Cell Voltage Tests on AGM Batteries

[SparWeb]

Hi,

After bringing my bank of absorbed glass mat batteries gradually up to float voltage, I've had an opportunity to measure individual cell voltages while the charge controller keeps the float charge steady at exactly 14.0V.  Interesting results follow:

(Click to see the input data - 141K JPG file)

Some of the readers may be noticing that the "corrected float" voltage (2.19V) is less than the manufacturer's recommended float voltage (2.25V).  That's because it's very cold these days, and cold batteries must have their voltage raised higher and higher.  However, there is a limit to how high the voltage can go.  I'm not supposed to raise the actual cell voltage above 2.35V.  Correct that from below freezing to 25 Celsius standard and it's not actually "full".

There is a fair bit of scatter in the cell voltages in cells 1-12, while cells 13-24 are much more consistent.  Have a theory why, which I'll get to later, once I've described the banks in more detail.

The 24 cells are stacked in a 4x6 bank.  They're currently connected in 12V, but once I've upgraded my inverter, I'll have them re-wired for 24V.

Stats:

GNB Absolyte 90A-11 Cells, 2V each

Each cell's 8 hour rate is 432 Amp-hours

 6 Cells arranged together per battery (1-6) (7-12) (13-18) (19-24)

 4 batteries total

Total charge is 1730 Amp-hours at the 8-hour rate.

At a more modest rate, the capacity is much much more.

(I still can't get over what a lucky s.o.b. I am for finding these.)

Before installing the batteries, I did a discharge test on some of the sets.  I tested sets 13-18 and 19-24.  I drew both sets down below 10V each using an array of auto headlights.  Then I charged them back up using a 6A auto charger.  Each cell is >400 Ah, so it took a really long time to drain and subsequently recharge them.  I was satisfied after testing the two batteries, so I didn't bother testing the two other ones.

Surprisingly, it's the sets I tested that are much more consistent, now that I test their float charge voltages.  So I'm speculating that I should un-couple the top two batteries for a while, and suck them down below 10V, too.

Any thoughts on this idea?

Does a deep discharge cycle do AGM's any good?

Can I read any meaning into the fact that the first and last cells are the bad ones?

Thanks!

[SparWeb]

A picture's worth a kilo-word:

(ignore the + and - stickers - I hadn't moved them before taking the picture).

[commanda]

Assuming that the batteries have been sitting for a while, I'd say your assumption could be correct.

Interesting read here.

http://www.users.bigpond.com/solarbbq/batteriesrejuvenating.htm

Amanda

[Flux]

I agree with Amanda, batteries are generally not considered to need cycling to maintain performance but They do get lazy even maintained on float for long periods.

If they have been standing then cycling may help. Those cells with the high voltage would benefit most from attention.

Flux

[SparWeb]

Thanks for the interesting link - both for the data he's found, and for the fact that he's done the same kind of test I did - his discharge chart looks just like mine, too (except scaled differently).

[DanG]

Good to see a renewing massive battery post. Yet the equally huge GNB pdf on their latest strings pre-empted me searching for charge rates but I am sure your string is lusting after high current maintenance!!

AGM batteries usually have a thickened electrolyte to better trap any gas production near where it was formed to allow it to recombine and avoid moisture loss - there is excess in every case to allow for some abuse (carry it out of warranty & charge backs) but it takes hours if not days for true circulation to occur at recommended charge rates.

With only a six amp charger and such huge case I suspect not all the plate & electrolyte surface has been addressed and the plotted values have a lot of room for drift either way with any continuing exercise...

[SparWeb]

Yes, did I mention that a C/8 charge rate on this batt bank would be 200 Amps?!?!

They are humungous compared to the size of my windmill, but I'm happy to have them anyway.

I have my string of lights set up - I will turn them on this weekend and monitor on a regular basis.  It will take a couple of days to finish the pair of batteries off, and several more days to charge them back up again!

[SparWeb]

Well, that took a while.  36 hours at an averate 20 Amps = 720 Amp-hours.  Which is a fair bit lower than the manufacturer's performance specs, but considering the penalties I'm incurring, not bad.  It was -20 Celsius all weekend, and some cells were quite low to begin with.  Now begins the long, slow, climb back out of discharge valley, hopefully to a more consistent state of being.  More battery zen later, thanks.

[commanda]

Hey Spar,

please redo that chart of battery voltages after you've cycled the rest of the batteries. I'm really interested.

Can I read any meaning into the fact that the first and last cells are the bad ones?

In my experience, it's usually the most negative cell in a string that will fail first. Being that electrons travel from negative to posistive, the electrons absorbed by all the cells up the chain (charging) have to travel through that first most negative cell. Conversely, when discharging, all the electrons have to travel through the most positive cell.

Can't say I've seen it mentioned anywhere, but I'm sure some sort of rotation scheme would help prolong the battery array's life.

Amanda

[ghurd]

Cool! Strange, but cool.  I wondered about that, related to 'surface charge' almost actually being at the surface, then electrons running in circles in that cell.

I never saw a word about it either.

Say 24V system, 4 X 6V, would rotation be from 1,2,3,4 to 4,3,2,1?

Because 95% of what I do is a single 12V, a bad cell means a new battery.  Never bothered to keep track of which was which.

G-

[commanda]

I saw this recently with my electric scooter. 4 x 12 volt batteries in series. Charger is 2 x 24 volt chargers in series. The charger got rained on and over-charged the (VRLA) batteries. The first 2 to die were the most negative batteries in each pair.

Say 24V system, 4 X 6V, would rotation be from 1,2,3,4 to 4,3,2,1?

I would rotate 2,1,4,3 so the 2 end units go into the middle.

Or,

4,1,2,3

then

3,4,1,2

then

2,3,4,1

Amanda

[SparWeb]

Back by popular demand:

(just click on the thumbnail - it's 24k)

To explain the differences between the two plots on the chart, these are two tests done on different dates under different conditions.  The absolute and average values are not as important as the variability between the cells.  

The first test, top, was measured during a windy night, and the battery bank was being maintained at float voltage by the C40 diversion controller at about 2.25 volts per cell, and the temperature sensor was compensating for the -4C temperature.  The scatter is so bad that many cells are over-charging while others are hardly more than half full.

After that test, I recognized that many cells, particularly those that had not been cycled through a discharge and a recharge.  To seek more consistency among the batteries, I put those cells through their own full cycle of discharge to recharge.

The second test, bottom, was measured on a calm afternoon, and the battery bank had been resting at a "mostly-full" state all day, after slowly being recharged over the past couple of weeks.  There is only about a 1% difference between the highest and lowest.

Why go to the trouble of running them flat?   Well, these are valve-regulated lead-acid "VRLA" batteries, which lose water only when they are over-charged.  They don't take well to equalization charging.  Since equalization charging is the standard method of balancing out the differences between cells in a flooded lead acid battery bank, standard methods won't work for a VRLA.

The alternative is to drain it completely.  Then charge it back up, and when the cells get back up to float voltage, differences between them will be reduced.  The actual mechanism by which this works, is still unknown to me.  I suspect that there was a sulfate build-up which was removed by the cycle.