BATTERY VOLTGE ISSUE

[ASHISHRAMAN]

We have a battery bank for UPS system with Battery Voltage = 396 V .
Earlier when we take the voltage readings we were getting voltage half and half , like (positive to ground 198 V and Negative to ground 198 V) but recently voltage is showing difference like (Positive to ground -180 V and Negative to ground 206 V) but total battery voltage still showing the same. What would be the reason for this ? And any corrective measures need to be taken ?

 

[crshears]

Time to go on a hunt for a high-resistance ground somewhere on the positive side of your system, viz., failing or degraded insulation, or perhaps surface contamination consisting of conductive particles.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[Compositepro]

If you used to read exactly the same voltage to ground, there was probably an intentional ground connection at the mid-point of the battery bank. If so, that connection is now broken, or was open when you were taking readings.

 

[waross]

I'm with CR on this.
It may also be a piece of connected equipment on the system developing a high resistance to ground path.
Taking a broad view of CR's post will include this.
Even if the system is, was, or may be center grounded it is important to identify the source of the unbalance.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[IRstuff]

Are the reported measurements correct? 180+206 = 386, not 396

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

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[crshears]

If the neutral of the system is grounded through a resistor the readings would not be expected to sum to 396 volts.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[IRstuff]

The OP reports that they got 198+198=396 originally, hence the question

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

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[crshears]

OP, please provide more information: is the centre point of the battery bank [1] solidly grounded, [2] grounded through a resistance of some value, or [3] not intentionally grounded at all?

Depending on your answers, Ohm's / Kirchhoff's / Thévenin's will either lead to different conclusions or indicate error in the voltage measurements supplied.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[waross]

Given the resistances that would cause that reading, and the somewhat random chance of insulation leakage to ground, I suspect:
1: A typo.
2. A center point ground resistance has gone open.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[crshears]

ASHISHRAMAN:

OP = Original Poster; that's you.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[JWR01]

What make/model/size of UPS?

 

[itsmoked]

With IR's point about this new voltage sum a reduced sum could easily mean a shorted battery cell and no ground shifts at all.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[crshears]

Hey Keith, OP stated in part,

"total battery voltage still showing the same"

That would not be the case in the event of an internally shorted cell.

There also seems to be some "post and ghost" happening...

 

[che12345]

Dear Mr. ASHISHRAMAN,

Please tell us:
1. Whether the battery bank is earthed at the center point. If it is earthed, is it solidly earthed or earthed through a resistor. If it is through a resistor, check whether the resistor is open-circuited or with bad connection.
2. If the bank center is isolated from earth:
a) Connect a digital multi-meter between the battery + to earth. Take the reading. With the digital multi-meter connected, connect a filament lamp >200V say 60-100W between the battery + and earth i.e. across the digital multi-meter. The lamp shall NOT light up and the digital multi-meter shall read zero or very much lower than it was.
b) Likewise carry out the same measurement as above between battery bank - and earth .
c) If the digital multi-meter voltage drops to zero or very much low value and the filament lamp is NOT lighted, the problem is with the digital multi-meter which has very high input impedance, giving a false reading. This may NOT happen if you use an OLD moving-iron volt meter.
3. Please carryout the above procedure and advise the observation. We may be able to suggest other opinions/reasons with your input.
Che Kuan Yau (Singapore)