Sound like a bad cell to you?

techntrek · March 2011

Three times a year I shut down my system and manually charge each individual battery with a good 3-stage charger. They are normally floated on a charger that was intended for gel-cel batts so I know it floats them a bit low, which is why I do the manual thing. Plus I use the opportunity to check the water. The system is for battery backup only, no PV, 192 volts nominal, 16 batteries in series. The bank is 2 years old and had date codes within a month when I bought them. I'm glad I checked because there was one battery on the rack that was a year old...

It had been a while since I tested each battery and this time just after taking off float 15 of them were between 13.73 to 13.79. Not perfect but not horrible. One really worried me - 13.3x volts (not sure of the tenths place). I can't find my hydrometer so I haven't checked the individual cells on that battery. This morning I was getting at-rest readings around 12.7x on most batteries (ones I haven't individually charged yet) and 12.3x on that one low batt. Still 0.4 volts difference. I should also mention I have no corrosion on any of the terminals on the other batts but I do have it on both + and - sides of the low batt. I took the wires off and wire brushed it off last night.

This sounds like I have a bad cell, right? I'm debating if I should bite the bullet and replace that batt with a new one. I know, the new one will degrade to match the rest of the string. Then I can use the "bad" battery on my camper where it won't matter if its a few tenths of a volt low. I'm going to wait on that decision until I individually charge it and see how it holds up at rest. Thoughts?

Cariboocoot · March 2011

Re: Sound like a bad cell to you?

Although Voltage isn't the best way to judge, it sounds like most of them are in pretty good shape. The one low battery may just be low and might respond to equalization. A bad cell usually throws the Voltage off by around 2 Volts, not a few tenths (as that cell would contribute just about nothing to the over-all output).

Anyway, it's hydrometer time!

If that one battery doesn't respond to a EQ cycle, the rest are probably in good enough shape that a new battery won't lose much from being added to the mix.

techntrek · March 2011

Re: Sound like a bad cell to you?

I'm planning on working on that battery tonight, I'll leave it for my last one of the evening so it gets a longer float charge overnight. If it still reads low in the morning I'll restart the charge cycle and see how it looks tomorrow when I get home.

What do you think about the corrosion on both terminals? A symptom or a cause?

BB. · March 2011

Re: Sound like a bad cell to you?

Perhaps, over charging? More Acid "fog" from excessive bubbling.

Also look very closely at the positive terminal and see if it is "pushing up" relative to the body/cover of the battery (positive grid corrosion from too much charging/equalization).

Also a "shorted cell" with several parallel strings of batteries could cause that one string to over charge.

Check for plugged vent holes in caps forcing electrolyte onto battery top.

-Bill

techntrek · March 2011

Re: Sound like a bad cell to you?

I doubt its from overcharging, the charger floats at 13.5 v per battery (216 v for the string) over the summer when its hot and 14 v (225 v for the string) when its cold. Only one string of 16 batts, nothing in parallel.

Cariboocoot · March 2011

Re: Sound like a bad cell to you?

Keep in mind that "Float" isn't the critical Voltage. Absorb is more important for battery health. Equalization is an altogether different process which involves even higher Voltage.
It also helps if your charge is temp compensated.

techntrek · March 2011

Re: Sound like a bad cell to you?

Unfortunately the built-in charger isn't much for bulk or absorb - it peaks at 3.5 amps early usually at no more than 225 volts, and ramps down to 2-2.5 amps fairly quickly even when they are deeply discharged. Again, a downside of using a charger intended for gel-cell because it doesn't want to vent them. Its ok for getting some bulk charge done while the genset is running for a few hours but horrible for the finish charge after the outage is over - it can take up to a week before it levels off if I've had a long outage. I've thought about trying to short the temperature sensor probe to force the charge algorithm to its highest voltage all the time.

Yes I really should get a proper charger(s) but it is discharged shallowly for most outages so for now its good enough. I don't mind the manual EQ to compensate for its shortcomings. Eventually the whole system will be upgraded so I won't be spending much on this one.

techntrek · March 2011

Re: Sound like a bad cell to you?

Update on this issue. I finished my round of maintenance last night, and that "bad" battery still reads about 0.4 volts lower than the rest. The feedback I've received here so far is "don't worry about it", so I'm going to leave it in place for now. If anyone feels the opposite let me know.

niel · March 2011

Re: Sound like a bad cell to you?

keep that battery to the side for a day or 2 to observe what voltage it settles to. if it has a bad cell you'll see a big further dipping in the voltage as it would try to settle into the normalized voltage for the 5 good cells, 5 x 2v =10v. odds are a bad cell may not go all of the way to 0v right away either.
when one has a bad cell during normal charging it isn't always readily detected because the 5 good cells are overcharged while the bad cell may temporarily rise a bit. sg readings on a bad cell are not always low as it may reflect the same sg as when the failure occurred in some instances. if the battery still looks ok but just a bit low then i would make an effort to give it further charging and more vigorously. yup, there's no substitute for a good proper charging. the long small charges may be leading to problems in some of the cells.

techntrek · April 2011

Re: Sound like a bad cell to you?

As suggested I let them sit, until last night. Finally got around to watering and getting the system back online (took 6 gallons). Here are my long-term at-rest voltages:

1 battery = 13.23
14 batteries = 13.18 +/- 0.02
1 battery = 12.78

So that one battery is still 0.4 volts lower.

Cariboocoot · April 2011

Re: Sound like a bad cell to you?

techntrek wrote: »

As suggested I let them sit, until last night. Finally got around to watering and getting the system back online (took 6 gallons). Here are my long-term at-rest voltages:

1 battery = 13.23
14 batteries = 13.18 +/- 0.02
1 battery = 12.78

So that one battery is still 0.4 volts lower.

That's the one that really needs equalization.
Maybe it's time to invest in a new charger. Maybe it's past time to invest in a new charger.

RCinFLA · April 2011

Re: Sound like a bad cell to you?

Perhaps that battery has developed higher leakage (self discharge).

If this battery is connected in series with other batteries it will force a higher charge voltage on the other cells eventually damaging them.

A gel cell charger float level is normally a bit higher then a regular lead acid battery, in the range of 13.8 vdc versus 13.2 to 13.5 vdc.

Do a web search for lead acid rest voltage versus state of charge.

Put a load on battery to discharge it to expected 20% AH capacity. Check the rested voltage to see if it matches what it should for 80% SOC.

If access to electrolyte (not a sealed battery) measure specific gravity to see if it matches expected SOC.

techntrek · April 2011

Re: Sound like a bad cell to you?

The one I'm using takes the voltage up to 15.5 volts for some period of time, I haven't sat there and stared at it to know how long. That voltage is also based on its own readout and not my voltmeter so its possible it isn't going quite that high. This battery has been through this cycle 3 times now in the last 2 weeks. I also waited to water all of the batteries until after charging so there was a reduced volume of liquid in the cells - which should have made it easier to mix the electrolyte. They were quite low, I added about 1/3 of a gallon to each one. No exposed plates on any of them although this time around they were close.

techntrek · April 2011

Re: Sound like a bad cell to you?

RCinFLA wrote: »

A gel cell charger float level is normally a bit higher then a regular lead acid battery, in the range of 13.8 vdc versus 13.2 to 13.5 vdc.

Other way around. Gel cells float at a lower voltage to ensure they don't boil electrolyte since there is no way to replace it. They get no more than 13.8 volts per 12 volt battery (which is what mine does) vs. a flooded cell float of up to 14.7.

The worry about damaging the other good batts is why I started this thread, earlier I mentioned the possibility of swapping in a new battery (knowing it will come down to the level of the rest) and using this "bad" one on my camper.

niel · April 2011

Re: Sound like a bad cell to you?

i realize your concern with that battery, but have you noticed any reduction in performance of the system overall, aka not lasting as long? it is hard to say for sure what is going on there, but i would continue to use it while keeping a very close eye on it. do get yourself another hydrometer and see what you observe that way. it may even operate fine at a reduced capacity in one cell and that, as are all other comments, could be pure speculation until you/we know more about it.

ps-you didn't have to wait this long to get the at rest voltage readings as no more than a day would've revealed much.

techntrek · April 2011

Re: Sound like a bad cell to you?

No noticeable performance issues, I was mostly interested in someone saying "oh, a half-volt drop is likely due to x". Sounds like the mystery will continue. I'll make sure I get some SG readings sooner or later. Yeah, I know a day would have been enough but with Spring in high gear I've been pretty busy. :-)

Sound like a bad cell to you?

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