[T3] Batteries.

[Jim Adney]

On 4 Jul 2018 at 20:43, William J wrote:

>  When I brought up the issue with low voltage driving at night . I
> charged the battery in 2 3 hour 2 amp charges in 2 days , 6 hours. it
> came from 12.32 V to 12.57 V. Then after that I ran  the engine and
> checked the battery voltgae with lamps all on and off. I only ran the
> engine at most for 1 hour . I felt well this had to drain it some , I
> gave it another 3 hour  2amp charge and read 12.66 volts . 

Those voltage readings are all perfectly normal. The person who told you 
that a resting battery should measure above 13 V didn't know what he was 
talking about. Voltages that high will only be achieved DURING, or right 
after, charging

>  I wait at least 15 hours after a charge to check it. 

Not necessary to wait that long. In that time, the clock will have drawn the 
voltage down very slightly. An hour is usually sufficient. What you're waiting 
for is the battery acid to equilize. Right after charging, the sulfate ion 
concentration right next to the plate will be higher than normal, because 
those sulfate ions came from the plate during charging. Once the ions have 
diffused evenly throughout the acid, you will get an accurate measure of the 
state of your battery.

The battery voltage is proportional to the sulfate ion concentration. Standard 
battery acid is supplied at a concentration that gives sufficient energy 
storage without being so concentrated (strongly acidic) that it can damage 
the battery. It's a compromise that was made and agreed to decades before 
our cars were built.
 
>  I don't feel I need to charge it every day it sits. And certainly
> don't feel I need to check battery voltage before every drive I make .
> I would if I had a mounted voltmeter I could flip a switch and see or
> spend the 5 minutes it takes to connect my DVOM and check . 

You're obsessing about this, as you've said you tend to do. If the battery 
starts the engine normally, it's good. Once the engine is started, a properly 
working charging system recharges the battery; after that, all the car's 
electrical components are running off the generator, except when the engine 
is at idle.

At idle, the battery is supplying the power to keep things going, but it gets 
recharged quickly once you get off idle. This all happens automatically 
without our intervention.

So here's the short story.

If you get in the car and the voltmeter reads above 12 V, your battery is 
PROBABLY okay. If it starts the engine, you know it IS okay.

If you can see the voltmeter go above 13 V when driving, you know the 
charging system is working. It should get above 14 V after driving a bit.

At idle, the voltage should be above 12 V. The actual voltage depends on so 
many things that it's not important, so there's no point in measuring it 
accurately.




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Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
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