Author Topic: Shorted cell in a series/parallel battery string  (Read 8245 times)

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lifer

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Shorted cell in a series/parallel battery string
« on: July 17, 2015, 07:55:54 PM »
I have a 24V battery bank consisting of 8 x 12V/225A lead acid truck batteries (four 12V+12V strings in parallel).

While doing the regular maintenance I found out that one of the 24V string has unbalanced voltage readings: one battery reads 11.1V while the other reads 13.1V.
The rest of 24V strings are perfectly balanced (12.2V + 12.2V).

All the batteries are identical (same brand/model). I suspect the low voltage battery has a short-circuited cell. Both batteries in the string were bought at the same time (9 months ago) thus they are still covered by the warranty.

Anyway, I wonder what could have been the cause of the failing cell (there was no mechanical shock, no over/undercharge and the maximum charging current was 15A).

Also, I would like to ask what was the effect on the second battery (the one reading 13.1V)? It just has been kept charged all these time?

dnix71

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Re: Shorted cell in a series/parallel battery string
« Reply #1 on: July 17, 2015, 09:15:21 PM »
11.1 may not be shorted. 10v would be. Get a 12v battery charger and try to bring up the weak battery.

lifer

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Re: Shorted cell in a series/parallel battery string
« Reply #2 on: July 17, 2015, 10:03:49 PM »
2.4V (fully charged cell)  X 5 (remaining) cells it's still 12V thus it could be shorted for anything bellow 12V.

Anyway, I can't do much testing for now as I have to leave home for few days thus I've just disconnected both batteries. My main concern is what caused this damage as the batteries were carefully controlled (charge/discharge) all the time. Well, not each and every battery but the whole bank.

I'll be more than happy if ithe affected battery it's just discharged but how could that happened? Wasn't the same charge/discharge current for both of them?

One thing's for sure: for now on, I'll monitor the voltage across every single battery.

Frank S

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Re: Shorted cell in a series/parallel battery string
« Reply #3 on: July 17, 2015, 11:39:45 PM »
[[ I'll be more than happy if ithe affected battery it's just discharged but how could that happened? Wasn't the same charge/discharge current for both of them?

One thing's for sure: for now on, I'll monitor the voltage across every single battery.]]

 Not always. if there is any corrosion on the terminals the battery would have a higher resistance since your bank is a 24v version of what I have  almost that is. I have a total of 12 6v 225 AH and 2  12v 200 ah
 when I do routine maintenance on my bank due to where it is mounted in my motor coach I have to pull my batteries out when I check the SG there are nearly always a few cells  that are slightly lower than the others. my bank is beginning to age all batteries are nearly 3 years old now and some much older the recovered trogen 105 s are +5 years anyway I notice that the ones with a lower sg in 1 ,2,or all 3 cells will always have just a little more corrosion on the terminals than the others
 My connections cables are all 000  2 in series and 6 parallels connected to a single point to a 500 MCM  this make it have  the 6 series parallel and 2 individual 12 v all connected to the main cables.
 After wattering and cleaning then allowing the bank to be brought up to 14.8 for a few hours most will re-balance my problem is my bank never rests it is always in use either while charging or discharging.except that now with the summer the generator gives the bank a rest for a few hours each day while running the AC units
I live so far outside of the box, when I die they will stretch my carcass over the coffin

Ungrounded Lightning Rod

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Re: Shorted cell in a series/parallel battery string
« Reply #4 on: July 17, 2015, 11:47:07 PM »
Are you doing equalizing charges occasionally?  (I'll explain this for any newbies who happen to be watching.)

Various cells will have slightly different amounts of leakage.  If you do nothing about it, the leaker cells will discharge a bit more than the least leaky and the difference will accumulate.  Ordinary charging will stop when the most-charged cells are fully charged, ordinary discharging (with undercharge cut-off) will stop when the least charged cells are near their low point - and being more deeply discharged than the others tends to damage them and increase the leakage.  So your pack's capacity drops and your weaest cells get weaker.

An equalizing charge overcharges the battery a bit, gently forcing a little extra current through the fully charged cells to raise the charge state of the undercharged cells.  (This tends to make the fully charged cells electrolyze a little extra water into gas, so you don't do it all the time, to avoid running the good cells dry.)

If you have a low cell, the first thing to do is try an equalizing charge, just to see if the "bad cell" just happens to have been a bit leaky and got low.  If that works, and you catch it early enough, you've saved the battery.  You've also discovered that you need to adjust your charge controller to equalize a bit more than it had been, or see about adding some more generation or reduce your load a bit so your charging system will have enough surplus power to do the equalization on schedule.

dnix71

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Re: Shorted cell in a series/parallel battery string
« Reply #5 on: July 18, 2015, 12:03:21 AM »
The reason I say 11.1 is not necessarily a dead cel is that I just got two 12v 17ah sla's back from my boss. He had an old beat up 3-wheel mobility scooter that he wanted gone. I had put those batteries in years ago for him and he didn't keep them properly charged. When I got the scooter to scrap, the 2 batteries both had about 11.3v on them, but no swelling or obvious damage.

I borrowed a 12v wall wart (0.4 amps) that was intended to charge a cordless drill and used that to boost both batteries. After a bunch of charge and overnight rest cycles, both batteries now rest at 12.8v+

The slas have printed specs that say cycling charge of 14.5 to 15 volts at less than 1 amp. The wall wart puts out 17v no load, but I pulled it when the batteries got to about 15.5v and let them cool down and rest over night.

One battery was weaker than the other and took a couple of extra charges, but both recovered. The trick was to get the voltage way up but keep the current down so as not to drive off a lot of water, and to let the battery rest between charges.

Ungrounded Lightning Rod

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Re: Shorted cell in a series/parallel battery string
« Reply #6 on: July 18, 2015, 12:04:06 AM »
[[ I'll be more than happy if ithe affected battery it's just discharged but how could that happened? Wasn't the same charge/discharge current for both of them?

One thing's for sure: for now on, I'll monitor the voltage across every single battery.]]

 Not always. if there is any corrosion on the terminals the battery would have a higher resistance since your bank is a 24v version of what I have  almost that is. I have a total of 12 6v 225 AH and 2  12v 200 ah

Note that, at the hight currents and low battery internal resistances involved in a battery bank, the small resistances of the interconnecting cabling can steer a LOT of current, drastically unbalancing the paralleled strings so that some are heavily loaded and do most of the charging/discharging while others see a lighter load and take less wear.  Getting these interconnections right with four strings paralleled is NOT straightforward.  (With two you can just use identical wire lengths and hook the charging system up kiddy-corner, but that doesn't work with three or more.)

You can look around the site for any of several discussions of this.  Or let us know when you're back and ready to hack on things, and we can rehash the part applicable to your situation.


lifer

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Re: Shorted cell in a series/parallel battery string
« Reply #7 on: July 18, 2015, 12:22:49 AM »
Thank you all for your valuable advices! I still have few hours left till I leave home thus I could start a recovery procedure.

In this regard, I would like to ask you what's the best way to recharge that battery? Once I used to use double rectified (not filtered) AC voltage to rejuvenate my old sulfated batteries. That's it, it was kind of a high "pulse" (100Hz) charger.

Is it suitable in this situation? Or should I try to recharge it slowly or just using a regular (three phase) charging scheme?

Btw, I could remotely control(!) my wife to take care of the charging procedure (turn on/off).

Thanks again for your kind support.

PS: I didn't see any corrosion on terminals but even if that was the case, how come only one of the two batteries in the series string has been discharged? The parasitic resistance generated by the corrosion should have been affecting the charging/discharging current for both batteries. Am I missing something?

lifer

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Re: Shorted cell in a series/parallel battery string
« Reply #8 on: July 18, 2015, 06:06:40 AM »
A short feedback: when I've connected the affected battery to a charger, the voltage has rised very quick to 15V and it started to bubble (I was feeding 20A constant current). I reduced the current to 5A but the voltage still remained high.

Then I put the battery back in the circuit (charged by the solar array) and the bubbling has stopped but the voltage still remained higher than the other (good) battery. After 3 hours, the voltages seemed to become balanced though. I'm going to let them (over)charge a little and see what will happening next.

Does it looks like it's sulphated (isn't it the common behaviour in such a situation)?

lifer

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Re: Shorted cell in a series/parallel battery string
« Reply #9 on: July 18, 2015, 06:31:30 AM »
Btw, this is how I've connected my batteries (I have two such a banks in parallel).

As you can see, the wires length are kept to the minimum and are perfectly balanced. I'm already away from home but I hope to see it "healthy" when I'll return. ;)

Once again, thank you very much for your prompt advices.
« Last Edit: July 18, 2015, 06:39:14 AM by lifer »

Flux

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Re: Shorted cell in a series/parallel battery string
« Reply #10 on: July 18, 2015, 02:51:06 PM »
I understand your frustration as  you seem to have been careful with everything.

I think you are unlucky to have a battery with a duff cell and I suspect it is not as uncommon as many think. It does on the face of it seem like sulphation but I don't think it is. I think is a connection problem between the plate material and the grid or a problem on the link bars. I think this is the type of failure that those people who play with desulphators have some luck with, possibly the repeated pulsing improves the contact.

I think that battery will cause you endless problems and if you equalise it, it may behave itself for a while and repeat the process later on.

By all means try to get it back in step but keep a very watchful eye on that pair. It may fail a heavy discharge test so you may be able to get something done under warranty but the more you pay the better warranty you get, some very poor batteries have a good warranty but it seems to be included in the first cost.

Check all the obvious things with connections but I suspect you have a rogue battery.

Flux

lifer

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Re: Shorted cell in a series/parallel battery string
« Reply #11 on: July 18, 2015, 03:58:38 PM »
Thanks for your valuable input, Flux! Chances are your suppositions could be real.

After being charged for about 8 hrs (in series with its healty pair) the stand-by voltage of the affected battery has rised to 11.5V. It might have been better to let it charge alone but that's it, I'm going to make it as soon as I'll return home. For now, I'm going to let it disconnected.

I've tried to read about this stuff lately yet haven't found an answer: could someone explain the behaviour of these two (12V) series batteries with different SOC (state of  charge) connected in parallel to a large (24V) bank?

The voltage across the series string is forced by the rest of (healty) batteries of the bank (24V in this case). So the discharged battery has 11V and the other has 13V.

What happen when you try to charge/discharge this series pair (the current flowing through both of them being the same)? I wonder if that could have been affecting the "healty" one, too.
« Last Edit: July 18, 2015, 04:04:30 PM by lifer »

lifer

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Re: Shorted cell in a series/parallel battery string
« Reply #12 on: July 21, 2015, 02:34:03 AM »
No luck so far with battery recovery operations. I've tried to let it slow charge (2A) during 10 hours and it seemed to behave normally (the voltage has rised slowly to 13V). I left it open to "sleep" overnight but I found it at 10.7V in the morning.

Now I'm trying to charge it again, with a small increase in current (4A). This time, I hear bubbling while the voltage is around 13V. For the rest of my batteries, I only heard bubbling before whent the battery voltage was over 14V.

What does it mean? I really have a bad cell and the rest ones are overcharged (13V / 5 cells = 2.6V) during this procedure? Or is it a normal charging behaviour? (I must admit I've never "listen" to a battery as much before).

If it's a bad cell, how to prove that to the vendor? How to distinguish between whole battery sulphation and a bad cell?   

Flux

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Re: Shorted cell in a series/parallel battery string
« Reply #13 on: July 21, 2015, 04:45:01 AM »
Proving to the vendor is the tricky bit, but as it is a truck battery they will likely go for a heavy discharge test. With a bit of luck the dead cell will boil violently during the test and there will be sudden drop in volts as that cell gives up.

You are only getting to 13v by forcing the 5 good cells over voltage. This would have been difficult to spot with a parallel string but you get a chance to detect the unbalance with series parallel. As you spotted it early the other series member will likely just be well equalized and ok.

Flux

lifer

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Re: Shorted cell in a series/parallel battery string
« Reply #14 on: July 21, 2015, 07:15:29 AM »
Proving to the vendor is the tricky bit, but as it is a truck battery they will likely go for a heavy discharge test. With a bit of luck the dead cell will boil violently during the test and there will be sudden drop in volts as that cell gives up.

It was specified in the certificate of guarantee that it will be used in a solar system. What else could have lead to a defective (shorted) cell beside a manufacturing issue? There was no vibrations or mechanical shocks (though it has a HVR (high vibrations resistant) feature anyway). 

the other series member will likely just be well equalized and ok.

Yes, the other one is absolutely fine (it now reads 12.8V, after being disconnected few days ago).

Anyway, the affected one still can't rise up over 13.2V (and is continuously bubbling). I'm charging it with 4A (the battery is rated at 225A). I have disconnected it right now and I'll wait to read its voltage after some rest. If it falls down again to 10.8V I'm out of (recovering) ideas.

lifer

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Re: Shorted cell in a series/parallel battery string
« Reply #15 on: July 22, 2015, 06:04:03 AM »
Long story.. SHORT(-circuited cell).

After trying for few hours to resuscitate the battery, the vendor has offered a brand new (identical) replacement.

After they have removed the seal (it was a "maintenance free" battery) and have taken the lid apart, I could see the electrolyte bubbling in 5 of the 6 cells and standing still in the last one (there was a 10A charging current). The electrolyte was perfectly clear and the level was optimal, no sign of sulphation or other "bad treatments".

Now that the story has a happy end, I wonder what should I do to prevent any similar situation, beside monitoring the voltage and current for every single battery to rise an early alarm.

Flux

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Re: Shorted cell in a series/parallel battery string
« Reply #16 on: July 22, 2015, 10:31:45 AM »
Pleased you got it sorted pleasantly, obviously a decent supplier.

As you say, you will have to monitor the balance of the series strings regularly. Perhaps a weekly check for a while to see how things settle down and if all is well you can increase the time interval.  As the batteries get older the risk increases so you will have to be more watchful in their later life.

Flux

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Re: Shorted cell in a series/parallel battery string
« Reply #17 on: July 22, 2015, 10:42:50 AM »
As a suggestion I would be sure and record on paper what happened and also record the battery testing each time. Three years from now you may need a few guide lines to follow and some records would maybe help you find another problem. Just an idea 13
MntMnROY 13

lifer

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Re: Shorted cell in a series/parallel battery string
« Reply #18 on: July 22, 2015, 11:37:21 AM »
Thanks both of you for your suggestions. As a matter of fact, when I was talking about "monitoring" I actually meant "logging" both the voltage and current for each battery.

For now, I'm just monitoring/logging (by writing them in a daily generated file) only the whole bank voltage/current. I'll add a few current sensors (one for each series string) and I'll read the voltage across every battery, too. I'm already using an arduino-like board for battery monitoring/logging and I have many ADC ports available.

Being a real-time monitoring, there will be an instantaneous early warning each time the currents/voltages don't match (within a reasonable amount).

I'm just reading about battery equalizing and I guess I'll have to modify the charger software to allow a slight overcharging (15V) once per month.

Anyway, my next step is to reconfigure the battery bank from 24V to 48V (as I have a 6kW inverter) thus I have to put four batteries in a series string. In this situation, a perfect balance is a must.