T-Pain Is On A Boat: [Electric Boats] Re: Battery mystery

Thursday, August 13, 2009

[Electric Boats] Re: Battery mystery

Thanks Mark,

Based on some more googling, I'm 90% convinced this is classic cell-reversal where that cell got depleted, then when I charged the battery, it got completely discharged and the other cells overcharged until the charger went into float. At least that's my only working theory as to why the battery voltage was reasonable. This means each of the good cells (5) had to have gone to 2.5V, which doesn't seem impossible.

The concensus seems to be that you can potentially recover by completely discharging the battery using some load, then re-charging (but doesn't the bad cell get charged when discharging the others? I guess until the others hit bottom and then it starts dis-charging in reverse until you have a completely dead battery)

I went ahead and turned that battery in vs messing with it more, and this afternoon will check SG on the remaining 66 cells (ugh, ruined a pair of jeans yesterday...). If I find another reversal I will try the discharge trick and go from there.

Moral of the story - when a lead-acid bank starts getting unruly it's hard to recover.

-Keith

--- In electricboats@yahoogroups.com, "Mark Stafford" <mstafford@...> wrote:
>
> Keith,
>
> Open circuit voltage readings can be misleading, though the numbers you quote appear good (25 amp-hrs sucked out of a battery, then measure voltage). Voltage under load gives more info... too bad we can't measure cell voltage, only whole battery voltage. That is where LiFePO4 batteries with a BMS excel (measuring each cell all the time).
>
> I'm still learning about batteries, and hope to provide some quantified results from my desulfator experiments. I think a completely discharged cell has nearly all water and no sulfuric acid remaining... a very difficult situation to recover from. Swapping electrolyte is probably a last resort in trying to restart the chemistry. Perhaps the sulfur ions are locked up in lead-sulfate crystals, depriving the electrolyte of their chemistry.
>
> Mark Stafford
>
>
> --- In electricboats@yahoogroups.com, "aweekdaysailor" <aweekdaysailor@> wrote:
> >
> > I'm still tracking down a gremlin in my battery pack and have a strange one.
> >
> > One of my batteries reads a more-or-less normal voltage. But a specific gravity test reveals 1 cell which is basically water. How can that happen? I moved some electrolyte around between cells and it gave a brief improvement in total voltage.
> >
> > Is this a short? I thought that would drop total voltage?
> >
> > Could it be a cell-reversal? That should lower voltage too, right?
> >
> > Why is the voltage near-normal (open-circuit reading 12.14 after 25AH discharge)
> >
> > the cell-to-cell voltage is also erratic.
> >
> > Basic symptom that started it all is that this battery is 1 of 2 in the pack that show lower-than-normal voltage after a moderate discharge.
> >
> >
> > Any help from the battery wizards appreciated.
> >
> > -Keith
> >
>