I tested the cells of my battery. Some were about 1.23, some about 1.27, but 1, the fifth from the outside was about 1.15. The battery is 10 years old. They’re used in series: how can one wear out so much more?
The longest guarantee I could find on a battery was 3 years. I remember buying a Duralast from Auto Zone that had an 8 year warranty - 20 years ago.
6 cells averaging 1.25 volts is only 7.5 volts, that is a DEAD battery
Tester
I’m guessing @RandomTroll is testing the specific gravity of the electrolyte in each cell and not the voltage. 1.260 indicates a good, fully charged cell.
I remember doing that back when batteries had service caps. Never really understood why though. Either a battery is good or it’s not. You can’t replace just one cell in a 12V car battery.
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Yeah, that was back when batteries had a pro-rata warranty. If a 6 year battery failed at 4 years, you would get a new one for 2/3 the price. Now batteries are “rated” for X years of service, with free replacement if they fail before 3 years, that’s it.
Can’t tell you how many battery adjustment forms I filled out back in the old service station days.
How could you test individual cells on a modern battery when they no longer have exposed cell connectors?
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I haventvseen any batteries with service caps in years.
Why do the cells of a battery wear out at a different rate?
Even better, why do 2 head lights installed at the same time rarely die at the same time?
My answer is, there are some things mortal men are just never going to know. 
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Back when batteries were vented through the caps and chargers were not very “intelligent” you’d lose some water to get a full recharge. It “boils off” a bit of the water. Then you’d need to add a bit to bring the level up. Also the cell are not perfectly uniform…to address Random Troll’s question as well… so they charge and discharge at slightly unequal rates.
A large format battery for a forklift… a 36 volt battery still needs to be watered even after “intelligent” charging to equalize each of, say 108 cells (three 36V parallel strings of 18 cells).
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Interstate was still doing caps (mostly) a couple of years ago, haven’t seen any new ones lately so not sure, but when I was still selling them most had caps… I have a interstate from 2019-2020 that has caps on it… I’m sure others do as well…
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IMO it’s variability, both random and process related, in manufacturing.
Most new batteries, despite being called sealed, still have them, they just aren’t as easily accessed. Just another way of making things appear not to need any service at all. Here’s some images of the type of battery I have in most of my vehicles. You can see the cap and vent (the label covering the ports has already been removed from the cap):
Now here’s one of the ports after its cap has been removed:
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After 10 years any slight manufacturing differences will become clearly apparent, as they have here. The cells started off different, age exaggerates those differences.
On a related note, does anyone remember when buying batteries, they’d arrive dry with the acid shipped in separate boxes?
I filled many batteries with acid in the gas stations I worked in.
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Sometimes I still buy motorcycle batteries that are packaged that way.
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It wasn’t just chargers, it was the cars themselves. Back when cars had mechanical voltage regulators and that alternator was banging out 14.8 volts on the freeway you’d lose water. At the old service station we had those battery fillers that would top off the water without overfilling it.
But with the advent of electronic battery testers I never saw the need to check each cell with a hydrometer Either a battery is good or it isn’t.
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Ok, I curious and I am willing to show my ignorance and even tolerate ridicule… Back in the early '60s our local garage said the you can rejuvenate an old battery by draining all the old acid out and refilling it with new battery acid. They claimed that over the years the acid had been diluted with water and that was why each cell was weak. I did not put any stake in this and I figured that they were just trying to sell their excess battery acid and it might even cause the battery to short when you flipped it upside down…
This was the same garage that “short-sticked” the dipstick to sell you a quart of “very airy oil…” from an empty can…
Is there even a shred of truth to this?
Now, I will temper this question with a really weird way they used to find a leak in a tire. This garage also did tire repair, but the tire tub got driven over and was useless. The mechanic mounted the tire on the tire machine and only broke the bead on top. He then poured a lot of water into the tire, and then re-inflated it with a lot of water inside. He then blew the tire dry with an air hose and then slowly rolled the tire looking for water leaking/squirting out. When he found it, he marked the tire, broke it down, drained the water out and then patched it with one of those Hot Volcanizing tire patches…
I questioned that during my starting/charging class back in the day. The instructor said that variations over .050 between cells indicates a battery that is approaching failure. It’s not accurate if the electrolyte solution is faulty in any way. So, yeah, not sure what good it would do to measure it. I have never measured SG in the shop.
Measuring SG tells you whether the entire battery is low and just needs a charge, or if a cell is bad and the battery needs to be replaced.
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