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I had a 3 year old 12V battery in our XC90. One morning it would not crank the car after ~3 days of non-use. Open circuit voltage was 4V. After full recharge (about 7 hours) open circuit voltage was ≥12.6V after interface charge had dissipated. BUT the specific gravity in each cell was about the same and each showed the need for charging!

It seemed to me that this connoted an insufficiency of sulfate as sulfuric acid in the electrolyte consequent to loss of accessible sulfide on the plates. But I could find nothing to support my idea, nor could various local authorities (engineers, auto mechanics) help.

I talked to a person at Batteries Plus who said that the findings indicated that there was a loss of cranking capacity (i.e., amperage) and that the battery would fail this coming winter when heavy loads were placed upon it.

I bought a Schumacher 100 amp battery load tester (Amazon $27) and applied it to the fully charged battery. The indicator immediately went to "weak'" and held steady, the explanation of which by the tester manufacturer follows:

"The battery capacity is not satisfactory. The battery may either be defective or not fully charged. Check the specific gravity to see which condition exists. If charging does not bring the specific gravity to the full charge level, the battery should be replaced."

The conclusion seems clear. The man at Batteries Plus was right. All available sulfide on the plates had been converted to sulfate (as H2SO4) during the charging process and the voltage was as it should have been for a full charge. But there was not enough available sulfide to make the concentration of sulfuric acid (sulfate) reach full capacity (amperage) levels. After all, a system comprised of but a few molecules could reach full voltage while having trivial amperage.

So, the question has been answered. I replaced the battery today.

My practice in the future will be different. After two years of service I will test the battery with a load tester every month or so. If capacity starts to drop I will know to replace the battery before it fails. And, to be doubly safe, I will buy a portable jumper battery to keep in the trunk on trips out of town.

An interesting datum is that only about 30% of automotive batteries reach their nominal 48 month service lives.
 

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Your post is mostly valid, but might I suggest layman's terminology for your average Joe? Incidentally, you don't need to just test your battery, you need to maintain it. This includes not only keeping it fully charged, but keeping the electrolyte levels full and the terminals clean. That is unless you have a maintenance free battery. In that case you should never attempt to access the cells.
For anyone who's interested, here is a great battery link:
http://www.batterystuff.com/kb/articles/battery-articles/battery-basics.html
 
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