HP:The suspect string of batteries are scheduled to be load tested asap. My guess still is that they are fine. I suspect the charger, and the was it handles voltage flucuations on the input.
HP, I don't understand why you think the battery string isn't the problem. Our chargers are all fed by 208 single phase and have an AC input and DC output circuit breaker. We use a dual knife leading to 60 cells. We use much larger conductors to feed a DC distribution panel. The specified float range is 2.21-2.25 volts per cell. I set our chargers to 2.23 volts/cell or 133.8 volts total. We use 140 volts for an equalizing charge. Is this something similar to your system?
It would be almost impossible for the problem you described to be anything but the batteries. If you had a major fault across your battery load, you would likely trip a breaker on the DC distribution side. If you had a dead short in the output of your charger, you would almost certainly trip the output breaker of your charger. A typical problem in a charger will just reduce its output to zero, or overdrive it, or perhaps cause its output voltage to drift over time( the hardest ones to fix.) But none of these problems will be able to load a healthy string down to 70 some volts.
Our substation electricians record cell voltage and specific gravity once a month. I would bet that you have at least one cell that reads higher than the others under charging conditions but will let you down big-time if needed to feed a load. When I have tested our batteries, by turning off the AC breaker, I tend to notice the battery voltage increase over the first 15 to 20 minutes. It's as though they are waking up and getting used to flowing the 6-10 amps of normal substation load. (no controls) Then they start a slow discharge process. We don't use deep discharge testing of batteries due to concerns over shortening their lives.
I didn't realize how little I knew about batteries until I had to commission a charger and batteries in a new substation. The batteries were shipped dry and the electrolyte was added on site. I was shocked when I learned that I should bring the string up to 156 volts slowly by current limiting to 8 amps, and then leave it there for 3 days solid. I had never heard of using more than about 140 volts for an equalize, but it was in the C&D battery book. I followed the instructions and ended up with a perfect string, all cells within .01 volt and stark light/dark plate structure. We had another location where a substation had a new set of dry batteries. I advised the substation foreman to call me as soon as they activated them but he forgot. They were at normal float voltage for at least 2 weeks before I heard about it. I tried to undo the damage done by repeating the procedure described above but it was too late. The cells looked terrible and the voltages were all over the place. We will probably get less than optimal life out of them.
If you have never had a chance to cozy up to your battery manual, you might be surprised by what you'll find in there. The voltage that you say you float yours at sounds a tad low to me if you run a 60 cell string. Too low a float voltage will shorten useful life. Finally, when you replace your defective cell (s), it would be wise to prep the new cell(s) with a single cell charger or suitable supply.
Good luck with it!
Joe
[This message has been edited for typos.
[This message has been edited by JoeTestingEngr (edited 02-25-2006).]
