What causes a battery cell to go bad?

softdown · August 2016

We all know that batteries degrade with time and usage. I have seen some batteries expires prematurely because one cell essentially died. We talking specific gravity of 1.00 - distilled water. My current theory would be a manufacturing defect.

Eight batteries in 48 volt array:
With bad cell - overnight discharge to 47 volts.
After replacement - " " discharge to 49 volts.

FWIW: Checking voltage of each battery will flag down a bad battery. I have a year old battery that discharges to 12 volts instead of 12.25 volts. Thinking that I should replace it soon. Of course the S.G. is lower....especially in one cell.

BB. · August 2016

It is hard to say--There are many possible causes... We have reports of new (or virtually new) batteries that have a cell with (what appears to be) 1.0 SG (water)--The reality is most battery hydrometers cannot read down to 1.0 specific gravity--So, while it is possible somebody filled one cell with pure water, we really do not know, and I would not expect that to be a wide problem (somebody mixing up raw acid, mixed electrolyte, and distilled water).

So that leaves us with the cell being shorted (shorted cell takes sulfur out of the electrolyte and puts it into the battery plates). A Cell shorts could be from a plate to plate short, older battery shedding plate material and shorting at the bottom of the cell, bus bar fails, etc.

Here is a little more info:

http://batteryuniversity.com/learn/article/corrosion_shedding_and_internal_short

The measured capacity will also be low because self-discharge has consumed some of the stored energy. According to the 2010 BCI Failure Mode Study, shorted batteries accounted for 18 percent of battery failures, a drop from 31 percent 5 years earlier. Improved manufacturing methods may account for this reduction.

Another form of soft short is mossing. This occurs when the separators and plates are slightly misaligned as a result of poor manufacturing practices and they cause parts of the plates to become naked. Such exposure promotes the formation of conductive crystal moss around the edges, which leads to elevated self-discharge.

Lead drop is another cause of short in which chunks of lead break loose from the welded bars connecting the plates. Unlike a soft short that develops with wear and tear, a lead drop often occurs early in battery life due to a manufacturing defect. This can lead to a serious electrical short with a permanent voltage drop that could result in thermal runaway.

The most radical and serious form of short is a mechanical failure in which the suspended plates become loose and touch each other. This results in a sudden high discharge current that can lead to excessive heat buildup and thermal runaway. Sloppy manufacturing as well as excessive shock and vibration are the most common contributors to this failure.

-Bill

softdown · August 2016

Thanks BB. Been awhile since I visited. Guess I welcomed a break from worrying about fine tuning my little power plant.

Here is my theory. A superb memory may recall that I once had four 8Ds hooked up and then incurred low voltage. Used an old generator that was over revving and a battery charger that seems to have lost its ability to moderate charging voltage. I don't recall the exact charging voltage and amperage but it was a lot. Two of the 8Ds died within months. Two of the 8Ds received a little less charge because the interconnects were #4 AWG at the time. Took them longer to expire.

Why do I think it was the overcharge? Because the problem cell occurred right next to the negative post in three of the expirations.I think that bus bar suffered significant damage.

All of these batteries were mfd. in 2011. Then stored "cold" for three years. I wonder how cold storage effects lead acid vs AGM vs gel...assuming that voltage was always kept above 12.5.

Why the cold storage? Building took a long time...I kept expanding. Paid off with an almost perfect solar panel location.

BB. · August 2016

In general, lead acid batteries like to be stored "cold"... The standard engineering formula is for every 10C (18F) reduction in temperature, you have a 2x increase in life (20C cooler, 4x longer life). Also, self discharge is lower, so batteries can last longer between charges (call it recharging once over 30 days for standard flooded cell batteries at ~25C/room temperature). If you do not recharge the batteries in storage, self discharge will take them into faster sulfation and eventual "death".

AGM and GEL are lead acid chemistry too... With roughly the same performance issues. AGM and GEL usually have lower self discharge rates and should be able to go upwards of 6 months of storage before needing recharging.

Our friends in Canada can see batteries that last ~2x longer (for those with cabins that are usually not occupied during winter/cold months/unheated battery installations).

Flooded cell batteries do take over charging better than AGM and GEL batteries--As long as you keep the water levels above the plates... If the electrolyte drops and the plates are exposed--That is also another way to murder the batteries.

-Bill

softdown · August 2016

Turns out that my forklift battery has a very high evaporation rate. It is on the north side of a shed and outside. Climate is alpine desert. I was shocked to see my plates very exposed in spite of no usage other than periodic charges.

Thinking of sealing 1/2 the holes in the yellow "cell plugs".

As opposed to my 8Ds...never come close to seeing the plates in those in spite of heavy usage and rare top offs.

Yes....my gels have a very, very low self discharge rate.

My 8Ds are usually fully charged by 10-11am. What does that say? Thinking that I could safely add a chest freezer kept in a cool room.

I am rusty as hell...

BB. · August 2016

Charging (gassing) is what (mostly) causes water loss... And from what little I have read, forklift batteries tend to have less capacity for "extra" electrolyte vs batteries typically designed for solar power.

And charging an almost full battery (over 90% state of charge) generates more gas per amp*hour vs charging a battery at less than 80% SoC.

-Bill

softdown · August 2016

Only have two panels dedicated to that battery now.....so 360 watts of power. I would be surprised if that made significant gas. I have yet to hook this bank up for any work. Was going to add some panels and get some work out of it yesterday. Then my realtor got an offer on one of the eight land parcels I have for sale.

What causes a battery cell to go bad?

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