Lead Acid Batteries 1

[zaza123]

We have Battery Banks for UPS which has critical load. The battery are Power Safe Lead acid Sealed batteries. THe Installation is 5 yrs old.There are two banks each having 66 cells.Each bank has two strings of 33 cells each

During last year Battery discharge test we found 70 batteries out of 132 faulty. The same were replaced with the new batteries. The Secification of the batteries was same as before.During the commissioning of the new batteries again we dd load bank dischage test of all batteries.we found that out of old batteries ( which were healthy last time) 10 batteries have failed.

The battery type is 12V80.We have made one complete bank of new cells. My questions are
1. We are doing the Load Bank discharge test of Batteries every year. Is this frequency ok
2. At this rate each year we may find some old batteries faulty so what is the solution.
I have read that the Life of Lead Acid cells are 5-7 yrs.

 

[alehman]

Life depends greatly on the application and how much they are cycled, average temperature, etc. For this type of valve-regulated lead-acid, after 5 years, I would pretty much count on total replacement though. Some less expensive varieties, more like 3 years. Some heavy-duty telecom type VRLA's will go 10 years.

You may want to take a look at your warranty.

Annual discharge testing is no doubt stressful for the batteries. Some warranties specify a maximum number of discharges per year, so you might check that. Better they failed during a test than when you really needed them!

VRLA batteries tend to fail suddenly, as you have observed. As they get older, I would typically want to do internal impedance measurements quarterly.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[DanEE]

We recommend at least internal resistance measurements* every 6 months. Some hospitals to meet Joint Commission certification requirements mandate load testing every quarter.

All the below is assuming you are using AGM VRLA sealed batteries, not wet lead acid.

The longest life battery we have found in the AH range you are using are the Deka Unigy 24HR3000 batteries manufactured by East Penn Manufacturing in Pennsylvania.

http://www.eastpenn-deka.com/assets/base/0965.pdf

Also assuming you are in the US. Batteries manufactured off shore (and many are these days) may spend a long time in transit and storage before you receive them as "new" batteries.

The Deka batteries have a 10 year design life, but for critical applications such as yours, we recommend replacement every 3 years. We have yet to experience an early life failure with these batteries and we buy and install them by the pallet load.

* Test methodology (other than a load test) is a much debated subject. After an extensive reading on various methodologies and discussion with the engineering staff with 2 major manufacturers IMO, the Alber testers are the most reliable, quick test in that they do an actual DC load test versus some other debatable test methods.

http://www.alber.com/ProdPageCellcorderCRT400.htm

The other thing you might want to look into is whether the float voltage on the batteries is at the correct level for the ambient battery temperature environment. AGM VRLA sealed batteries need to be right on the float voltage ambient battery curve to avoid on one hand sulfation due to float being on the low side and positive plate corrosion due the the float being on the high side.

Lastly what is the ambient temperature the batteries are exposed to? We have seen faster deterioration at 80 deg F and higher temps, and of course they should never be allowed to be at or above 95 deg ambient due the the risk of thermal runaway.

 

[Kiribanda]

zaza123,
Per IEEE STD-450 a two year interval for performance test is recommended for Lead Acid flooded batteries.

 

[raithrovers1]

I agree with Kiribanda. 2 year discharge intervals are sufficient.
VRLA batteries are notorious for being extremely fussy when it comes to temperature, ac ripple and actual float voltage.
For your Powersafe cells the float voltage should be 2.28Vpc @ 20 degrees centigrade. The life of this type of cell is reduced by half for every 10 degrees above this.
The AC ripple current seen by your battery should be less than 6Arms.
I have found Powersafe batteries that last 10 years and some that do not make it to 5 years.
6 monthly internal resistance checks are advised but these only highlight cells that may be failing. A discharge is the only way to prove battery capacity.
Also, when I find more than 50% of a battery string failing I would usually recommend that the whole string is replaced.

UPS engineer

http://www.calibrepower.co.uk

 

[rbulsara]

I have read that the Life of Lead Acid cells are 5-7 yrs.

Yes, at best. Usually 4-5 yrs.

The battery are Power Safe Lead acid Sealed batteries. The Installation is 5 yrs old.

.

You have the answer in your own post. It is never advisable to mix and match batteries of different lives, unless in emergency situation for a short period of time.

There is no point in load discharge test of VRLA batteries after initial test at the time of installation. From reliability stand point of view and even from economical point of view, plan to replace the entire string.
If the load is "really' critical, consider flooded cells.

Also work with reputed vendors and service (testing) companies and consultants.


Rafiq Bulsara

http://www.srengineersct.com

 

[zaza123]

Is there any IEC standard which mentions the Disharge Test frquency

 

[cherry2000]

Some battery vendors have Battery monitoring system which monitor the Battery Internal resistances and give online values, which can be easily accessed. I had seen the results in one of the Battery vendor's presentations and I liked the graphics. As far as Battery warranty goes, it is one of those things with many fine prints. Unless you have kept meticulous records as per the Battery manufacturer's expectations, warranty is a blind alley. As correctly said by the experts above, battery life is a very undetermined thing and is like the weather which depends on many variable parameters. The fact of life is Battery unlike other electrical equipment is a Electro chemical equipment, with fictitious life to start with.