Voltage as an indicator of battery state

[JHZR2]

Hello,

Per the other issues with my alternator, I put in a new group 49 in place of the 91 in my 91 BMW 318i.

I put the old battery on a charger, which used a good profile to get it up to full. Over the last few days, sitting in my garage at roughly 60-70F, the battery has dropped to 12.5V.

Similarly, I put my 2004 saab 9-3 on my battery minder. It charged it up to full, and was on maintenance mode for the better part of a day. Taking the leads off, the battery read about 13.07V - surface charged obviously.

Letting it rest only a few hours, I went back, and re-measured. Instead of reading 12.8V, as Id expect... or at least 12.6V at open circuit, which would indicate "full" voltage, instead it was already down to 12.52V, as read on my fluke 179 - so I assume it is reasonably accurate.

So, is such a loss in voltage in such a short time indicative of a slightly shorted cell or other issue (highly sulfated plates, high internal resistance, etc.)?

A fully charged batter should sit at 12.6-12.8V for a relatively long period of time, right????

Thanks,

JMH

 

[Steve S]

You need to do a load test. Most likly the voltage drop does indicate a weak batters. That is what a load tester does ,it puts a load on the batters then the battery should hold a certain voltge fot a whilt ., Usually the batteries I have check drop volts to below the good range on the loadtester immediatly.

 

[JHZR2]

Well, the car starts perfectly, every time and there are no issues (on the battery in the saab, that is). It is merely an odd thing to see the voltage relatively low after being trickle and then maintenance charged for a while.

Removal of the battery is a big hassle when there is no indication that anything is broke... plus then I loose all my settings, different custom programming in the car, etc. Without an issue, I can justify the time and effort.

I thought there might be a basic correlation that would shout "fully charged and below 12.6 in less than a week... thats a sign of trouble" kind of thing.

Thanks... I might take the time to do the load test in the future, but not now.

JMH

 

[Steve S]

make up a thingie to hook up a 9v battery to a plug that plugs in to the cigarret lighter to keep the memory alive with the battery disconnected "you can also buy them".

 

[Donald]

You can do a load test with the battery in the vehicle.

 

[oilboy123]

I take it the batteries are off the floor? Batteries can leak voltage around the out side case. It is like a very slight short.
This is a proven fact. I have used a volt meter on the low setting and found leaking voltage in the past on batteries.
This is if the battery is dirty with an acid layer around it. The fact it reads close to 12.6 then dissipates leads me to suspect this. After a charge pop the caps if needed and wipe the case down a couple of times real well. Use baking soda on a moist paper towel without getting it in the cells. Then let the battery sit and test the voltage again.

It should be close to 12.6 but a regular battery will dissipate over time. How much is up in the air. AGM batteries are supposed to hold a charge over time better. And have a resting voltage of 12.6-12-8 according to optima.
I worked in a couple of shops many years ago. I was in charge of the battery room but can't remember how long the batteries held a certain voltage for how long.

What I do remember was as standard wet cell batteries were getting near the end of their life they tended to have a resting voltage heading up to near the 13 volt range.

This might not make a difference but it's JMO. Good luck

 

[oilboy123]

I don't know if you guys have heard of or use the new NIMH batteries. Like the Eneloop or hybrid RayOvac.

They are a new rechargeable battery that holds a charge for a much longer period than the old style NIMH types that have been on the market for years. I just bought a kit at Costco.

So the time has come where they will work for remote controls and many other devices.

 

[JHZR2]

I use them in my calculator and camera flash. too bad I cant use such a thing in my car!

 

[oilboy123]

I have not had a chance to really put them to the test yet. Hope they are as good as they are claimed to be.......