Gel cell battery failure

[rick hart]

Can someone point me in a direction that could explain why 2 bateries in a string of 18 would burn and meltdown? The bateries are less than six months old, installed in an external UPS cabinet in a controlled environment but failed in glorious fashion. You should have seen the smoke.
There was no sign of actual fire damage- the cabinet was not scorched and the sprinlers did not open- other than two batteries and one that looks like was next to one burning. It was not until the smoke cleared and the dry chemical was swept up that the lack of fire damage was apparent. The fire department removed the batteries from the cabinet so actual location that each battery was in (other than the two distroyed0 cannot be determined. The UPS is disconnected pending post mortem. What causes bateries to fail like this?


Second question: although power was removed from the both the AC line and battery cabinet to the UPS, the batteries were actually where the problem occured. The firemen had to cut the cables connecting the batteries before the archng and source of ignition was removed. The question is did we miss something by not providing a battery string interrupt? I have never seen one before but today, they look like a real good idea.

 

[dereckbc]

Re: Gel cell battery failure

My first guess is thermal run away. It is a nasty problem the valve regulated batteries exhibit. But it never seen a fire result. Thermal runaway is when the battery starts to short internally, as it heats up the impedance gets lower, therby drawing more rectifier current and heating up more. The process of heat generation continues until all the rectifier current is consumed.

Breakers, fusses, disconnect is always a hot topic with battery plants. Most will not use them, some will. There are pros and cons of both methods.

Upper end plants use thermal senseor mounted on VLRA battereis to detect thermal runaway. When heat is detected the rectifier voltage is cut back to stop the process and an alarm sounded.

 

[Flyersfan]

Re: Gel cell battery failure

Were they discharging (during a power failure) when they fried? I have seen bad cells within new battery strings before. It doesn't happen too often, but it does happen. Another question. Do the damaged batteries look as though the strap and posts melted down or the casing? Sometimes an improper installation of the battery string can lead to a high resistance connection that could fry. As far as the battery string disconnects, they are not a bad idea for the isolation of different strings, but it's not going to stop an internal cell meltdown once that begins. Batteries put on quite a show. When you do put the system back on line I recommend that you have somebody do a base line check of all cells to see how healthy they are. I use a Midtronics meter to do just that. I can check each of them while the unit is up and running. Maintenance is the key. Even on "maintenance free" systems.

 

[rick hart]

Re: Gel cell battery failure

The casings are burned mostly from the bottom up. There really wasn't a fire so much as arching and popping- no visible scorching on anything except the battery. The terminals were clean and tight. The UPS (Best Unity 360) was in standby and not being discharged.

If power was removed from the UPS, could the thermal run away still continue? Or once things start shorting it's off we go?

What can start the thermal run away? If the room is too hot or cold can that start the thermal run away? What temperature can this happen? I am starting to suspect that the room- which is a converted store room/electrical room was warmer than it should have been. I try for 78F

We are going to relocate the UPS away from the critical area into a large electrical room to keep the excitement down in the future.

 

[Flyersfan]

Re: Gel cell battery failure

Looks like a bad cell to me. It happens. I would perform the test with the Midtronics meter to make sure the rest of your cells are healthy and replace the damaged ones. A disconnect will isolate the string from other strings, but once a runaway starts there is nothing to stop it really. Removing the power source does very little to stop the internal fault of the cell. It's going to feed itself until it burns out or you extinguish it. In a gel cell as you have described there was more than likely something wrong internally to that particular cell. If you had a wet cell battery, sediment lying on the bottom of the cell could cause a short across the plates and the same thing would have happened. Room temperature as you have described shouldn't have caused any problems. Too hot and too cold are both not good for batteries. Hot is worse, but I have had plants run in the high 90's for days without problems. Not ideal, mind you, but not catastrophic. Moving the plant away from the critical load is a good option if you have the real estate. Call the manufacturer of the battery and tell them what happened. They may have had similar problems elsewhere.

 

[dereckbc]

Re: Gel cell battery failure

Originally posted by Flyersfan:
A disconnect will isolate the string from other strings, but once a runaway starts there is nothing to stop it really. Removing the power source does very little to stop the internal fault of the cell.

Sorry but this is completely accurate. There are two ways to stop a runaway.

1. Disconnect the batteries from the rectifiers and other parallel strings if installed.
2. Lower the voltage.

I will try to explain a little better this time. VLRA (valve regulated batteries) have a gel or paste as an electrolyte rather than water/acid solution. They are manufactured completely different from wet or flooded cells. The plates are laid on top of each other with the gel or paste separating them, then rolled like a jelly roll, or stacked like a sandwich. This make them very compact and have lower internal impedance than there flooded cousins. This is why VLRA have higher discharge rates in smaller/lighter packages. The down side of VLRA is their life span is significantly shorter and prone to internal shorts if by the likely chance they ever vent pressure from to high float voltages.

OK so now what happens? If one of the batteries in a string of 12, 18, 24, 190 what ever develops short or low impedance, its voltage drops in proportion. As the voltage drops, the rectifiers (and any parallel strings) then try to equalize the voltage by supplying more current to the affected string to get its voltage back up. As more current flows, the affected cell (and surrounding cells) heats up. As the batteries heats up, the internal impedance get lower, which causes more current, more heat, and the regenerative process continues unless something happens to break the cycle.

Modern rectifiers for VLRA batteries use heat sensors mounted to the batteries. As the batteries heat up, the rectifiers lower the float voltage, thus reducing the charge current, heating, and breaking the chain.

Look at it this way. Let?s say we have a 24 VDC plant using 12 batteries in series. The plant normally floats at 27 VDC, or 2.25 volts per cell. Now let?s assume one battery shorts out. What is the voltage across the shorted cell? 0 volts maybe? What does that do to the remaining 11 cells voltage with a plant supplying 27 volts? 27/11 = 2.45 volts per cell. That is way over voltage across the cells and they will take on all the current the rectifiers will supply to try to get to 2.45, plus any that parallel strings will supply. Since a battery can never charge to that level, it will heat up, lower the internal impedance of the remaining good cells, current increases and off to the race we go, thermal runaway.

So if we had a thermal sensor that folded back the voltage from the rectifiers to say 24.75 volts, what happens? Well 24.75/11 good cells = 2.25 volts per cell. So you end up with 11 happy cells operating normally, one dead shorted out cell, voltage output of 24.75 volts from the plant, and a low voltage alarm. Will 24 volt equipment operate @ 24.75 volts? You bet it will, it should work down to 20 or 21.

Now some food for thought, I can prove battery disconnects can cause explosions, fires, extended outages, and massive damage to equipment. I can also prove it can prevent the same but not as likely.

[ October 28, 2005, 07:56 PM: Message edited by: dereckbc ]

 

[Flyersfan]

Re: Gel cell battery failure

I stand corrected on the disconnect removing the rest of the strings from the system thus preventing the entire system from discharging into the damaged cell. I have had such emergencies happen on telecom plants and the on site engineers have thrown the disconnects and the cell continued to simmer. The string that the cell was in continued to pump into the short. Individual cell disconnects would be ideal I guess but the $$ would be staggering if they even exist. I haven't come across them. Rick, are the batteries on this system VRLA's or are they "just" gel cells? Do you have temperature sensors on the cells to control runaways? From your first post, I don't believe that you do. Those systems are the Cadillacs. I only seem to run into Chevrolets.

 

[rick hart]

Re: Gel cell battery failure

I do not have sensors but I can see the advantage from Dereks explantion.
Is there an after market retrofit that you know of? Shutting down the charger would be difficult but sounding an alrarm would have a benefit.
The batteries were VRLA of very high recommendation- don't want to get into brand names here in an open forum.

 

[dereckbc]

Re: Gel cell battery failure

Originally posted by Flyersfan:
I have had such emergencies happen on telecom plants and the on site engineers have thrown the disconnects and the cell continued to simmer. The string that the cell was in continued to pump into the short.

Don't mean to be a know it all, just interested. How is this even remotely possible. Open circuit = no current.

 

[petersonra]

Re: Gel cell battery failure

i suppose you could have an internal short,

[ October 31, 2005, 04:36 PM: Message edited by: petersonra ]

 

[rick hart]

Re: Gel cell battery failure

Looking at the remains, it looks like once the arching started it simply fed itself. The EPO was hit which opened the DC and AC inputs to the UPS leaving just the stored energy of the batteries. Only when the string was cut in several places- under load- did the arching stop. The batteries that burned were located on separate shelves in a cabinet.
So....open circuit did not equal no current since it was the storage of current that was faulting to something- that is what I am still trying to determine.

 

[Flyersfan]

Re: Gel cell battery failure

Dereck,
You have educated me already. I never claim to know too much (far from it) and our Q&A is enjoyable. The string containing the damaged cell discharged into the fault in that one bad cell. The disconnects just separated the different shelves(strings) on the rack from one another. Just like Rick's experience.

 

[rick hart]

Re: Gel cell battery failure

Ditto on that education thing.
Thanks Dereck

 

[dereckbc]

Re: Gel cell battery failure

This thread prompted me to call on one of my battery manufacturing reps and throw this question out to him.

First words out of his mouth before I could finnish my question was thermal runaway. He said it is rare for it continue after disconnected, but they can continue to discharge upon itself until the gel evaporated and opened the internal fault. He said he didn't have exact figures but thought maybe 1 in 20 cases of runaway the battery will continue to runaway and no way to stop it.

I have seen around 9 or 10 cases of runaway, but never one that continued after disconnected. So I learned something from the topic.

Jim also confirmed the steps to protect oneself:

1. When batteries are purchased, put them in service right away and do not let them set. If there is a delay put them on a float charger in a environmentally controlled area. Be sure the manufactures initial charge procedures are followed to the letter to form the plates.

2. Once batteries receive their initial charge, use a DLRO meter and record internal battery resistance to set a base line to spot problems. Then check monthly for any changes. In addition record cell voltages monthly to help aid problems.

3, Keep the batteries at a constant temp as near to ideal as possible around 75 degrees.

4. Set float voltages at the proper level. Most people set it too high thinking it increases reserve time, WRONG. Manufactures use a window to specify float voltages. For example something like 2.20 to 2.30. The ideal voltage is 2.25. This gives the maximum range for the batteries to fall with in tolerance.

5. Never equalize, never equalize, never equalize unless specifically instructed by the manufacture.

Finally a bit of irony. When VRLA were first introduced they were marketed as "Maintenance Free". This was a huge joke because as it turns out VRLA require more maintenance and do not last near as long as their flooded counter parts. I know of flooded batteries that have been in service for 50 years and still test at 90% of rated capacity. You would be very lucky to test a 5-year VRLA that still has 75% rated capacity. Where I used to work at MCI it is policy to replace VRLA's every 7 years.