Batteries: single failed battery in the house bank, impact for other batteries

David Vogel

Greetings all,

Whilst trouble-shooting an unserviceable ONAN (failure to crank), I checked out the health of the 105Ah 800CCA AGM start battery (good), and house-bank (12x105Ah AGMs). Start and house batteries were all AGM, same model and date-of-install (Sep'16).

All tested OK, with the exception of one battery in the house bank, which failed the State-of-Charge test with an Open-Circuit-Voltage (OCV) of 11.00V (as measured, so no surprises here), unstable; and failed State-of-Health of CCA of 35.0A).
The other battery in the pair containing the failed battery surprisingly tested OK, with SoC 12.72V [92%] and SoH CCA of 642A [91%].

As an interim step, whilst considering broader options ...
... I replaced the good start-battery with a 100Ah/700CCA flooded lead-acid battery; removed the failed AGM battery from the house bank, and replaced it with the 'good' AGM start battery. The performance of the house battery bank immediately improved, I guess due to the absence of the parasitic load of the failed battery.

However, concerned about the remaining battery in the new pair contained the failed battery, I have been keeping a close eye on the charge volts, current, and temps of all batteries. 24 hours after the swap-out/in, during the second charge cycle, I noted high charging current to the new battery-pair containing the old-start battery.

The smart-charger was ordering ~150Amps from the 175A/24V Leece-Neville (normal for the start-of-charge-cycle) - 5 battery-pairs were accepting about 20Amps each (OK and as expected, at ~20% of the '20-hour rate' of 105Ah), but the 'at risk' pair was accepting 40 to 50Amps. Temps for the 5 'good' pairs were about 1ºC above ambient and stable; but the temp of the questionable pair was ambient +2ºC and rising. I stopped the charge cycle after 30 minutes; and isolated the questionable pair from the house-bank by removing the bridging strap between the batteries in the pair. Shortly after the cessation of the interrupted charge cycle, the temperature of the high-current pair peaked at 36.5ºC (ambient other batteries +2.5ºC).

I am seeking clarification regarding: Is is likely that the remaining battery from the 'old pair' had already suffered irreversible damage (such as an internal short), thereby reducing internal resistance, thereby accepting a higher charge (than the other 'good' battery pairs).

On the basis of temperature alone , I do not think I have suffered temperature-related damage to the old start battery. However:

Question - is it likely that 30-minutes of charge at twice the maximum recommend bulk/absorption current charge-rate has resulted in permanent damage to the previously 'good' start-battery?

This discussion shared for the learning, and also for the benefit of trouble-shooting by other unfortunates who may in future find themselves in a similar predicament.

Thanks in anticipation ...

On anchor, Brisas, Pacific side of Panama
Departure for the Marquesas pending the
return of the ONAN to service

Diagnostic tools: Magneti Marelli BT002 Battery Tester; FLIR TG165 Spot Thermal Camera; Voltcraft VC-595OLED Digital Clamp Multimeter.
Note: the OLED display on the Voltcraft multimeter is impossible to read in direct sunlight, so next time I would choose a model with a different type of display. Otherwise, this is a great tool, although I do not use the BlueTooth functionality.

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