Battery testing

System

Hi everyone!! I have pretty well just observed and read various posts on this forum. However this last week I had an incident at work that I feel is worth passing on. I work for a large chemical company along the Texas Gulf Coast between Freeport and Texas City.
We have close to 20 various size industrial UPS's with large battery banks. Anywhere from 7.2KVA upwards to 37.5KVA.  When power is lost we try to feed 120VAC to all the control systems in the complex to safely bring the DCS,PLC's, and critical control machines to a safe shutdown. We usually load test the battery banks to last a minimum of 30-60 min. and expect this to happen also during a power outage. We have had 1 hurricane and 1 138KV transformer fail where the UPS's were under huge demand and strain within the last 3-4 years. This has played havoc on the batteries on all the UPS's because we actually pulled them down under their voltage limits enough to trip the DC breakers when the current draw tried to keep up with the sagging voltages.
We received a summary alarm on one of our 37.5 machines and the process operators responded. Upon entry to the UPS room the heat and sulphuric acid gasses were terrible.(this ups has 30 wetted jars) and the charger was trying to output up to 60-70Amps. Also to add misery to the situation the AC had been down. We immediately IRed the batteries and connections and the jars themselves were running anywhere from 140degF to 165degf. We had thermal degradation and why they did not explode is beyond me. We immediately brought in a spare battery bank and isolated the hot batteries and saw the charger come back down to 1-2 amps. Beautiful. This UPS bank was exactly 55 months old.
We then started testing them by doing the following:
Read total jar voltage from + to -.
Read each cell (4 cells to jar)(these cells have external connected posts.
Read resistance readings on total jar resistance.
Read each cell resistance.
When the batteries read upward of 2 volts we had high meg(millions)ngs or "opens".
When the cells were below 2 volts or lower the meg readings decreased.
This was amazing to me and we could tell immediately what condition the cells/jars were in by taking the voltage/ohm readings.
We only found 6 batteries that were eading high megohms and the voltages were up but not totally beacuse of the drain.
I was curious so I hooked a paper clip across an AA battery and the readings do the same. If the volage is there the resistance is high high or close to open. Once the battery voltage was below.8 to .7 the meg readings went down. I am talking with some battery manufacturers to see if this is common readings for all various makes and types of batteries.
We did find out also that most ambient standards for batteries is @ 77degf and for every17/18degf above this the decay and degradation increases 50%.  So, with the 2 previous drains and the A/C failure our batteries on this UPS were doomed when the temperatures went up and shorting cells began.
Everyone please be careful and monitor all your systems and especially ambient battery atmospheres.
The warranty on this particular brand was 10years. We will be lucky just to recover the reclamation costs.
The IR scans are also invaluable for the connections and jars and also on the UPS and other electrical items. A 100.00$ laser Temp hand held  meter are available if you cannot afford our get your hands on a IR camera. They work well and my wife and I use one for cooking and around the house when temperatures are needed.
Also, be sure to monitor the charging system amps for various readings after load draws or charging outages. The industrial grade UPS can take the heat but not the batteries. A good preventative maintenance and prdictive maintenance routine also will go a long way.
Everyone please be careful !!

niel

Re: Battery testing

i'm sorry for the loss you guys had, but i'm not quite following these resistance measurements you talk of. how is it you can measure the resistances? this is not internal resistance as that would be low for a good cell and most meters or fuses for them would blow out trying to measure it as current and voltage would pass through the meter. tell me exactly the meter type along with how and where it's placed to measure. are you sure you were measuring ohms or resistance?
i would be very carefull of the electronics if it was in the same room as the batteries for the sulfuric acid became airborne. the equipment may work now, but the acid will eat it eventually and lead to another failure when you need it the most. i would at least take each piece of equipment and open it up for inspection. i don't know how you would determine if acid residue is present inside or not as it would be so minute and difficult to see unless corroded. maybe determine ph with litmus paper somehow. or maybe somebody else could give their thoughts on this, but i would not trust your ups system even with new batteries because of that airborne acid.

BB.

Re: Battery testing

The 17/18F 1/2 life rule is derived from material physics with an assumed (average) activation energy, and not just applied to batteries but to almost anything that ages (electronics, etc.)--The commonly accepted rule of thumb for generic accelerated life (thermal) stress testing is for every 10 degrees C increase, life will be shorted by half. So, 10 C rise (18F), a 10 year life would be 5 years. A 20C rise would be 1/2*1/2 or 2.5 year life, etc.

Here is a good summation of how this is used (sorry, this is a link to a PDF file):

http://www.keithley.com/data?asset=10016#search=%22activation%20energy%20life%20electronics%2010%20C%22

Second, while I believe in operational tests of backup systems... Make sure that the systems are sized and designed to survive multiple testings (once per week, per month, per year, whatever)... For example, you would want to make sure that you don't draw the battery bank down more than 50% (for common storage batteries) and for specialized storage batteries (like used in the Telephone Companies--also a form of Lead Acid--(if I recall correctly) are only cycled between down by 15-25%% of capacity. They have long lives (multiple decades) but cannot stand deep cycling. Other battery chemistries are supposed to give up to 5 year life even with 100% discharge cycles.

Also, high rate discharges can very easily cause damage to batteries too.

I too am not quite sure what resistance you are measuring... If you are using a DMM (Digital Multimeter), they will usually give very strange readings if there is voltage present (like a battery). I am guessing that the DMM puts out a calibrated current and measures the voltage--in your case, the current is probably swamped by the cell voltage and you are just reading the voltage of the cell (something like 2.0 vdc of cell is reading as 2.00 MegOhms or something). In any case, using a DMM directly is not going to give you cell resistance.

Given the fact that everything was running (boiling batteries, tripping breakers, etc.), I would sincerely doubt that you have multiple opens in your battery bank.

The usual method of measuring battery resistance is to place a small load on the battery, measure the voltage and current, then place a heavier load on the battery and again measure voltage and current. And use this equation (light and heavy currents are relative to capacity of battery being measured):

R=V/I=(V1-V2)/I2-I1)= (change in voltage divided by change in current = Resistance)

Note, you can measure the current anywhere--but measure the voltage right at the cell, not somewhere down the current carrying wires--otherwise you will also be measuring the resistance of the cables too.

I also agree with Niel--unless the UPS's and their wiring are sealed (possible if this is an explosion resistant installation sharing space with the batteries), I would not expect any equipment exposed to the fuming acid to last long. You may be able to salvage the enclosures with some sort of neutralization/pressure-washing--but you would have to consult a chemist or other speciallist to see what is best here. Some of this may be covered by insurance, so contacting your carrier and/or a recovery specialist may be a good way to try and preserve your claim for damages.

In the end, you probably need to contact several engineers/vendors and have them review your current installation and see if it needs upgrading and/or replacing (either due to the chemical fumes and/or if it is correctly sized for the needs of your plant).

Sounds like a very interesting place to work.

-Bill

System2

Re: Battery testing

Good points!!
We are now constantly checking the UPS internals for the acid fumes. luckily the roof top ventilator exhausted the Hydrogen fumes during the crisis, The acid gas was so bad we had to replace the A/C thermostat because the bimetal strip was damaged. The 20yr old UPS is slated for replacement in 2008 if it makes it.
We have had two deep draining cycles that added to this mess and probably several load tests dropping down to 25% of capacity. We usually do these yearly, Also we randomly cycle the UPS's to go to the batteries,back to an alternate feed and then back to the inverter/rectifier. The alternate feed comes from a different plant feeder.
The resistance readings were taken with a Fluke Digital Muti meter.
We were aware of the more accurate means of measuring the cells/jars but to quickly see what damage there is we chose the method I mentioned previously.
To me the readings are like measuring a huge capacitor. If the megohms was high are the meter read OL then we considered the jar/cell to be good. When this reading was high or read OL the cell/jar was also read to read the correct voltage.
If the ohm readings were hi/low with correct meter polarity we would reverse the leads and they would read inverted. Just like a capacitor would read.
The AA battery is hooked to the paper clip(shorted) for over the weekend so I am anxious to read what it reads monday morning.
The last voltage readings on the bank or string was 140vdc on the Dc UPS buss. Monday we will check to see if this is down or remaining the same with the string disconnected and no charging systen connected.

kc8adu

Re: Battery testing

megohms????
i have tested individual cells with an esr meter but we are reading milliohms!
sounds like you need temp sensors on your batteries to prevent a meltdown!