Battery Bank Balancing Questions

Andrew Mackey

Moderator
Several things to do: After charging, let set a few hours and check electrolyte with a hygrometer. All cels sjould be equal in readings. A big differential means a bad cell. Do the cells have external links or are they inside the case? If external, you can read each individual cell for voltage. If you load test the batteries, use a mirror and look at the top of the electrolyte in each cell. it should be clear. if grey or black, the plates are bad - discard the bettary. If you look at the electrolyte and see one or 2 cells gasing off and the others are more quiet, then the gassing cells are bad. discard the battery. iff all cells are gassing evenly, battery probably OK. In old battery packs, that had individual cells, the man in charge would shift cell position to bring up lower charged cells and lessen charge to higher ones, using a hygrometer to check the electrolyte. Old school but it worked. cant do that with sealed batteries.
 

Radiomike

Member
The operation of a VRLA cell depends on the gases evolved during charging being recombined. For this to happen the separators must be porous and not totally flooded. This enables the gases to recombine on the plates. During normal use very small amounts of hydrogen can be evolved, the pressure relief valve opens to allow the gas to escape and then reseals to prevent oxygen getting into the cell. I have stripped down lots of VRLA cells and the AGM separators require hard wringing to get out sufficient acid for a test.

https://www.tms.org/pubs/journals/jom/0101/nelson-0101.html

It will take a long time for the added water to reach equilibrium with the acid in the AGM mats. The usual acid density (SG) is around 1.290 fully charged. but will vary with each make.

Now you have added the water, and I assume it was de-ionised or distilled, you can only float charge the cells and see what happens. With great care very this insulated prods can be inserted into each cell to read individual voltages.

Your cells are indeed VRLA AGM lead acid and the 150a/hr is at the 10 hour rate. The cells look to be very similar to many used in the telecoms industry for footprint. This looks to be a close match https://www.power-sonic.com/wp-content/uploads/2018/12/PGFT-12V160 technical specifications_US.pdf this data sheet gives lots of discharge data
http://www2.exide.com/Media/files/Downloads/IndustEuro/Marathon_M_FT_en.pdf and https://www.sbsbattery.com/media/PDFs/Battery-12AFT150.pdf

Mike
 

mihit

New member
Several things to do: After charging, let set a few hours and check electrolyte with a hygrometer. All cels sjould be equal in readings. A big differential means a bad cell. Do the cells have external links or are they inside the case? If external, you can read each individual cell for voltage. If you load test the batteries, use a mirror and look at the top of the electrolyte in each cell. it should be clear. if grey or black, the plates are bad - discard the bettary. If you look at the electrolyte and see one or 2 cells gasing off and the others are more quiet, then the gassing cells are bad. discard the battery. iff all cells are gassing evenly, battery probably OK. In old battery packs, that had individual cells, the man in charge would shift cell position to bring up lower charged cells and lessen charge to higher ones, using a hygrometer to check the electrolyte. Old school but it worked. cant do that with sealed batteries.
Hydrometer.

Yup been reading up on this. I have a beer hydrometer and a spirits one, not a battery one. Plus would mean sucking each cell out, into a test jar. measuring, then injecting it back in. 132 cells, I'll pass! I will do the voltage/cell test tho as I can get a wire into the electrolyte. Depending how many low cells I have I may split off into a main good bank for heavier loads, and a secondary for LED lights, radio etc.

I read through "the battery bible" Good old days. A shame everything's supposed to be throwaway now.
 

mihit

New member
. During normal use very small amounts of hydrogen can be evolved, the pressure relief valve opens to allow the gas to escape and then reseals to prevent oxygen getting into the cell. I have stripped down lots of VRLA cells and the AGM separators require hard wringing to get out sufficient acid for a test.
Valve of sorts. a rubber cap on a pillar/tube with slits in it.
.

Now you have added the water, and I assume it was de-ionised or distilled, you can only float charge the cells and see what happens. With great care very this insulated prods can be inserted into each cell to read individual voltages.
Rain, collected and settled in a barrel.
Insulated prods?? as in "don't touch the plates"?
Your cells are indeed VRLA AGM lead acid and the 150a/hr is at the 10 hour rate. The cells look to be very similar to many used in the telecoms industry for footprint. This looks to be a close match https://www.power-sonic.com/wp-content/uploads/2018/12/PGFT-12V160 technical specifications_US.pdf this data sheet gives lots of discharge data
http://www2.exide.com/Media/files/Downloads/IndustEuro/Marathon_M_FT_en.pdf and https://www.sbsbattery.com/media/PDFs/Battery-12AFT150.pdf

Mike
Far more than i ever wanted to know! but thanks for your time on this.
Second link shows equalise voltage at 28.2, but that's lower than their cycle rating of 29.8...
 

Radiomike

Member
With care two insulated bare tip prods can be inserted into each cell and placed on a +VE and a -VE plate to get cell voltages. I have done it and it takes a steady hand. Insulated and thin, as a short between plates through the probes can be dangerous.

The small cap you removed is a valve. It is designed to open at low pressure to relieve gas build up inside. It must seal to prevent oxygen ingress as well. It is a critical component of the design. It is held in place by the cover.

To measure the acid SG in VRLA cells we used an optical refractometer that only needed a small drop of acid; like these https://www.amazon.co.uk/Refractometers/b?ie=UTF8&node=3579771031 But for our flooded cells it was the traditional float type.
 

mihit

New member
Well, some kind of progress. The batteries I had put aside for scrap and not taken to the shack, after topping up, 3 out of 4 have come up from <11 to 13+ on a trickle charge. I will disconnect it and see how far they fall. I'll get out at the weekend and see how the solar charge has done.

With care two insulated bare tip prods can be inserted into each cell and placed on a +VE and a -VE plate to get cell voltages. I have done it and it takes a steady hand. Insulated and thin, as a short between plates through the probes can be dangerous.
I don't see that happening on these. My hand's aren't steady, and the hole is less than 3/8" and I can't see in, I'd guess the top of the plates is more than an inch below.

One method I read was a lead on the -ve terminal, and dip the other probe into the electrolyte, but thinking about it, this may have been for a resistance measurement...

To measure the acid SG in VRLA cells we used an optical refractometer that only needed a small drop of acid; like these https://www.amazon.co.uk/Refractometers/b?ie=UTF8&node=3579771031 But for our flooded cells it was the traditional float type.
Ah good to know. They start at about $50 down here, but I suppose it's an investment in certainty.
Might hold off just yet.
 

mihit

New member
At this stage I'm still going to pair them off to 24v

But thinking, paralleling them at 12 and series-ing those strings may provide better levelling/balancing between batteries...

Or is it more likely that the two batteries closest to the load are going to make the current path, and so be stressed more than those "behind" them...
 

Attachments

Power

Active member
Or is it more likely that the two batteries closest to the load are going to make the current path, and so be stressed more than those "behind" them...
Yup.
That is why you connect a power lead to opposite ends of the bank, not both at same end.
 

mihit

New member
Well, I'm into Testing.
All batts had been on slow charge for long time and were showing reasonable SoC (12.4V+)
All batts had been left off-charge to dissipate any surface charge.
The timer keeps better time off the inverter than on mains!
I decided to hurry it by doing a 2h test per battery at the 5h rate (30A) Rather than "sucking the guts" out of them to 10.5 or whatever.
Immediately 6 batteries put inverter in low-volt fault.
So far 5 batteries show promise, all dropped to ~11.6V under load, but supplied the 30A for 2hrs. I will let them rest and check voltage again, then pair them off to charge.

Of the 6 to fault, I put one on the solar charger at high rate (set to 14.9V absorb, 13.8 float) as a sort of equalise charge, though I noticed the voltage spiked to ~17V a few times! The controller tells me it got 15Ah in today, it'll either come back or die.

2 more left to test.

My thinking is that I'll set the Cut out to 11.4V initially (1.9Vpc), then raise it 0.2/ week or something as the batteries get "reformatted" It seems most of their capacity is at that level rather than 12.4-13 as I would expect.(Or the 13.6-13.8 specified on the case)
 
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