Aging battery bank - What happens when sulfation doesn't kill a battery bank?

benthere · June 2015

Hi, all.

I'm trying to decide if it's time to give up on my six-year old bank of eight Rolls S600 (450AH) batteries.

From the beginning, these batteries have seemed to like a high charge voltage and I set out to wear them out rather than under-charge them. I've been charging at the maximum listed 2.5-volts per cell while ignoring the fact that that value is only for very cold temperatures. It served me well enough for years.

Lately, though, my batteries have been getting hot and using more water. I wonder if the "natural" voltage of my batteries is dropping causing the high absorb setting to now be a real overcharge. If so, it still isn't enough to keep my SG values in line as they range from 1.240 to 1.270 after temp correction. The amount of power input for a full charge has also increased. I saw that last year and assumed that our loads had risen but now I'm starting to wonder if that was the point of departure. We have been charging with about 7KWH/Day and now charge with 10KWH/Day.

What can I expect to see as the bank further deteriorates? Can anyone point me at a document listing the behavior of batteries as they age? It seems that the entire discussion of battery behavior centers around sulfation. If one dodges that bullet, what happens?

Edit to add: Absorb setting is 60V for 6-hours.

Thanks!

benthere · June 2015

I should add that my bank does not seem to have a shorted cell. Each battery contributes roughly 6.3 volts to the pool.

Dave Angelini · June 2015

How is SG after EQ ? Any significant improvement?

The 7KWH to 10KWH is your total for the home or just the battery charging?

In answer to your main question, The SG will get worse and no amount of EQ will help. You then write a check when you can't put up with the reduced capacity. Your current situation could stay like this for quite a while longer.

Vic · June 2015

Hi beenthere,

Six years of service from these L-16s is not bad service.

Are you using a Remote Temperature Sensor with the FM CC? If you are not using an RTS in the system, this is really a MUST-have option for good battery health.

And, it is not uncommon to need to set the Vabs higher in the Winter, to get batteries recharged during the usually shorter solar days, and the more-common inclement weather. In the Spring/Summer/Fall, it is fairly common to need to set the Vabs somewhat lower, as the batteries are more easily charged with longer days. Realize that AZ Summers can have significant cloudy days, too.

As you know, it is fairly common for aging batteries to consume more water, so none of this may be out of the ordinary.

You really do not need to try to get to 1.270 SG, unless these cells were a bit low on water when measured, IMO.

FWIW, Vic

benthere · June 2015

Dave,

7 vs. 10KWH is from the log on my FM80 so total charge. We are off grid so presumably about half of that is our usage and the other half is losses.

I'm going to try an EQ today but I think battery temp will stop me. We shall see.

I can live with the capacity, in fact don't feel a reduced capacity. What I'm having trouble with is a full charge doesn't cook the batteries. The last couple days I have not reached my desired 6-hour absorb and batt temp has reached 120F. I think I'm going to have to drop the absorb voltage.

benthere · June 2015

For a little more back-story, we had a cloudy week but I didn't run the generator due to problems with it. Now I'm trying to recover from that and finding it hard to do. Guess I should have gotten the generator issues sorted earlier.

benthere · June 2015

Vic,

This is my first battery bank so I only know about increased water consumption from reading about it. I do use a RTS and the compensation set to limited with 59V as the lower limit and 61V as the upper. I have now gone to wide compensation.

Vic · June 2015

OK beenthere,

Do not know how Wide Compensation plays with Limits on range of compensation. You do want Wide compensation, and for these Flooded batteries, do not know why one would want to limit the compensation range as narrow as 59 - 61 volts.

Personally would either avoid setting any Limits, or, at least set the upper limit to 64-ish V, or so, and something like 56.5 or so for the lower. Believe that the VFX inverters can tolerate 64-ish V, and depending on just how hot your batteries get should dictate the lower Limit, IMO.

Please make sure that the VFX also uses an RTS, and that your EQ voltage is also temp compensated (believe that this is an option on the FM CC -- it was not available on the older MX CCs).

My opinions, Good Luck. All of the Surrette banks here are ten years old FWIW. Vic

BB. · June 2015

Start backing down on the charging voltage a bit--High water consumption is not good and indicates over charging.

Sulfation takes "sulfur" out of the electrolyte. As a start, I would measure the specific gravity of the full charged bank (after equalization)... If 1.270 is the "fill" specific gravity, and your average 100% full reading is ~1..255 sg units. Then, I would suggest backing down charging voltage by (as a first step):

24 cells * (1.270-1.255) = 0.36 volts

In (my) theory, that should account for the lower resting voltage of your full bank... 60 volts is a fairly high charging voltage--But for Surrett/Rolls batteries, it is probably pretty typical.

Regarding the 1.240 to 1.270 spread--That may take many hours or several days of equalization (at ~2.5 to 5.0% charging current) and even some cycling to reduce the spread.

Make sure you do not equalize with too much current. Check the "low cells" every 60 minutes for SG reading--When that cell's SG stops rising between readings (watch for bubbles on float)--Then that cell is at its new 100% full SG. Equalization is finished when all cells stop rising (remember to log temperature compensated SG readings).

Equalization needs to be stopped if battery/cell temperature gets too high. From a Rolls battery manual:

http://rollsbattery.com/public/docs/user_manual/Rolls_Battery_Manual.pdf

Do not let the cell temperature exceed 125°F (52°C).
*Do not place on charge until electrolyte temperature is below 35°C

High battery temperatures, excessive bubbling/gassing, too much water usage are usually signs of over charging.

-Bill

benthere · June 2015

Thanks, Bill. Your advice agrees with my limited experience. What is surprising to me is that I didn't change the charge settings yet the battery bank behavior seems to have changed. I intend to drop the charge voltage if the condition continues after EQ.

BB. · June 2015

Keep an eye on battery temperature... if you assume -5mV/cell/C:

-0.005 volts * 24 cells * 10 C rise = -1.2 volt drop in required charging voltage (assume ~25C is "standard temp", 25C = 95F).

You can get into thermal run away if the cells start getting hot (age, sulfation, etc.) and the battery battery starts to run "hot"... Which, as equation, means that the charging voltage falls, and charge controller pumps in more current, battery gets hotter, etc..., and fails.

More or less, if you are >~2% rate of charge (uncontrolled), you can overheat a battery bank. Less than 1% "uncontrolled" charging, the battery bank should not overheat (although, it will still gas and lose water).

Of course, when batteries get cold--Then you need to jack charging voltage up.

-Bill

Vic · June 2015

BB. wrote: »

... If 1.270 is the "fill" specific gravity
-Bill

Almost all Surrette Flooded batteries have electrolyte that is 1.265, except Locomotive Starting batteries.

It can be a bit misleading when looking at the Surrette spec sheets on specific batteries, as the Capacity is almost always shown as if the batteries are filled with 1.280 SG electrolyte.

This difference between fill and spec sheet represents a 5% reduction in actual Capacity, FWIW. Vic

benthere · June 2015

BB. wrote: »

More or less, if you are >~2% rate of charge (uncontrolled), you can overheat a battery bank. Less than 1% "uncontrolled" charging, the battery bank should not overheat (although, it will still gas and lose water).

Bill, I'm not quite following this statement. When you say "rate of charge" is that the charge rate compared to the 20AH capacity of the battery and thus even at 2% of the 20-hour rate a battery can be overheated. The control you speak of is presumably temperature adjustment?

Overheating the bank is exactly my fear.

BB. · June 2015

More or less, 2% of 20 hour battery capacity or C₂₀ Battery Capacity--The "standard" battery capacity number we use around here (i.e., a 400 AH battery bank would be 8 Amp @ 2% rate of charge). There are other capacity numbers like C₁₀₀₀ C₂₄ C₈ and such... Really depends on the application. I suggest that 5 hours of battery usage per night, for two night, with 50% discharge = 20 Hour rate for our off grid power needs.

Equalization is, more or less, "uncontrolled" charging--If you leave the battery on "equalize" long enough, at high enough current, it will (probably) overheat... The 2% and 1% are rules of thumbs used by me and some others in industry regarding battery charging.

Many "cheap systems" do not have regulators to charge batteries (portable electronics, etc.), they just limit the charging current with a resistor or similar.

For Lead Acid batteries, it is typical to put a 1% rate of charge solar panel (no regulator) on a battery that is otherwise in long term storage (flooded cell lead acid batteries will typically self discharge after 1 month to state of charge levels where sulfation becomes a problem).

And on the other side, there was an interesting "sales article" by a battery maintenance company (for large UPS/Backup installations), and their experience was that batteries that had a self discharge rate of >2% (i.e., steady date float current) were in trouble and could fail in the future if problems were not addressed.

Add that 2.5 to 5% is the typical recommended Equalization rate of charge, and people here have seen their banks get hot/overheated during "extended" equalization--Just another warning that a rule of thumb is highlighting.

Notice that I am saying a battery at 100% state of charge could over heat >~2% rate of charge (will take many hours typically). A lead acid battery at less than 80% State of Charge can take many times that current and not overheat or even get warm (25% rate of charge or higher).

During Equalization, roughly 50% of the current is turned into Hydrogen and Oxygen and 50% of the current is turned into heat. At 80% or less state of charge, you are probably looking at 5% to 2% or so of current being turned into heat and no gassing (the vast majority of charging current goes into actually charging).

If you are not equalizing and/or have forced air cooling of the battery bank in cool room, of course, the actual conditions that can cause overheating will be different.

-Bill

benthere · June 2015

http://www.evdl.org/pages/hartcharge.html

http://www.evdl.org/pages/hartcharge.html wrote:

[FONT=arial, sans serif]The voltages mentioned above all will change with temperature. Voltage falls 0.003 volts per cell for each degree C increase, and vice versa for a decrease in temperature. If you ignore this, or your "automatic" charger does, you will apply too much voltage to hot batteries and fry them, or insufficient voltage to cold batteries and so not fully charge them.[/FONT]

[FONT=arial, sans serif]As your batteries age, their end-of charge voltage gradually falls, their end-of charge current gradually rises, and their amp-hour capacity slowly drops. Barring abuse, this is normal, inevitable, and not reversible.[/FONT] [FONT=arial, sans serif]If your charger blindly tries to bring old batteries up to the same voltage or current as new ones, it will charge too long, and so overcharge them. So, the best charging algorithms for old batteries are ones that count amp-hours, or look for the voltage to stop rising or for the current to stop falling.[/FONT]

This sounds pretty close to what I am experiencing. The question, then, is how to select my newly required charge controller settings other than guess?

benthere · June 2015

Can a person find the condition-dependent absorb voltage by the voltage at which the cells start to gas?

Vic · June 2015

benthere wrote: »

Can a person find the condition-dependent absorb voltage by the voltage at which the cells start to gas?

The Gassing voltage is probably subjective, if one is watching for bubbles. Surrette might be able to tell you just what is the official Gassing V for your batteries, but, bet that things do NOT need to be this complex.

You should probably look for the lowest Vabs that will fully charge your batteries on a typical day. This will probably change with the seasons.

Watching the actual battery charge current during Absorb, will generally indicate when there is no more charging to be done. This amount of Finishing current will probably vary with temperature, and definitely change with a change in Vabs.

BUT, the good news is that your batteries are Flooded, and you can actually measure just how well your batteries are being charged.

Perhaps you need to work out from under the weather effects, and try to get the bank as fully-charged as you can, and then try EQing. Be sure to Temp Comp the EQ voltage, As you know, Surrette has recommended a minimum Veq of 61.9 Volts ... a bit higher should be OK.

Aso, BB Bill mentioned (I think), that a bit deeper cycling the battery can help SG Disparity, especially on cells that have the recommended fill Level, but are still a bit high in SG.

In the above quote from the evdl.org states a Temp Comp value of -- 3mV/C. This is common for AGM batteries, but is INCORRECT for Lead/Antinomy plated Flooded batteries (as are your Surrettes), which need -- 5mV/C.

Again, would recommend widening the set voltage Limits in the FM CC. Mostly these voltage Limits are either for funky inverters that will not tolerate cold or EQing Flooded batteries, or for Gel/AGM batteries that cannot take the higher voltages needed for Flooded batteries, IMO.

FWIW, off to work, Good Luck, Vic

BB. · June 2015

Regarding Gassing, from this link:

http://www.powerstream.com/SLA.htm

Anything above 2.15 volts per cell will charge a lead acid battery, this is the voltage of the basic chemistry. This also means than nothing below 2.15 volts per cell will do any charging (12.9V for a 12V battery) However, most of the time a higher voltage is used because it forces the charging reaction at a higher rate. Charging at the minimum voltage will take a long long time. As you increase the voltage to get faster charging, the voltage to avoid is the gassing voltage, which limits how high the voltage can go before undesirable chemical reactions take place. The typical charging voltage is between 2.15 volts per cell (12.9 volts for a 6 cell battery) and 2.35 volts per cell (14.1 volts for a 6 cell battery). These voltages are appropriate to apply to a fully charged battery without overcharging or damage. If the battery is not fully charged you can use much higher voltages without damage because the charging reaction takes precedence over any over-charge chemical reactions until the battery is fully charged. This is why a battery charger can operate at 14.4 to 15 volts during the bulk-charge phase of the charge cycle.

The basic lead acid battery is ancient and a lot of different charge methods have been used. In the old days, when voltage was difficult to regulate accurately flooded lead acid batteries were important because the water can be replaced. The lead acid chemistry is fairly tolerant of overcharging, which allows marketing organizations to get to extremely cheap chargers, even sealed lead acid batteries can recycle the gasses produced to prevent damage to the battery as long as the charge rate is slow. We offer a range of chargers from inexpensive to very sophisticated, depending on the requirements of the customer, but all of the chargers we sell off-the-shelf are highly regulated sophisticated chargers that cannot overcharge the battery.

-Bill

benthere · June 2015

Vic wrote: »

Again, would recommend widening the set voltage Limits in the FM CC. Mostly these voltage Limits are either for funky inverters that will not tolerate cold or EQing Flooded batteries, or for Gel/AGM batteries that cannot take the higher voltages needed for Flooded batteries, IMO.

I have enabled the wide temp compensation and think I will be further reducing the absorb setpoint checking SG as I go. Thanks.

Vic · June 2015

benthere wrote: »

I have enabled the wide temp compensation and think I will be further reducing the absorb setpoint checking SG as I go. Thanks.

OK, BUT, it is quite possible that regardless of weather you have limits Wide, the CC may well still be LIMITING charge, and possibly EQ voltage excursions, ONLY to the relatively LOW high limit. If this Limiting is applied to EQ voltages, you might never really be performing an EQ, except for warm/hot batteries. On a very warm battery, the 59 volt limit might be too high ... do not know how the FM CC handles Wide, and specific Limits.

In the Winter, with cold batteries (possibly), then, even possibly the daily Absorb voltage could be relatively too low, at 61 V.

Could you check the FM CC Manual, to confirm the interaction of Wide Limits and charging voltage limits that are set?

You would also be able to observe how the CC behaves by changing the absolute voltage limits you have set ... think that these Limits are really there to try to keep an over-sensitive Inverter from shutting down when EQing or with cold batteries, AND for to limit charge voltages to Sealed batteries, and, IMO neither your Inverter, nor your Flooded batteries need this "protection".

These are my opinions, and these may not be of interest to you. FWIW, Vic

zoneblue · June 2015

Around here the flooded guys increase the absorb setpoint by 0.1v for every year of battery age. Im not sure what the rationale is exactly.

Couple of other things to consider:
- 6 hour absorbs. If you have the ability to use end amps, that will better terminate absorb with less over charge.
- some controllers auto EQ doesnt wait for absorb to finish first. Doing EQ after absorb means less current and less heat.

Vic · June 2015

BB. wrote: »

Regarding Gassing, from this link:
http://www.powerstream.com/SLA.htm
-Bill

Thanks for that Quote, Bill, but, that exact quote appears to deal with the voltage where charging just begins. Seems to me that Gassing V is quite a bit higher. Have not looked at that link, so there may be much more about gassing V, etc.

Believe that the customary Absorb voltage listed for Gel batteries, is just below gassing V.

And, also, believe that not only Chemistry, but Metalurgy IS important (this may be a distinction without a difference) affects gassing quantities and Vgas.

But, I am not a Chemist, FWIW, Vic

benthere · June 2015

flexmax_80.pdf wrote:

During cold weather, a battery often requires a higher recharging voltage. Lower quality inverters might not accommodate these higher voltages and can shut down during recharging, cutting off power to their loads. The FLEXmax 80 allows the user to lower the compensated voltage in the Absorb cycle so these inverters will remain operating. Also, some batteries have an absolute voltage limit that should not be exceeded and the WIDE/LMIT option allows the user to control this during recharging. WIDE allows the RTS full control over recharging; LIMIT sets the ceiling and floor voltages for the RTS.
During hot weather, the LIMIT feature set point assures recharging will continue at a high enough voltage rather than dropping too low in reaction to a higher ambient temperature. This assures the recharging voltage ad- equately charges, but should be monitored according to the battery manufacturer’s recommendations.
The RTS default compensated voltages apply if the WIDE/ LIMIT option is set to WIDE. To change these values, press the <WIDE> soft key to bring up the next screen which allows user-determined limits. Press the <SET> soft key to adjust these values.

I read this to indicate that when in the Wide RTS Mode, the upper and lower Limit settings are disregarded.

benthere · June 2015

Another thought to locate the absorb setpoint as the point at which the input amps = 2% of the 20AH capacity on a fully charged battery. Any thoughts on that? That would put me at 56.5, which to me is a very low setting but it may very well be what the bank now requires.

benthere · June 2015

I have started a case with Surrette to see what they have to say on the issue.

Photowhit · June 2015

I saw your rather high battery temps and ponder if you have your battery bank outside in AZ?

If so your long absorb with the local hot temps may have aged your batteries quite a bit. You might consider building a berm shelter for your battery bank, to see if you can help moderate the temps.

benthere · June 2015

The batteries are not outside but they aren't ground-coupled either.

Photowhit · June 2015

Sorry, I didn't mean sitting outside, but rather in un air conditioned space. Being constantly at a high temp shortens the battery life. It's basically a an electro-chemical reaction, warmer temps speed up the reaction. Being at 120 degrees daily would be harmful.

BB. · June 2015

More or less, the "gassing voltage" applies when the battery cell is nearly fully charged (as I understand the documentation and how I see batteries react).

When the batteries are not fully charged (especially below, say, 80% State of Charge), you can run them "over the gassing voltage", but the recharging chemistry takes priority over gassing chemistry regarding current flow (i.e., it recharges rather than gassing at that point).

-Bill

benthere · June 2015

Update

One battery runs about 5F warmer than the others. Today I noted that the same battery has a cell that seems to be self discharging. I'm looking forward to confirming the diagnosis with a battery autopsy but will have to wait until the new bank of Trojan L16RE-B batteries is installed.
I am not unhappy with the six-year run we had on the Surrette S600 battery bank; I set out to wear out my first set of batteries rather than lose them to sulfation and that seems to be what I have done.

Surrette tech support had a significant impact on my decision to go with Trojan for the next bank.

inetdog · June 2015

benthere wrote: »

What can I expect to see as the bank further deteriorates? Can anyone point me at a document listing the behavior of batteries as they age? It seems that the entire discussion of battery behavior centers around sulfation. If one dodges that bullet, what happens?

!

One of the other things that happens to batteries is erosion of the plates. Loss of active material (falling to the bottom of the battery case) causes loss of capacity and increase in internal resistance. (Higher terminal voltage for same charging current, lower terminal voltage under identical load current after full charge. )

PS: Try Battery University.

Aging battery bank - What happens when sulfation doesn't kill a battery bank?

Comments

Categories