Battery Specific Gravity

sawmill

Since I have a 24 volt battery system it is always of great interest to read all posts concerning care and maintaining any and all types of batteries.

One thing that has been mentioned in previous posts concerns the first activation of a flooded battery. I understand that different specific gravity levels can be used. If one used 1.300 and 1.265 level in two identical batteries, how does the battery know to recharge to the 1.300 level and only recharge to the 1.265 in the other battery in order to reach full charge?

It would appear that the first activation sets the standard for the full charge level for the life of the battery. How is this accomplished?

BB.

Re: Battery Specific Gravity

Pretty much... One reason an "absorb" timer and/or charging current cut/off level.

-Bill

sawmill

Re: Battery Specific Gravity

Thanks for the reply BB

Would it be possible to "overcharge" the 1.265 battery until it reached the 1.300 level? If this is possible it would appear that the initial specific gravity could be increased if so desired. I'm sure this would have an effect of the life of the battery. I am still unclear the purpose of having different specific gravity levels for flooded batteries.

BB.

Re: Battery Specific Gravity

Not a battery chemist--But once you have all of the sulfur off the plates and back into solution (electrolyte)--All you are doing is generating Oxygen and Hydrogen by electrolyzing the water. Plus, the O² being created at the Positive plate is being driven into the plate frame and causing corrosion there.

Stop once the battery fully charged, further charging is just damaging to the plates.

-Bill

Blackcherry04

Re: Battery Specific Gravity

If a gallon of pure water is the " 1 " the .265 is the Specific Gravity of the electrolyte solution ( weight ). As the battery discharges the Sulfuric Acid separates and attaches it's self to the lead in the plates. When you reverse the current you are driving the Sulfuric Acid back into the solution of electrolyte. The question you had it there a way for overcharging to increase the SG to 1.300, the answer would be no, unless the mix was changed by removing some of the 1.265 and adding some more Sulfuric Acid. This why you only add distilled water when you replace lost water when you top off. Using the SG always gives you a Constant to return to when charging.

My airplane batteries use 1.285, It is supposed to make them keep a more stable and have a lower current drop when discharging. A lot of Traction Batteries use 1.300, again because of loading and discharge.

vtmaps

Re: Battery Specific Gravity

sawmill wrote: »

It would appear that the first activation sets the standard for the full charge level for the life of the battery. How is this accomplished?

If the batteries are otherwise identical, it is accomplished by adding electrolyte of differing SG to each battery.

If one used 1.300 and 1.265 level in two identical batteries, how does the battery know to recharge to the 1.300 level and only recharge to the 1.265 in the other battery in order to reach full charge?

The batteries don't know. It is up to you to set their charging parameters. By the way, the batteries are not identical if they were manufactured with different SG. You should not mix non-identical batteries in a single battery bank. That includes identical batteries that are of different ages because they are no longer identical.

Would it be possible to "overcharge" the 1.265 battery until it reached the 1.300 level?

As Blackcherry04 answered, no. To begin with there was not enough sulfuric acid to reach 1.300.

If the batteries (otherwise identical) were manufactured with different SG, the higher SG battery would have a greater capacity and a shorter lifespan.

It occurs to me, are you asking your question because of what you read in this recent sticky post:
http://forum.solar-electric.com/showthread.php?p=109069#post109069
If so, I'm not sure that the answers given in this thread are correct. In that post I believe that the batteries were manufactured with identical SG and the author of the HP article was suggesting that you might run them at a lower SG to save on operating costs.
--vtMaps

Volvo Farmer

Re: Battery Specific Gravity

vtmaps wrote: »

You should not mix non-identical batteries in a single battery bank. That includes identical batteries that are of different ages because they are no longer identical.

Oh... like this?



Luckilly, lead acid batteries are somewhat forgiving. In my case, I am hoping that Jesus is looking over my battery bank, because I am in serious need of forgiveness!

sawmill

Re: Battery Specific Gravity

I must admit that I'm still a little confused as to what constitues a full charge. Is it considered a full charge once the battery reaches the initial specific gravity that it was activated with in the beginning? Or is a full charge reached once the specific gravity will not rise any more such as used in equilization? It seems that these issues would need to be clear in knowing how to set the Absorb/time and or ending amps. I fully realize that absorb times have to obtained by some trial & error in the beginning of commissioning a new bank.

offgrid me

Re: Battery Specific Gravity

Specific gravity is an indicator of charge level. When a battery bank is first installed one should measure, adjust for temp and record the sg of each cell. These are your reference #'s for all future charging. Different battery manufacturers and different chemistry will have different sg's from the factory. No amount of over charging will increase this once it is set.
Ned

Blackcherry04

Re: Battery Specific Gravity

Everything with a Battery is a trade-off. The day they are activated they are dying and the lead is sacrificial . The 3 stage charging routine + equalization has been proven to give you the best results and the longest life in the FLA Battery. By using sponge lead I think you are some what limited at ever getting back to exactly 100 % sg. Getting to 99. point something has reversed the process is about as good you'll ever get. In a true Absorption Stage the Voltage is Maintained and the amps are decreasing. The Voltage is the test. The charger will keep dropping the Amps ( current ) and see if the voltage is maintained, when it backs off the current if the Voltage drops it will add back more current until the battery no longer drops the Voltage it's trying to Maintain. Ending amps Is a point that that the Voltage will stay steady with a minimum amount of current. I like the Ending AMP's / Current to be 7-10 Amps and let the Float stage use that as the trigger to raise the Voltage as needed to maintain the charge. If you don't have it high enough then the charger will just short cycle over and over and shorten the life of the battery. Ending Amps on one Battery may not be what you'd want to use on a Large bank of batteries. 5 Amps constantly going into a single battery to overcome a parasitic drain will just cook it to death if the ending amps are to low. It's kind of a test and see what your charger is doing and make the adjustments where you can. Sg's will give you the Constant and everything else can be altered.

vtmaps

Re: Battery Specific Gravity

sawmill wrote: »

Is it considered a full charge once the battery reaches the initial specific gravity that it was activated with in the beginning?

Yes

sawmill wrote: »

Or is a full charge reached once the specific gravity will not rise any more such as used in equilization?

also yes.
To explain: When the battery is manufactured there is a certain amount of sulfuric acid. It has a certain SG. You can't make the SG go higher. Later on, some of that sulfuric acid may be converted into hard sulfate crystals. At that point you cannot get the crystals back into solution no matter how much you charge the battery. The sulfated battery now has a lower SG when fully charged because there is less sulfur available to go back into solution.
--vtMaps

Vic

Re: Battery Specific Gravity

My few cents,

First Blackcherry, think that your explanation of regulation of Asorb voltage is more complex than needed. As long as the CC can maintain the Asorb voltage, that is where it stays, fairly closely. If the charge source cannot maintain that target voltage, then the current goes to the max available, and if that voltage cannot be maintained, the voltage begins to fall.

The End or Return Amp value varies with the size and type of battery. I usually use 16 Amps for the two main banks, but someone else may need 20, or 6 all depends.

Regarding SG never being able to go above the initial value (at the mfg recommended fill level), Some of the cells in may banks DO exceed this value when correctly filled, and Temp Compensated. Technically, to me, these are Overcharged cells. In my experience, there is some ability of the cell to exceed the initial SG of the initial "target" of the manufacturer.

YMMV, Vic

Blackcherry04

Re: Battery Specific Gravity

Vic wrote: »

My few cents,

First Blackcherry, think that your explanation of regulation of Asorb voltage is more complex than needed. As long as the CC can maintain the Asorb voltage, that is where it stays, fairly closely. If the charge source cannot maintain that target voltage, then the current goes to the max available, and if that voltage cannot be maintained, the voltage begins to fall.

The End or Return Amp value varies with the size and type of battery. I usually use 16 Amps for the two main banks, but someone else may need 20, or 6 all depends.

Regarding SG never being able to go above the initial value (at the mfg recommended fill level), Some of the cells in may banks DO exceed this value when correctly filled, and Temp Compensated. Technically, to me, these are Overcharged cells. In my experience, there is some ability of the cell to exceed the initial SG of the initial "target" of the manufacturer.

YMMV, Vic

I guess this is where two different people with two different opinions will differ. I am sure there are Charge Controllers with algorithms all over the place just like Battery Chargers. The term 2 Stage, 3 Stage and More, means a lot of different things to different Manufacturers. If there are not TAPERING AMPS in the absorb stage then I wouldn't own it because it's just a 2 Stage charge with some kind of a " Me too. "added to it.

If I put in 1.265 electrolyte into a pot on a stove and boils some water off could I change the ratio ( SG )of the electrolyte, of course I could raise the SG above the original electrolyte. If I added back the same exact amount of water that boiled off would the ratio be back to where I started ?? Actually with sponge lead it's really hard to ever get back to the same exact ratio, as the lead will always have some sulfuric acid trapped.

Vic

Re: Battery Specific Gravity

The charge current tapers because, at a constant Asorb voltage, the batts will ACCEPT less and less current, as they become more fully charged.

It is not really the charger tapering the current per se, it is that to keep a constant asorb V, the current needs to be reduced. THis is not a strategy of the charger, it is because the charger is regulating the voltage to be a constant.

The observations about overcharged cells having higher than original SG, are just what I've seen over the years here. compensated, and with electrolyte at speced levels.

Vic

oliline

Thank you all for the posts. Very instructive and interesting. I am confused on the way your 3 stage charging works.
On many solar charge controller, "bulk" allows all current in; "Absorption voltage" limits the current to not exceed the absorption set voltage and "Absorption time" limits the time it stays at this voltage... Finally "Float" comes in when the "time of absorption" finishes.
Our experience in solar indicates that many times the SG indicates not all sulphur is recovered (battery not 100% full when Float is reached).
To my understanding the "absorption voltage" needs to be increased a bit, or the "time absorption" needs to be increased...
My questions are:
In your 3 stage charger, does time of absorption exist at all?? It looks like you don't care about the time of absorption but the "Ending Current" of absorption... true??
In your case? Is there a formula to know the "Ending Current" according to the capacity of the battery we use?
What does really happen if we increase the "absorption voltage" (not exceeding let's say 15V for a 12V system)?

Estragon

Something like 1-2% of capacity is the range often used for transition from absorb to float, so about 4a for a 350ah bank. You can get close to, but never really reach 100% SOC, so the end-amp value is a bit arbitrary, a trade-off between not quite full, and heating and gassing for very little gain.

Increasing Vabs is sort of a mild equalization. It will take longer to reach Vabs from bulk, and take longer for current to taper to the arbitrary end-amps. Again, a trade-off.

oliline

Thanks for your quick answer Estragon!
Does it mean that in our Outback FM we can set the absorption time to infinite (a very high amount of hours) and concentrate more on the transition current from absorption to float??
What would happen if the transition current from absorption to float is set at 0A (meaning the battery would stay in the the Vabs for a long period of time, or even forever)? Overcharge? Extra boiling only? Damaging of the active material of the plates?
Oli

mcgivor

You have to consider the sun sets every day so the absorption will terminate as the ability, or availability of output diminishes later in the day, using an end amps of 2% is to prevent overcharge, but if the loads exceed that then the absorption will not terminate, hense the time limit. Depending on time of year the time limit can be adjusted to suit loads and solar availability, there is no one setting fits all, careful observation of battery gassing will indicate where settings need to be. Once in float, the PV will support the loads, usually a recharge voltage set-point is programed to prevent over discharge before the sun sets, it's a fine ballance that is specific to each system, only the operator can see where these transitions and setpoints need to be.

oliline

Thanks for clarifying slowly our grey-areas
Let's say the Abs to Flo Current is set to 2%. Let's say "the loads exceed that then the absorption will not terminate", what would be the problem of that? Can the absorption not be continued next day?
We know gassing occurs after 2,4 V/cell. We know it's good to avoid stratification of the electrolyte, and they say it can even be beneficial break/dissolve lead-sufate crystals. But MCGIVOR, you say "careful observation of battery gassing will indicate where settings need to be". In what sense do you mean it?
And back to the question I already made: What would happen if the transition current from absorption to float is set at 0A (meaning the battery would stay in the the Vabs for a long period of time, or even forever)? Exceed the desired SG?? Extra gassing/more refill required only? Damage of the active material of the plates?
Thanks full

706jim

Have to comment here: Last night I had dinner with a friend who works for Bell Canada. In charge of servicing remote transmission stations. They equalize their batteries for a MONTH at a time! They say this works best although they have to keep on top of acid levels. I forgot to ask when they repeat this process but apparently it works best for their batteries. Cells that weigh 550 lb.

jonr

oliline: overcharging causes plate corrosion and boils away water.  On the other hand, if you set Vabs to a float voltage, you will be fine if it continues forever (but there are good reasons for not doing this).

Estragon

@706jim - are the telco batteries cycling, or more typically, floating for most of their lives in a UPS application? Floating with little use, they'll tend to get replaced based only on age, so grid corrosion isn't likely much of a factor.

Also, what do the use for EQ voltage? If I EQd my bank for a month at my normal Veq, even just during daylight hours, I'm pretty sure I'd be adding water daily at least.

mcgivor

A careful observation of what the electrolyte is doing is what I'm trying to explain, mild gassing is normal, pushing beyond is detrimental, for example you can hear the hissing, accompanied with the small of sulphur, it's probably beyond where you should be, every system is different, but I would highly recommend staying within the 2 % end amps, adjusting for seasonal changes, for example in fall/winter an open ended absorption may be required due to limitations of sun, personally I adjust frequently depending on daily changes, but I depend on my system so careful monitoring is paramount.

706jim

I will try and get specifics as to battery type and voltages used. The discussion I mentioned was in a restaurant last night and the noise level was getting louder by the minute. When I do I'll post the details. I certainly never knew that batteries would like equalization voltages for this sort of duration.

oliline

Are you saying that by leaving a battery in Vflo indefinitely will also take us to the desired SG? I know that we need to take advantage of the hours of solar radiation as much as possible and that's why we better charge at higher voltages, etc. But just asking, Can Float be set as final voltage if we have all the time in front?

Would that be a good idea if we want to give a slow charge to a battery?

arby

From what I have read on these forums the battery bank needs a good kick in the ass every so often.
 I have a small 12 volt system at my cottage which pretty well only runs a car radio. Most of the time during the day the radio is taking much less than the panels are producing. I would say that this bank is in float almost perpetually and when I have checked the SG , it is 1265. 10 years now, BUT, I have no idea if these batteries could stand a load. Four Sam's Club batteries.

oliline

mcgivor said:

 I would highly recommend staying within the 2 % end amps, adjusting for seasonal changes, for example in fall/winter an open ended absorption may be required due to limitations of sun, ...

I forgot to ask, this 2% should be taken as a reference to what discharge rate capacity: C10; C20; C100 ? Thks

mcgivor

Whatever the battery capacity is, take 2% of that, eg. 100Ah  would be 2 amps, 1000Ah would be 20 amps etcetera, so 2% of C100 if you like.

Estragon

The 1-2% end amps number is of capacity at a 20ish hour rate of discharge (C/20).

At recommended float voltage, there is essentially no charging going on. They can sit at that voltage pretty much indefinitely, although very long term float (months-years) can result in stratification and cell balance problems.

mcgivor

Estragon said:

The 1-2% end amps number is of capacity at a 20ish hour rate of discharge (C/20).

At recommended float voltage, there is essentially no charging going on. They can sit at that voltage pretty much indefinitely, although very long term float (months-years) can result in stratification and cell balance problems.

Sorry you lost me with the above @Estragon ,by end amps I was referring to charging, when to terminate absorption, so as not to over saturate /charge the battery and go into float, 2% charging amps of the battery Ah capacity is the accepted cutoff  for lead acid, unless I misunderstood your statement. 

jonr

As mvgivor says, the 2% is a number that should be tweaked anyway.  So the reference discharge rate ends up not mattering.