BU-804a: Corrosion, Shedding and Internal Short

Corrosion occurs primarily on the grid and is known as a softening and shedding of lead off the plates. This reaction cannot be avoided because the electrodes in a lead acid environment are always reactive. Lead shedding is a natural phenomenon that can only be slowed down and not eliminated. A battery that reaches the end of life through this failure mode has met or exceeded the anticipated life span. Limiting the depth of discharge, reducing the cycle count, operating at a moderate temperature and controlling overcharge are key in keeping corrosion in check. To reduce corrosion on long-life batteries, manufacturers keep the specific gravity at a moderate 1.200 when fully charged, compared to 1.265 and greater for high-performing  lead acid batteries Read about How to Measure State-of-charge. A lower specific gravity reduces the capacity the battery can hold.

Applying prolonged overcharge is another contributor to grid corrosion. This is especially damaging to sealed lead acid systems. While the flooded lead acid has some resiliency to overcharge, sealed units must operate at a correct float charge. Chargers with variable float voltages adjust to the prevailing temperature to help to keep grid corrosion in check. Such chargers are in common use for stationary batteries.

To attain maximum surface area, the lead on a starter battery is applied in a sponge-like form. With time and use, chunks of lead fall off and reduce the performance. Figure 1 illustrates the innards of a corroded lead acid battery.

Innards of a corroded lead acid battery

Figure 1: Innards of a corroded lead acid battery

Grid corrosion is unavoidable

The terminals of a battery can also corrode, and this is often visible in the form of white powder. The phenomenon is a result of oxidation between two different metals connecting the poles. Terminal corrosion can eventually lead to an open electrical connection. Changing the connecting terminals to lead, the same material as the battery pole of a starter battery, will solve most corrosion problems.

Internal Short

The term “short” is commonly used to describe a general battery fault when no other definition is available. As the colloquial term “memory” was the cause for all battery ills in the NiCd days, so do battery users often judge non-functioning lead acid batteries simply as being “shorted.” Let’s take a closer look and see what a shorted lead acid battery truly is.

The lead within a battery, especially in deep-cycle units, is mechanically active and when a battery discharges, the lead sulfate causes the plates to expand. This movement reverses during charge and the plates contract. The cells allow for some expansion but over time the growth of large sulfite crystals can result in a soft short that increases self-discharge. This mechanical action also causes shedding of the lead material. On a starter battery, the shedding is manageable because the lead plates are thin and the battery does not go through a deep discharge. On a deep-cycle battery, on the other hand, shedding is a concern.

As the battery sheds its lead to the bottom of the container, a conductive layer forms, and once the contaminated material fills the allotted space in the sediment trap, the now conductive liquid reaches the plates and creates a shorting effect. The term “short” is a misnomer and elevated self-discharge or a soft short would be a better term to describe the condition.

“Soft shorts” are difficult to detect because the battery appears normal immediately after a charge and everything seems to function as it should. In essence, the charge has wiped out all evidence of a soft short, except perhaps an elevated temperature on the battery housing. Once rested for 6–12 hours, the battery begins to show anomalies such as a lower open-circuit voltage and reduced specific gravity. The measured capacity will also be low because self-discharge has consumed some of the stored energy. According to the 2010 BCI Failure Modes Study, shorted batteries accounted for 18 percent of battery failures, a drop from 31 percent five 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. This causes parts of the plates to become naked. The 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 large 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. This causes a more serious short and is associated with a permanent voltage drop. The shorted cell may have little or no charge and the specific gravity of the electrolyte is close to 1.00. This is mostly a manufacturing defect and cannot be repaired.

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.


On March 8, 2012 at 3:53pm
Carl wrote:

In a flooded battery, is it possible for a cell to be open? If so, what can cause a open cell?

On March 8, 2012 at 7:45pm
Wayne Robey wrote:

The above statement users often judge non-functioning lead acid batteries simply as being “shorted.” does not apply to long life sealed cylindrical cells such as made by Gates. I have never seen one of these shorted but I have several that suddenly opened.  Of course sulfation over a long time will gradually produce a near open as well.

On March 10, 2012 at 9:28am
Sanjay wrote:

why lead used in battery

On April 8, 2012 at 8:35am
Chris C. wrote:

In response to Carl: many years ago I was building a powerful winch system for a pickup, designed to pull logs out of the bush. It ran on an 8D size battery and worked with with a huge, 130A alternator. It worked well, but it was hard on the components: batteries would only last a couple of years, the 10,000 lb ‘heavy duty’ winch had to be serviced after a year and the pickup needed new springs at the end of the first season. Anyway, we soon realized that our components were undersized for our needs and that there is a good reason skidders are needed for this kind or work wink But I digress…

The first battery we installed failed within a couple of weeks. Soon after installation we noticed the ammeter would fluctuate quite wildly and suspected the alternator, which we upgraded from 100A to 130A as a precaution, but the problem reappeared. Then one morning nothing, 0 volts. So we took the battery back to the dealer, he said “the battery is sulfated, it’s not covered under warranty” and was reluctant to give us a replacement. After we explained it had been connected to a 130A alternator that worked well (I had both an ammeter and a voltmeter installed to monitor the system), he realized we hadn’t abused it and when he tried to charge it there was absolutely no response, like I expected. Since he had actually worked on batteries before (in the old days you used to be able to remove the top), he said “looks like a weld cracked in one of the cells”. In other words, one of the connecting bars made of lead, which are soldered during manufacture to the internal posts each cell possesses, came lose or broke. Result, open circuit and unusable battery.

So there you have it, Carl. If you suddenly have no voltage whatsoever (I haven’t seen the problem since), one of the cell connections is probably cracked or came unsoldered or perhaps even was never properly soldered in the first place. This type of manufacturing defect will show up more if the battery is subject to a lot of shocks and vibration, like in our installation.

On May 6, 2012 at 12:28pm
Blair wrote:

Just got back from an aborted trip, stopped for lunch, back out to start the car and heard a thump, the battery had split open at the top. 4 year old original equipment. Water level was a little low, but not significantly. Had someone bring me a new battery, replaced it and drove back home. Any thoughts on why the battery failed? Was a little concerned about putting a new battery in that there could have been another reason the battery failed, decided to give it a try anyway.

On May 6, 2012 at 11:50pm
Chris C. wrote:

Blair, that sounds like an internal explosion from hydrogen and oxygen accumulation. Overcharging may have be involved. If the battery developed a short or internal crack, high demand may have caused the spark needed for the explosion.

Advice: check the charging system.

On June 20, 2012 at 2:59pm
John mc wrote:

My 4 year and 2 month old OE battery test readings seem odd. The car in that time has 120k. Lights are never left on when parked, gets lots of driving so should be well cahrged. The EN rating is 570. I’m getting a reading of circa 430 ish, once got 390 but can vary. The open circuit voltage rests approx 12.48 ish, it definately never gets to 12.6. Is my battery faulty? using the tester on other batteries always read higher than the rated 570 and voltage 12.7 at least..

One morning i pulled a fuse and done a crank test. Engine cold. It cranked good and fast. I kept it going for a good 15 secs and the speed of the cranking never declined.. What should i do? The readings say bad in comparison to other batteries but when tested under load from cranking it works great.. Could this fail suddenly?

On September 4, 2012 at 2:22am
karim nawaz wrote:

i am a beginer to learn about a lead acid battery, and want to know whether chemical re-actioin starts before applying dc current to charge the battery or it starts on charging.

On September 10, 2012 at 8:29pm
Manuel G. wrote:

Does a battery cadmium based aditive affecs a lead-acid battery? Could this additive make a battery to “explode”?

On September 11, 2012 at 6:52pm
Wayne Robey wrote:

I used a Cd containing additive several times over 20 years ago wit old engine starting batteries. It was supposed to improve performance by reducing sulfation but did not seem to do anything. I don’t see how it could cause explosion.

On October 27, 2012 at 11:30pm
John Fetter wrote:

Cadmium does nothing in a 12V lead-acid battery until the battery is charged up to 15.5 to 16 volts. It plates onto the negatives and in doing so reduces sulfation. Switch the charger on and off at intervals of a couple of hours, over a period of three weeks and your battery becomes as desulfated as it will ever get.
Putting cadmium into a battery CANNOT make it explode. Igniting the hydrogen-oxygen gas mixture that is given off during charging certainly can. Take care to NEVER create sparks near a battery. I have seen scars on someone’s face that were put there when he playfully made the battery spark by putting a piece of metal between the terminals.

On December 2, 2012 at 12:42am
John Fetter wrote:

Ask any acknowledged lead-acid battery expert this question - Assuming a lead-acid battery is correctly maintained, correctly used, why does it wear out?
Answer: The lead-acid system is subject to slow, progressive corrosion of the positive grids when correctly used. It is subject to sulfation when it is persistently undercharged, (incorrectly used).
A lead-acid battery can give between 4 and 25 years service when it regularly receives a small, controlled overcharge. It can fail within 2 years if persistently used below full state of charge.
A large percentage of leisure marine, light aircraft, truck and automobile batteries are operated almost permanently below optimum state of charge, hence become sulfated.
This does not mean lead-acid batteries will naturally become sulfated, as implied by desulfation merchants. They have products to sell, therefore they will say whatever it takes to sell their products
The dividing line between corrosion and sulfation is a knife edge. There is no safe area of operation. The only way to make batteries last is to subject them to regular, mild overcharging.
The corrosion process begins when a lead-acid battery is formed. The lead at the surface of the positive grids is converted into lead dioxide. Lead dioxide passivates the metal surface. After that it takes many years in service for the layer of passivation to very slowly work its way right into the grids and cause them to fail.
Corrosion is necessary. The process of charging “corrodes” lead sulfate of the discharged positive active material into lead dioxide. The lead-acid system cannot work without corrosion. Undercharging causes lead that is exposed to sulfuric acid to be converted into lead sulfate. If there is no “beneficial” corrosion, there will always be “harmful” sulfation..

On December 3, 2012 at 6:48pm
tiger wrote:

i have replace my meter with new four Ni Cd batteries but still the new one also fails and discharge suddenly. what can be the reason, is that possible that there is some issue in battery holder? as someone told me that holder can get corrosion and that also shorten battery life.

On December 3, 2012 at 7:56pm
Wayne Robey wrote:

Relating to John Fetter’s comment, I have some 220 AH Exide golf cart batteries used for 11 years often floated, often slightly discharged, occasional discharged to no more than 20% SOC use around 50 ml water/cell/year (about 100 ml first year). What I noticed is flaking off on the positive internal buses and swelling upward of the case around the positive terminal. I have not seen this on other batteries and it does not seem to correspond to wear out by total AH discharged over their life. I cannot see any sign of sulphation and the fully charged specific gravity indicates there is not much. What could be done to reduce this problem?

On December 3, 2012 at 11:51pm
John Fetter wrote:

tiger - There is a how long is a piece of string element in your question. I am not trying to be disrespectful. It is not possible for someone else to work out from the limited info.

Wayne Robey - What you describe is normal. The internal connector bars are likely made with an alloy that is sufficiently different to the grid alloy, thus attracting extra corrosion to themselves. When the grids in the positive plates become corroded, they increase their volume. When lead metal is turned into lead dioxide, the resulting material takes up more space. The grids of the plates are firmly connected to the top bar that in turn is firmly connected to the positive terminal. The lid is flexible. Hence the terminal gets pushed up. Ignore the stories about sulfation. They are written by people who have something to sell. When people get old, their faces become wrinkled. When batteries get old, they do what your batteries are doing. There is nothing wrong with your batteries.
As a matter of interest, when the total volume of water that is added over the years reaches the same volume of the electrolyte, your batteries will most likely stop working. I commissioned testing on two sets of Trojan golf cart batteries, two sets of Exide and two sets of Deka golf cart batteries years ago. They all expired after using this amount of water. Two-thirds of them lasted between 40 and 60 percent longer because we had given them something to make them last longer. Their water consumption was slower. I was amazed to find the water consumption so predictive.

On December 11, 2012 at 8:00am
econobiker wrote:

John Fetter wrote: “A lead-acid battery can give between 4 and 25 years service when it regularly receives a small, controlled overcharge.”

I can vouch for this fact of battery life.  I had to buy an automobile battery in order to transport a rented car back to an automobile dealer. I kept this battery on a maintenance charger (in series with 2 to 3 smaller motorcycle batteries) for about six years.  I occasionally used it to help start motorcycles with dead batteries but otherwise it was in a cool basement on the charger for the bulk of the time.  After about six years I installed the battery in a small 4 door car which used it another three years before the battery performance began to degrade. 

The funny part was when I went to buy a new battery.  As auto parts store technician removed the old one, he could not believe that the date code showed a nine year old battery! I allowed him to keep thinking that the battery had been in the car for 9 years…

On May 29, 2013 at 2:51pm
Jim Kincheloe wrote:

Great information !!!!!!!!!!!!!

On June 4, 2013 at 8:33am
Bill Wilson wrote:

I’m in the process of recharging 3 wet Interstate batteries that were barely used before laying idle for 4 - 12 years. I’m using an ancient charger with an amp dial and meter,. I start at 2 amps and it’ll drop to 1 then 1/2 amp then stay there for days with the battery warm and bubbling. The 4 year old lawn tractor battery came back to life with two charge/discharge cycles. Some sulfate remains visible one the plates. The 12 year old Honda Goldwing battery was bulged at the ends, heavily sulfated, took an initial charge and showed 12.6 volts. A fair admount of sulfate disappeared. It discharged in a few hours using a m/c turn signal bulb. Am doing the second charge and noticed that a piece of lead shelled off in one cell and two other cells have developed some lumpy stuff between the plates on top. The battery showed 13 volts, showed barely bad when load tested and went back to 12.3 volts after the test.  It’s back on an 1/2 amp charge to see if the lumpy stuff will dissolve. Will the shelled off lead do the same of should I try to fish it out thru the cap hole?
I haven’t bothered with the 9 year old auto battery yet. It’s been sitting in a car outside in NE Texas since new.

On June 5, 2013 at 10:34am
Wayne Robey wrote:

I have had some success and some failure with these neglected batteries. With continuous charge, it is necessary to maintain a slow charge for a long time, sometimes a month. When done, the capacity will be low and self discharge high. Pulse charging is another approach that I have not tried.

On June 5, 2013 at 2:35pm
John Fetter wrote:

Bill Wllson, Wayne Robey - We discussed cadmium in Sep-Oct. Wayne says he tried cadmium but it did not work. I tried it and it did work. I discovered that it is necessary to get the voltage right up to at least 15.5V by slow charging. Don’t try forcing it up. It must rise on its own account with just a few amps. I used a timer to switch the charger on for one hour, and off for one hour, for three weeks.
The cadmium plates onto the negatives during charge, goes back in solution during the idle periods. Each time progressively infiltrating the sufate. Nothing seems to happen for the first week. By the end of week two the battery begins to show signs of life.If at the end of week three nothing has happened, the battery is ready for recycling. Perhaps two out of three batteries cannot be recovered.

On February 25, 2014 at 10:22am
Patankar Sharad wrote:

What are the conditions that shrinks negative active material in 5 cells and 1cell having soft and mussy negative?

On March 9, 2014 at 11:19am
Mark Vanderkooi wrote:

I am interested in the comment at the beginning of this article ” To reduce corrosion on long-life batteries, manufacturers keep the specific gravity at a moderate 1.200 when fully charged, compared to 1.265 and greater for high-performing lead acid batteries.”  I just took delivery of 6 Surette dry-charged golf-cart style batteries, and duly filled them with 1.230 acid, which all we can get here in Chad, Africa where I work. I was little concerned about this, so wrote the manufacturer who told me I would lose “at least half the battery capacity” with such weak acid. Is that true? Seems a little excessive for only a 13% decrease in sulfate ions. Or is there some other mystery in the art of dry-charge such that the manufacturer did his magic assuming 1.265 and I messed all the chemistry up with my 1,230 acid? Actually, I would be happy for the extended life-expectancy - if sheer capacity is the only price I have paid, and I have not messed something else up.

On March 9, 2014 at 12:06pm
Chris C. wrote:

Mark, I am not an expert as John Fetter who posts here from time to time, but my understanding is that you are both right. Lower electrolyte density means less ions to transport electricity therefore less current producing capacity but on the other hand in a hot country such as Chad you would have greater chemical action therefore it should compensate.

What the Surette engineer is probably telling you is that you will have lower reserve capacity and you will exhaust the acid sooner, which could lead to premature sulfation, which is arguably problem number one with lead acid batteries.

Now if these batteries are constantly kept at a high charge level and properly balanced, you will minimize the risk of sulfation.

In other words, keeping a low acid density electrolyte, you should still be able to produce good current at your higher ambient temperatures while protecting the cells from long term corrosion, but you will probably experience have a lower reserve capacity as ions will exhaust sooner, and you would have to balance higher risks of sulfation with lower risks of internal corrosion.

From what I know of Surette batteries, although I am not sure about how the golf cart type are designed, is that they are very robust, which means they can take a lot of abuse that would kill lesser batteries. What is your usage scenario?

On March 9, 2014 at 1:45pm
Mark Vanderkooi wrote:

Overall, that sounds pretty encouraging. Thanks Chris. The trick is going to be equalizing on a regular basis - something I haven’t always been very good at. They are on their first charge since being filled right now. They run a 250 watt FM radio station which draws about 50 amps for an hour and a half each evening, and for 40 minutes each morning. They are recharged by solar at about 23 amps, You are right about the Surette batteries being pretty robust. I got 8+ years out of the last set on the radio station. Similar on my home.

On March 9, 2014 at 4:38pm
Chris C. wrote:

As long as you remain in C/10 or C/20 range for charge and discharge you should be fine in terms of draw versus battery capacity (in other words, 500 to 1000 AH of capacity)..

Your draw of a total of 2 hours at 50A should well be taken care of in 4 hours of direct sunshine at 25A or so, which I’m sure you should be able to get without any problems in your country wink

Equalization requires distilled water (which may be rare and expensive in your country - deionized water is another alternative), and careful monitoring of the electrolyte density of each cell.

In order to equalize you need to set your voltage such that you will force current through an already charged battery, which may require voltage to be raised to 16 or so, which can overheat the battery, so it’s important not only to monitor electolyte level (so as to keep the plates covered) but also temperature (so as to reduce corrosion and plate warpage - try not to exceed 45C).

I would suggest you do that when you know you will have plenty of sun and temperatures aren’t too high, while you don’t use power, because of the high voltages that may damage sensitive equipment. It can sometimes take several days, so it’s best to do on a regular basis, as soon as density differences between cells exceed 0.005, to minimize the time it will take to do it.

On my marine start/troll batteries, it needs to be done approximately every 3 months or so; Before I knew about equalization, my batteries would generally last 3-4 years, since then I’ve been able to get more than twice that.

During equalizing, already charged cells will bubble from outgassing while the lower density cell(s) take(s) on the charge, and those bubbling cells will have to get extra water as water evaporates from them in this process.

On March 9, 2014 at 4:44pm
John Fetter wrote:

Mark, Chris - The maximum summer temperature at the Surette factory, located far away from major cities, in the pine forests of Nova Scotia, is lower than the minimum temperature in Chad..Surette have absolutely no idea what a hot climate does to lead-acid batteries. Mark is 100% correct to use a lower SG.

On March 18, 2014 at 3:31pm
Wayne Robey wrote:

One thing to keep in mind is that the large stationary batteries have a large volume of electrolyte for the capacity so the range of specific gravity from fully charged to discharged is less than a typical golf cart battery which is physically smaller, so less electrolyte volume. Starting with a fully charged specific gravity of 1.23 seems quite satisfactory if you only discharge to the SG given by the manufacturer for the fully discharged battery then charge it. I can’t say if a stronger acid would be helpful to prevent sulfation during poor weather.

On March 18, 2014 at 4:45pm
John Fetter wrote:

An SG of 1.265 in a cold climate and an SG of 1.230 in a hot climate, at full state of charge, will provide corresponding minimum SGs at minimum state of charge. The battery operated in the hot climate does switch to a cold climate when it is discharged. It will be able to deliver the substantially the same current down to a correspondingly lower SG. The minimum SG given by the manufacturer applies to a cold climate.

On March 18, 2014 at 4:49pm
John Fetter wrote:

Correction, typo -  The battery operated in the hot climate does NOT switch to a cold climate .....

On March 19, 2014 at 3:57am
Mark Vanderkooi wrote:

It has been about 10 days since I filled the batteries (6 x 6 volts wired for 12 volts, about 1000 amp-hours total) with the 1.230 acid. I charged them for about a week up to 15.0 volts. They didn’t seem to want to go much above 15 volts, though I tried. We started actually using them two days ago, and they are performing very well. Funny thing though: many of the cells went all the way up to 1.250 or even 1.260 after charging.  Most stopped at 1.240. A few are stopped at 1.230. It sure seems that Surrete must put some sulfate in the plates in the dry charge process which go into solution when the charging cycle begins. Anyone know anything about the chemistry of dry charge? I happen to know that some of the bottles of acid I used were even less than 1.230 and suspect that the weakest cells are probably a result of that. I have evaporated some acid to make some super thick stuff - maybe 1.40 or more (haven’t tested it yet - it still out in the sun distilling) My idea is to try to balance the weakest cells with a calculated amount of this stuff. Good idea or bad idea? I know from experience in years past the in unbalanced cells just stronger get stronger at the weaker one’s expense.

On March 19, 2014 at 4:40am
John Fetter wrote:

Mark - You ARE doing it right. After filling, lengthy charging, you MUST balance the SGs. Dry-charged batteries are not totally dry. They are fully charged in the factory, most of the acid is then removed and the cells are plugged. The negatives tend to become discharged but not the positives. That is why the batteries must be charged after filling. Golf cart batteries have antimony alloy grids which have a relatively low gassing potential. You need plenty of amps to get the voltage above 15 volts.

On April 4, 2014 at 8:20pm
Santa Clara wrote:

I have problem that some batteries corroded in the internal connect (between cells) at negative side. Normally, the corrosion happen in the positive grid but why it only happen with the negative connection?

Could you please give some advice?

On June 30, 2014 at 11:20pm
David B wrote:

I have 5, 150 a/hr Gel batteries at my cabin connected to solar panels through a regulator, the batteries are 4 years old. I am finding that they show a good charge but, they do not seem to have a lot of a/hr. Why is that???  I do not live at my cabin, but use it as a weekender from time to time.

On July 1, 2014 at 7:14am
John Fetter wrote:

David - You are reading the full-charge voltage of gel batteries that are reaching the end of their useful life. If you want a reasonable battery life, use flooded batteries.