Bought a salvage auto battery showing 12.9 volts

softdown

The battery was made in 7/17 and appeared unused so I took a chance for $30 incl. core charge. Pretty surprised to see it measure 12.9 volts though the sun was hitting it and warming it up this morning.  Also bought a jump seat for $15 that had about $10 worth of change in it. Jump seats fit in between bucket seats. 
Used stuff - worth it? Hmmm.

NANOcontrol

I have on occasion bought "blem" batteries. These are ones that are returned from sellers racks when they get too old (1 year). These batteries are sold to used car dealers who want to move a car off the lot cheap and don't care if it fails a year later. That has been my experience with these, a year maybe two. Just sitting they are highly sulphated.  It only takes 10AH to start a car so they work fine for a while.  That said, I am using one of these batteries at camp for small loads two years after it wouldn't start a car.

softdown

Damaged batteries don't usually show 12.9 volts in my experience. In fact I rarely see 12,9 unless the charge is very fresh. Bad batteries charge to 12.5 volts or less - depending on temperature. "Bad" is relative of course. 

I'm not saying that salvage batteries are the way to go. I've been disappointed by initial strength that quickly declined many times. But I never had a 12.9 volt reading before. 

mike95490

12.9V indicates a bad meter.   As @softdown said, only a battery hot off a charger would read that voltage.

littleharbor2

I have handled about 65 of these batteries recently and even though they have been taken out of service months ago they are all reading 12.9x volts. These voltage readings have been taken with three different DVM's including my trusty Fluke meter. I thought it strange myself but these are the numbers I'm seeing.

BB.

AGM and GEL seem to have a higher resting voltage... In theory, since Lead Acid terminal voltage is proportional to the strength of the electrolyte, a higher resting voltage would indicate a higher initial Specific Gravity. Which is used for UPS batteries, higher storage capacity and typically a shorter life.

-Bill

softdown

As BB states, cell voltage may vary depending on manufacturer objectives. "Flashlight batteries" are normally charged to ~1.56 volts yet their lithium counterparts are charged to ~1.75 volts as memory serves. 

I use an Ideal meter that should be pretty good - cross checked with another Ideal meter from time to time.  

Is it also possible that a salvage yard has determined a way to crank up charged voltage a bit?

The morning sun was hitting the battery. It may have had an ~90F surface temp. being black would help.

Estragon

I haven't seen much, if any, effect from temp on resting voltage.  In operation, yes (capacity, charge voltage, etc), but not resting.  My banks run ~0°f at coldest to ~70°f warmest.  Sitting in the sun at 90°f would if anything speed self-discharge, but make a load test look better.

My guess FWIW is Bill is correct that the design of the battery accounts for the higher resting voltage.

BB.

With a sealed battery (especially if GEL), I don't see any method that they could do to make the battery look batter--Other than have placed it on a hot charger before you got a chance to measure them... 

-Bill "not a battery engineer" B.

softdown

Estragon said:

I haven't seen much, if any, effect from temp on resting voltage.  In operation, yes (capacity, charge voltage, etc), but not resting.  My banks run ~0°f at coldest to ~70°f warmest.  Sitting in the sun at 90°f would if anything speed self-discharge, but make a load test look better.

My guess FWIW is Bill is correct that the design of the battery accounts for the higher resting voltage.

I live in Colorado's second coldest region and will attest that temperature effects battery voltage readings. Not so familiar with warmer batteries but I am with colder batteries. 

Aren't you within 20 degrees of the equator? 

Estragon

I'm at ~49.5°N lat. in Canada.  Temps can get to -40° in winter, and >90°f in summer, but batteries don't get to those extremes.  Maybe there is a difference in resting voltage, but I've not seen it. 

Full and resting seems to be ~12.8/51v-ish winter or summer.  Absorb has a cold/warm range though, and voltage will drop faster in the cold with lower apparent capacity.