Battery Tests Fine, Low Dark Current, Yet Battery Low After Sitting Several Days

[Number_35]

I tried testing A-h on another battery, this one a 6- or 7-year-old AGM that Jr had replaced but that I thought was still OK due to it passing the 100-A-for-10-s load test. I watched this test on and off, and was present when the inverter quit due to low voltage. When the voltage drops to about 10.6 V, the inverter issues a "low voltage" alert, and then cuts out after the voltage drops to 10.5 V. I like this, as that's the specified voltage cutoff for an RC test.

This (the AGM) battery failed after 35 minutes.

What misled me with the other test was that after the inverter quits, the battery voltage rebounds to a higher no-load reading (11.6 V in the case of the first battery).

So, bottom line, I am happy with this set-up, and plan to add this A-h test to the vehicles I look after. I plan to continue to the 100 A/10-second load test as well. I typically test the batteries around October, as they are most likely to fail in the winter.

 

[Donald]

Some of these tests are not testing the ability of the battery to handle a 300 to 400 amp load of the starter.

That's what a carbon pile battery tester can do. Apply a huge load for 15 seconds while measuring the voltage.

 

[Number_35]

Some of these tests are not testing the ability of the battery to handle a 300 to 400 amp load of the starter.

That's what a carbon pile battery tester can do. Apply a huge load for 15 seconds while measuring the voltage.

400 A? Wow! I could see it, with a large cold-soaked engine with no block heater.

Assume 400 A for 10 seconds - that's just over 1.1 A-h of battery capacity used up.

Now imagine a car sitting for a week, with a dark current of 30 mA. 30 mA x 24 hours)day = 720 mA-h/day (0.72 A-h/day).

Over a week, that's about 5 A-h - an insignificant load on a health battery, with a rating of, for example, 60 A-h, but critical on the two batteries I tested (one with a capacity of about 2 A-h, the other about 2.5 A-h).

 

[JHZR2]

High impedance (low current) internal shorts are a thing. Plates in a cell can slough enough material to make conductive paths that drain the battery.

Inverting and returning the battery briefly might help. Might not. But the battery needs replacement.

 

[Number_35]

High impedance (low current) internal shorts are a thing. Plates in a cell can slough enough material to make conductive paths that drain the battery.

Inverting and returning the battery briefly might help. Might not. But the battery needs replacement.

Yup, the debris will settle on the bottom, and will eventually build up enough to provide a conductive path between the plates. Not always a dead short, but even a high-resistance path will take a toll.

It's interesting to see the old station batteries (ganged to provide 129 V) with their glass walls, and the sediment at the bottom. Because they are stationary, and kept at 25°C (77°F), they typically last well over 30 years. Ditto for telecom batteries (ganged to provide -48 V).

My friend has already bought a new 96R battery to replace the old one.

 

[SHOZ]

Yup, the debris will settle on the bottom, and will eventually build up enough to provide a conductive path between the plates. Not always a dead short, but even a high-resistance path will take a toll.

It's interesting to see the old station batteries (ganged to provide 129 V) with their glass walls, and the sediment at the bottom. Because they are stationary, and kept at 25°C (77°F), they typically last well over 30 years. Ditto for telecom batteries (ganged to provide -48 V).

My friend has already bought a new 96R battery to replace the old one.

we use to make the old style plante pure lead plate batteries. Very high maintenance with gassing but never wore out, good for 100 years.....

 

[Number_35]

we use to make the old style plante pure lead plate batteries. Very high maintenance with gassing but never wore out, good for 100 years.....

The station batteries our utility used were usually Varta, from Germany.

And yes, the electrical techs were kept busy testing the electrolyte's SG, and topping up with distilled water as required.