:
NutmegCT said:
load test = good idea.

What is the voltage from the "replacement" battery when the engine is off?

What was the result of the horn/headlights test?
Click to expand...

The other battery showed about 12.7 volts.

Using the original battery, engine off, headlights on, honking the horn doesn't change the light level that I can see. I added in the fog lights, still no change. Battery reads 12.2 volts with headlights and fog lights, 12.3 volts with just headlights. After that test, I shut the lights off and the battery read 12.6, but kept climbing. Got to 12.69 before I stopped. I'll check to see where it stabilizes after lunch.
 
OK, battery sat over lunch, now showing 12.95 volts.

However, checked with a different voltmeter, and got 12.4 volts!

Neither looks good!
 
NAPA says the battery is good. Read 13.5 volts on their tester, they didn't seem to think that was odd. They said it was a load test, and that it still had 625 CCA (out of the rated 650).
 
If they have O'Reillys in your area, they will test your battery for free (also the charging system if you take them the car).

I have seen batteries that would hold a higher voltage like that, that seemed OK otherwise. I was taught that you should switch on the headlights for 10 or 15 seconds and then back off before taking a "resting" voltage reading, to bleed off the "surface charge".

If it's any consolation, I'm having somewhat similar problems with my TR3. The last couple of times something like this has happened to me, it turned out to be a bad starter. (In one case, just tightening up the through-bolts 'fixed' it; in the other case the starter had to be replaced.)

O'Reillys said my battery was good, but that if I would bring it in when it wouldn't start the car, they would replace it under warranty. Naturally it hasn't refused to start since then.
 
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