Lead Acid Battery Discharge Curve - Strange

[kolbep]

Hi All.
I wanted to test a Generators battery, as the battery was completely flat (<1v), and the Intelligent Charger would not charge it.
I used another Intelligent Charger to get the charge going. A few hours later, the existing Intelligent Charger was happy to charge and maintain the battery again.

The Battery is a Varta 12v 40AH 330A Wet Battery (Similar to a Car battery, but half the size)

I decided to see if monitoring the discharge curve of the battery could point to a fault in the battery, so I calibrated a Wemos D1 Mini ADC and resistive divider to monitor the Voltage. I then put a 120R Resistor across the battery as a load and I got the attached Graph.



Does this graph point to a fault in the battery, and why does the voltage Gently Drop, then a Quick Drop, and then a Gradual Drop again?

Mainly Just Curious.....

If anybody wants to play with the Thingspeak data, the channel is :
https://thingspeak.com/channels/180710

Thanks
P

[The Soulman]

Your 12V battery is constructed from 6 2V batteries connected in series, you witnessed one cell being drained to zero..
If the voltage of a 12V lead acid sits below 10V or so for a substantial amount of time its dead..

[kolbep]

So, If I understand you correctly, one of the cells is draining faster than the others,
so then I assume that that one cell is faulty (and hence the battery can also be deemed as faulty)?

[SeanB]

One cell has reduced capacity ( either from low acid level, a cracked plate connector, shed plate material or a broken plate that is then isolated from the rest of the cell)  so as you discharge it it reaches zero capacity, then as you further discharge the cell it actually starts to reform in reverse polarity. This rapidly degrades the affected cell, and with a few more deep discharge cycles it will get to almost zero capacity and leave you with a 10V pack. The failed cell can also short out, or swell till it either ruptures the case or breaks the lead cell interconnects.

If you can open the cells check the water level, and then charge it and see which cell starts to bubble vigorously first, as it will be the faulty cell. This will be academic though, you will be taking a trip to Deltec to get a new one anyway, as this single cell failure will make it useless as a starter battery.  Careful on the roads, raining a lot at the moment.

[The Soulman]

So, If I understand you correctly, one of the cells is draining faster than the others,
so then I assume that that one cell is faulty (and hence the battery can also be deemed as faulty)?

Yes that sums it up nicely, according to your info it has 7ah of capacity remaining.
As it is a wet battery you could check the fluid level of the individual cells, warning it contains battery acid
and you don't want it anywhere else but in your battery!
If fluid levels are ok (if the battery is white and a bit see-thru you can see from the side, it should be between min and max, if not visible
from the side you can look on the top by unscrewing the filler caps -battery must be on a flat and stable surface-
and shine in the holes with a torch, the metallic plates must be completely submerged in acid .)
If levels are low they could be topped of with distilled water.
When fluid levels are ok charge it back up to 14,4V and discharge again to the moment the start of the drop off becomes visible.
Repeat charging/discharging to see if it improves.

Odds are slim but could be worth a try..

Edit: what Seanb says