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Battery Bank Balancing Questions


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  #1  
Old 09-05-2019, 10:08:01 PM
mihit mihit is offline
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Default Battery Bank Balancing Questions

I will try and be as clear as I can.

I have a remote cabin and I've acquired old 12V/150Ah SLA cells. I'm calling them cells because my setup is 24V, so two of them make a "battery"

They are all individually at various voltages (11.9-13.2, having been sitting off charge for probably months) Anything less than 11.9 I scrapped.

I have wired them series-parrallel for 24V, matching pairs by their standby voltages, and connected the solar controller.

Now the bank is at say 26, but the pairs are mostly unbalanced (a couple are within 0.5V), with one cell being <2V higher than the other.

So my question is, what is the best way to balance them?

Do I have to? I assume the imbalance will result in one cell over-charging and one under-charging, and imbalanced current paths between batteries, and some self-draining effect in the bank as high-charge cells/batteries bleed off to low-charge ones?
Do I re-pair them based on their charged/charging cell voltages?
Do I run an equalisation charge and hope they level off?
Is there a test I can do on each cell to determine the best pairing?

Or do I sell them (nominally 1200Ah) for scrap and buy new batteries, and get a guaranteed 50Ah...
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Old 09-05-2019, 11:58:41 PM
Birken Vogt Birken Vogt is offline
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Default Re: Battery Bank Balancing Questions

Sealed lead acid don't take kindly to abuse and you have had abuse. I would get rid of them and replace them with a single string of flooded whatever size you need. Others may have differing opinions but I live in the real world and if you want to enjoy the property and not meddle with batteries except to add water, that is what to do.
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  #3  
Old 09-06-2019, 02:11:28 AM
mihit mihit is offline
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Default Re: Battery Bank Balancing Questions

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Originally Posted by Birken Vogt View Post
Sealed lead acid don't take kindly to abuse and you have had abuse. I would get rid of them and replace them with a single string of flooded whatever size you need. Others may have differing opinions but I live in the real world and if you want to enjoy the property and not meddle with batteries except to add water, that is what to do.
I hear what you're saying, but this is only for intermittent and light use.
These are actually an upgrade from a half-dozen old mismatched 320-650CCA car batteries five of which measured alright after I pulled them out.
Ideal world, yeah I'd buy new. Right now I am looking to make the best of what I've got.
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Old 09-06-2019, 02:20:35 AM
Vanman Vanman is offline
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Default Re: Battery Bank Balancing Questions

Cells are the separate individual parts within a battery. So each 12 volt battery contains 6 cells. When you connect two of these in series, you now have a single battery of 24 cells. If you connect more of those groups in parallel, the whole thing is still a battery. Just bigger.

I have heard of these being sort of "brought back" by leaving them on a low charge for a long time. Like a month. They DO NOT tolerate over voltage, so the only way to equalize them is with the long period at a relatively low charge voltage such that no cell exceeds it's gassing voltage.

I'd imagine it helps immensely if they have not been too deeply discharged for too long.

Keith
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Old 09-06-2019, 07:44:06 AM
pegasuspinto pegasuspinto is offline
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Default Re: Battery Bank Balancing Questions

occasional equalizing is needed for battery life. It will be a bit of guesswork.

Take each battery out of the system individually and charge them up to about 14.5 volts for a couple hours or so, and then let them sit overnight and measure the voltage. You should be close to 12.6 or 7 on a fully charged battery. If it's still low, equalize it again. While equalizing, you will get some gassing and the battery will warm up. There is some guesswork. Do this with the battery outside where if it blows up, it won't damage anything.

If you try to run a bank of batteries all over the place on their cell charge, someone in the bunch is going to fail, so you really can't ignore this issue.
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  #6  
Old 09-06-2019, 11:03:12 AM
Radiomike Radiomike is offline
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Default Re: Battery Bank Balancing Questions

The manufacturer will have a data sheet on the cells and the recommended float and cyclic charging voltages. The usual float voltage is 2.27Volts per cell, this gives 13.62 per six cell battery. There will be a correction for ambient temperature as well.

The problem, and I have had troubles and experience wit this, is that the cells age and their internal impedance changes. This makes series parallel strings difficult to keep in balance.

I would float charge all the six cell blocks in parallel at the manufacturers recommended voltage for a week or so. Then let them sit for 24 hours and measure all the voltages. Charging at the float voltage can be done indefinitely, higher voltages need careful supervision.

Bear in mind that SLA batteries are not sealed they give off small amounts of hydrogen especially when being charged at higher voltages. The correct name for the cell is Valve Regulated Lead Acid batteries.

Mike
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  #7  
Old 09-06-2019, 12:10:32 PM
Vanman Vanman is offline
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Default Re: Battery Bank Balancing Questions

Ah, I had it in my mind that these were the lead gel variety.

So these should tolerate a wee bit of over voltage but must bear in mind that water lost cannot be replenished?
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Old 09-06-2019, 03:48:54 PM
mihit mihit is offline
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Default Re: Battery Bank Balancing Questions

Quote:
Originally Posted by Vanman View Post
I have heard of these being sort of "brought back" by leaving them on a low charge for a long time. Like a month. They DO NOT tolerate over voltage, so the only way to equalize them is with the long period at a relatively low charge voltage such that no cell exceeds it's gassing voltage.

I'd imagine it helps immensely if they have not been too deeply discharged for too long.

Keith
Yes low-charge-long-time seems to be the consensus. I doubt I'm able to get a vehicle in there to pull them out to mains power (It's end of winter and very muddy here) so hopefully that charge can be done with solar, not cycling them and the overnight-lull wont affect them badly.

I don't know their cycle history, but imagine they have suffered some big DoDs, but they were all at reasonable voltage given they've been sitting for months.

Last edited by mihit; 09-07-2019 at 02:36:25 PM.
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Old 09-06-2019, 03:54:13 PM
mihit mihit is offline
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Default Re: Battery Bank Balancing Questions

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Originally Posted by Radiomike View Post
The manufacturer will have a data sheet on the cells and the recommended float and cyclic charging voltages. The usual float voltage is 2.27Volts per cell, this gives 13.62 per six cell battery. There will be a correction for ambient temperature as well.

The problem, and I have had troubles and experience wit this, is that the cells age and their internal impedance changes. This makes series parallel strings difficult to keep in balance.

I would float charge all the six cell blocks in parallel at the manufacturers recommended voltage for a week or so. Then let them sit for 24 hours and measure all the voltages. Charging at the float voltage can be done indefinitely, higher voltages need careful supervision.

Bear in mind that SLA batteries are not sealed they give off small amounts of hydrogen especially when being charged at higher voltages. The correct name for the cell is Valve Regulated Lead Acid batteries.

Mike
I have got the specs. I think they're 13.4-13.8 for float usage, 14.2-14.9 for cycling. I'll re-wire them 12 and leave the controller on, set to float, with no load for a week or two and see what happens. I have the old dead ones so I might take to one with a screwdriver and see if I can't get at the cells to check/add water.
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Old 09-06-2019, 04:48:54 PM
Dan Barson Dan Barson is offline
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Default Re: Battery Bank Balancing Questions

I've had same problem. You need to charge each battery up and see if it holds. Next pair the closest voltage ones together. To keep equalization that was your first question. Tie the negatives together from each 24v battery. If you are charging with solar this will keep each set of batteries equal.
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Old 09-06-2019, 04:59:30 PM
Rich Mc Rich Mc is offline
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Default Re: Battery Bank Balancing Questions

Because you have no history of these batteries, I would charge them to let say 14.5 volts each then put a 10 hour discharge load on each battery measuring the time that it takes to discharge to let say 11 volts, then you have an idea of the relative quality of each battery in the bank then group them accordingly.
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Old 09-07-2019, 12:27:23 AM
mihit mihit is offline
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Default Re: Battery Bank Balancing Questions

Quote:
Originally Posted by Dan Barson View Post
I've had same problem. You need to charge each battery up and see if it holds. Next pair the closest voltage ones together. To keep equalization that was your first question. Tie the negatives together from each 24v battery. If you are charging with solar this will keep each set of batteries equal.
All the negatives would necessarily be tied when they get paralleled?

Thinking out loud here, but is there an advantage if I paralleled half of the batteries at 12V, then series the two strings...?
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Old 09-07-2019, 12:43:03 AM
mihit mihit is offline
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Default Re: Battery Bank Balancing Questions

Quote:
Originally Posted by Rich Mc View Post
Because you have no history of these batteries, I would charge them to let say 14.5 volts each then put a 10 hour discharge load on each battery measuring the time that it takes to discharge to let say 11 volts, then you have an idea of the relative quality of each battery in the bank then group them accordingly.
I feel a bit of a twit, I should have started with the basics. After prying off the top with hammer and chisel, the batteries were all basically dry/ running on acid

I spent today filling the cells (132 of the beggars)and now I have half the bank on 10A solar (which is a nudge over C/20) limited to 13.9v (the top of their float range) So I suppose I hurry up and wait to see >900Ah go in them, then do the same to the other half.

The batteries were originally stand-by UPS, I believe ganged to 72V.

Load testing... hell. It's a good idea but does add one more step, but I suppose better to get it right sooner rather than later.

I assume I just load them to whatever their 10H rate is and see how long they last? I'd rather speed it up to get done in one day when I'm there. Would doubling the load (for 5h) give a representative measure?

Last edited by mihit; 09-07-2019 at 01:22:24 AM.
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Old 09-07-2019, 12:55:43 AM
mihit mihit is offline
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Default Re: Battery Bank Balancing Questions

Also I assume being so low on electrolyte there will be heavy stratification, and probably sulfation on the worse of them. I guess the "remedy" there is to charge them up hot.
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Old 09-07-2019, 08:23:50 PM
Rich Mc Rich Mc is offline
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Default Re: Battery Bank Balancing Questions

Five hour discharge cycles would also be informative.
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Old 09-07-2019, 08:32:11 PM
Radiomike Radiomike is offline
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Default Re: Battery Bank Balancing Questions

It is possible to discharge a battery at rates other than 10hours. We often did ours at the 3 hour rate in the power plants I worked at.

The manufacturers data will give the 10, 3 and one hour rates and the recommended end voltages. At the three hour rate you will need to work with around 50 Amps. The voltage will need to recoded every 15 to 20 minutes at first and closer as you approach the end point. Be careful not to over discharge the cells as this is usually harmful.

The idea of the VRLA AGM type cell is the concept of the starved separator aiding the recombination of gases. The AGM separator should be damp to the touch, not saturated. When you stripped down the cell you must have removed the pressure relief valve. This relives pressure and prevents excess oxygen entering the cell. When the batteries are initially formed it is done without the relief valve, when the process is complete the valve and top are welded in place.

Mike

A basic review of the underlying chemistry is here https://www.tms.org/pubs/journals/jo...lson-0101.html
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Old 09-08-2019, 02:20:24 AM
mihit mihit is offline
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Default Re: Battery Bank Balancing Questions

Quote:
Originally Posted by Radiomike View Post
It is possible to discharge a battery at rates other than 10hours. We often did ours at the 3 hour rate in the power plants I worked at.

The manufacturers data will give the 10, 3 and one hour rates and the recommended end voltages. At the three hour rate you will need to work with around 50 Amps. The voltage will need to recoded every 15 to 20 minutes at first and closer as you approach the end point. Be careful not to over discharge the cells as this is usually harmful.

The idea of the VRLA AGM type cell is the concept of the starved separator aiding the recombination of gases. The AGM separator should be damp to the touch, not saturated. When you stripped down the cell you must have removed the pressure relief valve. This relives pressure and prevents excess oxygen entering the cell. When the batteries are initially formed it is done without the relief valve, when the process is complete the valve and top are welded in place.

Mike

A basic review of the underlying chemistry is here https://www.tms.org/pubs/journals/jo...lson-0101.html
Batteries are not AGM. I thought SLA, but as rightly pointed out, and after a bit of vandalism, they're VRLA.

Good to know about the quick discharge. My cunning plan was to get one of the 240VAC timers

after checking to make sure it keeps good time through the inverter, load up to whatever amperage draw, set the cutout to whatever voltage, and walk away. When I come back, wherever the timer stopped will tell me how long it took.
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Old 09-08-2019, 11:00:24 AM
Radiomike Radiomike is offline
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Default Re: Battery Bank Balancing Questions

With VRLA batteries the plates must be separated and this is done either with a silica based gel or a with chopped glass fibre mats AGM. Both of these then hold the electrolyte. A VRLA battery may be a GEL or AGM type. AGM is by far the most common and the AGM mats can be seen when the pressure relief valves are removed; looks like white blotting paper, between the plates.

Did you manage to get the manufacturers data sheet and instructions for these cells?

Mike
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Old 09-09-2019, 03:21:26 PM
mihit mihit is offline
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Default Re: Battery Bank Balancing Questions

Quote:
Originally Posted by Radiomike View Post
With VRLA batteries the plates must be separated and this is done either with a silica based gel or a with chopped glass fibre mats AGM. Both of these then hold the electrolyte. A VRLA battery may be a GEL or AGM type. AGM is by far the most common and the AGM mats can be seen when the pressure relief valves are removed; looks like white blotting paper, between the plates.

Did you manage to get the manufacturers data sheet and instructions for these cells?

Mike
What I know is what's printed on the units.
These: https://m.made-in-china.com/product/...618937112.html
I'll be heading back out at the weekend so will see what amps they've taken and where the voltages are.
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Old 09-09-2019, 03:26:55 PM
mihit mihit is offline
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Default Re: Battery Bank Balancing Questions

I can't see into the plates. They did have bits of white paper in the "tray" area, presumably to soak up then evaporate off any boil over. It would make sense if they were manufacturing off cuts and the same stuff was inside
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