Charging large Surrette/Rolls batteries

twistedtree · December 2010

Hi,

I've got an off grid system with 12 4KS25PS Surrette/Rolls batteries providing 1350Ah @ 48V (12 batteries, 4V each). In theory these are the cream of the crop in batteries, and the cost reflected that.

The rest of the system is Xantrex XW - 6048 inverter/charger and 2 MX60 MPPT chargers with 3200W of solar behind them. There is also a 14kW backup generator. Everything is about 2.5 yrs old.

On Solar, I get about 60A of charge current which is about 5% of C. That current has always been able to bring the batteries up to the absorb voltage, as well as the EQ voltage. From that I conclude that it's enough charge current. The generator provides another 100A, which is the max output of the XW6048.

The initial issue, 6 months after installation, were batteries not coming back to full charge as indicated by SG reading in the 1200 range. I worked through all the Surrette tech bulletins, and their tech support when I could get their attention. To get them to full charge ended up requiring a bulk/absorb voltage of 60.0V which is quite high, and at the upper limit of what Surrette recommends for solar charge. Absorb terminates when the charge current drops to about 16A which is around 1.3% of C. Note that Surrette says charge is complete at 2% of C, so I'm allowing Absorb to run longer than they claim is needed. A typical charge cycle appears as expected (at least to me), with Bulk running for an hour or so, then absorb for about 3 hrs. I don't have an Ah counting meter, but calculate the draw down to be about 100Ah prior to recharge.

Questions #1: Is this what others are experiencing with these batteries? What about other manufacturers with cells that are over 1000Ah?

Question #2: Are there long-term detrimental side effects to such a hot charge protocol? For example, plate corrosion which is aggravated by overcharging? I ask this particular question because I appear to have a bad cell or two. After about a 200Ah draw down, one battery's voltage suddenly plunges from 4V to 1.6V. I'm working with Surrette on a warranty replacement, but am wondering if other cells will follow. I'm assuming Surrette will stand behind their product, but cell replacement is a huge problem. Not only is it extremely difficult at this site, but I'm being quoted 8 week lead time to get a replacement cell! This is a big problem.

I'd be real interested to hear what other people ACTUALLY USING >1000Ah/cell batteries are experiencing.

Thanks

Dave Angelini · December 2010

Re: Charging large Surrette/Rolls batteries

Questions #1: Is this what others are experiencing with these batteries? What about other manufacturers with cells that are over 1000Ah?

I have a few systems with these and the if you are at the high end of Surrettes charge voltage that is pretty normal. What is your consumption?

Question #2: Are there long-term detrimental side effects to such a hot charge protocol? For example, plate corrosion which is aggravated by overcharging? I ask this particular question because I appear to have a bad cell or two. After about a 200Ah draw down, one battery's voltage suddenly plunges from 4V to 1.6V. I'm working with Surrette on a warranty replacement, but am wondering if other cells will follow. I'm assuming Surrette will stand behind their product, but cell replacement is a huge problem. Not only is it extremely difficult at this site, but I'm being quoted 8 week lead time to get a replacement cell! This is a big problem.

NO ! The goal is to get through absorption as quick as possible, without exceeding maximum current to avoid using too much water. Three hours or so and be at 1.265 SG is what I do, but my systems do not deeply cycle so your mileage may vary. Your problem has nothing to do with running absorption at 60V. I do not see any mention of EQ's which is your bible offgrid and possible the cause of your bad cell readings. More info please!

twistedtree · December 2010

Re: Charging large Surrette/Rolls batteries

Water consumption seems a bit high to me, considering I have watermisers. I just put 4 gal of water in across 12 batteries (24 cells) after 6 months. Is that "a lot"? I don't know - I'd love to see figures for a reasonable range of water consumption.

I check the SG pretty regularly, and EQ when anything drifts off. Things stay pretty happy over the summer, but in the winter I EQ more often. A couple of weeks ago I ran 3 hrs worth and cells came up about 5 pts then stabilized in the 1265 to 1270 range which is right where I'd expect them. That includes the bum battery. When the voltage collapsed on that battery, the SG was still 1180 and 1215 in the two cells. That's low, but not dead. The good cells averaged around 1225 at that time. So I think the cell failure is something other than sulfation. Would you agree?

halfcrazy · December 2010

Re: Charging large Surrette/Rolls batteries

I add 4 gallons every 6 months to my 16 trojan L16 batteries if that helps you compare

twistedtree · December 2010

Re: Charging large Surrette/Rolls batteries

halfcrazy wrote: »

I add 4 gallons every 6 months to my 16 trojan L16 batteries if that helps you compare

Do you have Watermisers or Hydro Caps on them? I have Watermisers which aren't 100% effective. I was thinking 4 gal was a lot given the Watermisers.

Dave Angelini · December 2010

Re: Charging large Surrette/Rolls batteries

Did you expose the plates? Adding water is usually done at 2 months +/-.
It is always checked every month!

Cariboocoot · December 2010

Re: Charging large Surrette/Rolls batteries

For what little it's worth, my POV:

5% charge rate is low, especially if the loads are still on while charging (which they almost always are). The effective charge rate may be much less than 5%.

If I were adding that much water I'd think the Voltage level for Absorb was too high.

I'd be looking at 135 Amps (you'd only manage 120 with the two MX60's) peak charge rate @ 57 Volts with Absorb time set to the max 4 hours.

But that's just me. :roll:

blackswan555 · December 2010

Re: Charging large Surrette/Rolls batteries

6 months of deficit charging, then

draw down to be about 100Ah prior to recharge.

so you are only using max 10% DOD,From new ? commissioned and EQ`d to start ? That points to sulfation, I would suggest doing a corrective EQ, Below from Roll`s,

After, I would recommend dropping you charge voltage to around 58.6>59 v and put absorb up to longer than the day (or float voltage to same, solar only) that should reduce your need to EQ a little, (Keep an eye on water usage to start, A rolls tech told me that one)
By the sound of your 100 amp hr a day usage, sounds like they never work hard ? they have to occasionally to keep the sponge lead`s "pores" clear, When you normally do EQ, may be worth giving them a good hard High load discharge down to 50% then charge till full > EQ till all stop rising

Equalization - Corrective

Corrective equalization needs to be performed if symptoms arise such as a constantly running generator (low capacity) or the battery bank will “not hold a charge”. These symptoms are typical of a heavily sulfated battery. If a battery is not being fully charged on a regular basis or limited equalization is performed using a generator (see Bulletin 611, Generators, Inverters and Equalization) sulfation will occur from “deficit” cycling. This undercharge condition can take months before it becomes a major and noticeable problem. This under charge condition is caused when batteries are deficit cycled. The bank receives less of a charge each cycle and starts to sulfate. Eventually the sulfate will cause a resistance to charge and a “false high voltage” reading will occur. The “false high voltage” is measured by the charge controller, which further lowers the charging current to maintain the voltage set point. This further increases the undercharge condition. This is one reason why specific gravity measurements are so important as “false high voltage” readings can be misleading. See Bulletin 609, Voltage, SG and State of Charge for information on how to correctly interpret voltage readings.

Amperage hour meters can compound the problem and cause people to believe they are returning the correct amount of energy back into the batteries to maintain a good state of charge. Amp-hr meters should be thought of as simply a fuel gauge that does not measure state of charge directly but indirectly. The state of charge is determined by using an equation (peukert's equation). Sometimes there can be fundamental errors with factors used in these calculations. You should always confirm, at least initially, state of charge by taking a specific gravity measurement of one cell when it is thought the bank is fully charged.
Corrective Equalization - Method

Corrective Equalization can take a very long time depending on the degree of sulfation. It is not recommended to equalize with a generator as some generators produce low grade AC that is not properly filtered by the inverter. This is especially true at higher voltages.

1. If you have hydrocaps remove during equalization.
2. Set charging controls to the highest voltage allowable by the charge controller (inverter). If the bank is severely sulfated or available current is very limited, charge control can be removed or by-passed. Temperature should be monitored very, very closely and keep below 125ºF.
3. Charge at a low DC current (5 A per 100 AH of battery capacity). If grid power is not available use solar panels or a good DC source when possible. At high voltages, charging with generator can be difficult and hard on the inverter.
4. Once an hour, measure and record the specific gravity and temperature of a test cell. If the temperature rises above 115ºF (46ºC) and approaches 125ºF (52ºC) remove the batteries from charge. (For temperature measurement choose a center cell, if applicable).
5. If severely sulfated, it may take many hours for the specific gravity to rise.
6. Once the specific gravity begins to rise the bank voltage will most likely drop or the charging current will increase. The charging current may need to be lowered if temperature approaches 125ºF (46ºC). If the charge controller was by passed, it should now be used or put back in line.
7. Continue measuring the specific gravity until 1.265 is reached.
8. Charge for another 3 hours. Add water to maintain the electrolyte above the plates.
9. Allow bank to cool and check and record the specific gravity of each cell. The gravities should be 1.265 ± 0.005 or lower. Check the cell electrolyte levels and add water IF necessary.

To avoid this situation it is recommended that a specific gravity reading of one pilot cell is measured and recorded on a regular basis when it is thought that the bank is fully charged. The measurement should be compared to previous readings. If the measurement is lower than the previous reading a longer absorption time and higher voltage setting should be used. Note as stated above, the longer the absorption time and the higher the bulk voltage, the more water will be consumed but less equalization will be required. Note: the specific gravity should rise as the cells use water. Look for trends in the specific gravity over a period of time and make very small adjustments as necessary.

Caution: If you have HYDROCAPS, remove during equalization.

Tim

To add, Do you have remote temp sensing on all charge sources and the sensors inside the batteries ?

twistedtree · December 2010

Re: Charging large Surrette/Rolls batteries

Dave Sparks wrote: »

Did you expose the plates?

No, not even close.

twistedtree · December 2010

Re: Charging large Surrette/Rolls batteries

A few months after commissioning the batteries I discovered that I did indeed have some sulfation due to deficit charging, and it was clearly reflected in the SG readings. This is what led me down the path that resulted in the high charge voltage that I have today. I checked SG VERY frequently after that for about a year until I became convinced that the batteries were reliably coming back to full charge. I continue to do periodic EQs and monitor how much more the SG comes up. In the summer the EQs do very little. After all, I'm darn near doing an EQ with every charge cycle at 60V:) Prior to this issue, the batteries were last EQ'd 4-6 weeks before.

The load I've described as sufficient to collapse the voltage on one battery, and it is indeed around 10% DOD which is light. My loads vary depending on whether the house is in use, and whether it's just me or my whole family. I'd say a typical one-day DOD when the place is in full swing is 20%. The sun doesn't shine every day here, and when it does it doesn't last long in the winter, so I can run 2-3 days with no appreciable solar charge. The generator kicks in once voltage is consistently <=48V, so the batteries get some exercise on a regular basis.

I'm thinking this is NOT sulfation, simple because the SG readings are good in all the cells, including the cells that die in a short time. In my experience, sulfation is always accompanied by low SG - after all, that's how the chemistry works to create sulfation.

Cariboocoot · December 2010

Re: Charging large Surrette/Rolls batteries

I'll just mention that the 4KS25PS is a "tall case" battery which is apt to suffer from electrolyte stratification if not charged at a rate sufficient to re-mix. These batteries really "prefer" a rate of 10-13%, and a real rate at that: not 10% minus the 3% being used to supply loads while charging.

Inconsistent SG readings between cells is the usual indication of sulphation. Over-all low readings usually indicates lack of charge time/current - and/or stratified electrolyte.

I must sound like a broken record. :roll:

twistedtree · December 2010

Re: Charging large Surrette/Rolls batteries

No broken record - I appreciate the dialog.

My cells read between 1265 and 1275 after a full charge cycle, and held there after 3 hrs of EQ. Would you consider that indicative of sulfation? I read it as healthy batteries, but welcome other interpretations.

When I was first having trouble getting the batteries charged properly, Surrette kept telling me I needed a C/10 charge rate and 6 hrs absorb. I wish that were part of their specs. Knowing that now, L16s might have been a better fit.

Dave Angelini · December 2010

Re: Charging large Surrette/Rolls batteries

Along with Marc's thoughts, and I always ask this because it tells me the useage and the charge source are matched well. What time do you go to float most days?

Cariboocoot · December 2010

Re: Charging large Surrette/Rolls batteries

SG readings of 1.265 to 1.275 consistent across all cells is good. BUT this is the thing with stratification; you are measuring the SG of the electrolyte on the top of the plates, not the bottom. Now you'd think that the "heavier" (higher SG) liquid would sink and so any reading at the top would be minimum SG, but if it's not mixed this, like the song says, ain't necessarily so.

MX60's won't go up to 6 hours Absorb time as far as I know, but Surrette's 10% charge rate recommendation makes sense to this old bird.

niel · December 2010

Re: Charging large Surrette/Rolls batteries

trojan l-16s will want about that same 10% rate of charge. 10% is a good rate for nearly any battery, but that isn't always feasible for many to fit or afford pv for that high of a rate of charge. the 5%-13% range is a somewhat arbitrary and generic rule of thumb and many try to go under that too because of costs rather than room for the pvs. it is a bit easier to mix the electrolyte in a 100ah battery at 5% than a 370ah battery at the same rate percentage. these high ah fla types of batteries work well, but are somewhat lacking on efficiency when charging.
now do not expect me to change the rule of thumb to fit all or i'll need to tell you to have between 10% and 13%. 5% is considered the 20 hour rate of discharge and many batteries will accept this low of a rate for charging as well and it wouldn't be fair to raise it because of some batteries not liking it that low. there is no substitute for checking with the manufacturer's charge recommendations if one wants to be sure to properly charge their batteries. my 104ah sunxtender agm for instance will charge fine at 3a or even at a whooping 50a and few flas can make this kind of claim. for the record, in most cases i still will not recommend those charges for my battery even though it will do just fine.
so can you put a lower rate on your batteries? most likely yes, but it should not be far off from the manufacturer's recommendations and realize that a smaller rate will need augmented at some times with a higher rate to mix the electrolyte and insure against sulfation. sulfation can happen if one brings their battery up to full charge too slowly and especially with those that rarely reach full charge, aka-deficit charging.

twistedtree · December 2010

Re: Charging large Surrette/Rolls batteries

Dave Sparks wrote: »

Along with Marc's thoughts, and I always ask this because it tells me the useage and the charge source are matched well. What time do you go to float most days?

Between 1-2pm probably 70% of the time. Other times it's done by 11 (long summer days), and others it never gets there if it's catching up after a few cloudy days.

twistedtree · December 2010

Re: Charging large Surrette/Rolls batteries

Just to focus on one particular point - would you guys agree that sulfation must be accompanied by reduce SG? This is assuming you have a good SG baseline, and have good electrolyte mixing. I think it does.

I take your point on possible stratification. Like you, my understanding is that the higher SG sinks and the lower rises so if anything I'd expect low readings, but who knows what other dynamics might come into play.

One advantage of my high Absorb voltage is that the batteries do bubble during absorb (remember my water consumption?), so there is definitely some mixing taking place. Also, after an hr or two of EQ I think I should be able to count on well mixed juice. Considering that my pre-SG readings and post SG readings are the same, I think it's unlikely that I have chronic stratification.

niel · December 2010

Re: Charging large Surrette/Rolls batteries

being on the high end of the charge range will create more gassing and as a result more maintenance, but would not be undercharged for sure and would give a good mix.

as to the sg being lower i'll agree because some of the sulfer is stuck to the plates. i would not totally go by the sg reading as a direct sulfation indicator though becuase it won't indicate what is normally on the plates and what is actually stuck on the plates. remember that sulfation starts as soon as a battery is not 100% charged. it doesn't become a problem until charging fails to remove it from the plates.

blackswan555 · December 2010

Re: Charging large Surrette/Rolls batteries

how long does EQ take you ? How many points is it out to start, best > worst cell ?

Cariboocoot · December 2010

Re: Charging large Surrette/Rolls batteries

I'll just mention a "worst-case scenario" here then everyone can point out how wrong I am. :roll:

Chronic under-charging & insufficient electrolyte mixing go hand-in-hand. Rate of charge isn't as important to mixing as length of time. In other words, c/5 over 6 hours is better than c/10 for 3 as far as mixing is concerned (but not for recharging, obviously).

So ...

The battery plates may be coated on the bottom but not on the top. In essence, they are reduced to a portion of their actual size and that whole portion is in the "upper level". The electrolyte below may be higher than 1.275, but that section of the plates is no longer functioning. Furthermore, this coating is not likely to be consistent across all plates, so some cells may be 90% and other 60% capacity even though the SG is reading properly in each cell. This will result in uneven charging, much like stringing together batteries of different capacity. Some will get hot, others won't. And then you've introduced the new problem of plate stress, which can cause pieces to break off or weld shorts.

It's impossible to see inside a battery and really know what's going on. You can only make a best guess based on external readings. It is also impossible to really fix one once something like this has happened.

And I'm sure I don't have to tell you that the potential solutions here are all expensive.

If it were me, I'd re-evaluate load needs and see about switching to parallel strings of L16's at about 640 Amp hours. The 60 Amps of current would be near 10%, even though they are also "tall case" units. But first I'd run those 4 Volt units 'til I couldn't get one more usable Amp hour out of them.

twistedtree · December 2010

Re: Charging large Surrette/Rolls batteries

This discussion about a MINIMUM bulk charge rate of C/20 versus C/10 is very interesting to me.

I always viewed the minimum bulk current requirement to be whatever it takes to reach the bulk/absorb voltage and complete an absorb cycle. That happens to be around C/20, but the ability to hit the voltage and hold it there is the objective. The only charge current limit that I've learned is the MAX charge rate which is typically C/5, and typically hard if not impossible to hit in an off-grid system. This is the fist time (with the Surrettes) that I've heard of C/10 (or more) as a MINIMUM bulk current for optimal charging.

It poses an interesting dilemma in sizing an array. If you apply the "standard" practice to size an array based on daily loads (200ah), de-rating for losses (.77), and using the ave hrs of sun for your location (4 hrs), you come up with a certain array size. In my case it's 3200W. If I take that same daily load and size the batteries to provide 3 days of power @ 50% DOD, I come up with a battery size. In my case it's 1200Ah. That sizing also places daily discharge at about 17% or so, and max discharge after 3 days at 50%, which is about right. When you then look at the array output compared to the battery size, the charge rate is C/20 (5% of C).

So to get C/10 as a charge rate, you need to either

a) Cut the batteries in half, resulting in 1.5 days of holdover instead of 3, and a daily discharge of about 35% which according to pretty much every battery manufacturer will significantly compromise lifespan.

b) Double the array size to get C/10 bulk rate, but otherwise throw away all that extra power since your batteries will still take the same number of kWh to recharge.

So what's a designer to do?

twistedtree · December 2010

Re: Charging large Surrette/Rolls batteries

niel wrote: »

as to the sg being lower i'll agree because some of the sulfer is stuck to the plates. i would not totally go by the sg reading as a direct sulfation indicator though becuase it won't indicate what is normally on the plates and what is actually stuck on the plates. remember that sulfation starts as soon as a battery is not 100% charged. it doesn't become a problem until charging fails to remove it from the plates.

Yes, I'm assuming the SG readings are at full charge, or as full as you can get it. At that point all the sulfer should be in the acid, and none on the plates.

niel · December 2010

Re: Charging large Surrette/Rolls batteries

twistedtree wrote: »

So to get C/10 as a charge rate, you need to either

a) Cut the batteries in half, resulting in 1.5 days of holdover instead of 3, and a daily discharge of about 35% which according to pretty much every battery manufacturer will significantly compromise lifespan.

b) Double the array size to get C/10 bulk rate, but otherwise throw away all that extra power since your batteries will still take the same number of kWh to recharge.

So what's a designer to do?

one possibility is to get agm batteries as they do accept 5% rates and are more efficient. 3 days worth of battery power is not so easy to come by without such a move or oversizing the array to properly charge the fla type batteries.

blackswan555 · December 2010

Re: Charging large Surrette/Rolls batteries

losses (.77),

Off grid, you need more like .50 >.55

Tim

BB. · December 2010

Re: Charging large Surrette/Rolls batteries

Pretty much Yep--Large bank means oversized array for many people... But it also depends on a person's location. For folks in the far north, coastal areas (lots of overcast/marine layer issues, etc.), then the panels do get oversized a bit anyway because of less available (more current into bank in a shorter amount of time).

-Bill

Cariboocoot · December 2010

Re: Charging large Surrette/Rolls batteries

Ah, well you see things change over time. Like recommendations for sizing arrays. Used to be the notion was you could recharge at any rate over any time (including several days) so long as the Amp hours "got put back". Subsequently it has been proven that recharging ASAP is better, hence the bigger arrays. Particularly if you take into account the notion of having more than one days' worth of capacity. That just means you need an even bigger array to recharge it in the small window of opportunity available (about 4 hours). If you had the same Amp hour capacity in a different battery design, the 10% rate would not be so critical.

If you need 200 Amp hours per day then you have to decide to what Depth Of Discharge you're "comfortable" with, knowing that deeper means shorter battery life over-all. With 50% maximum:

400 Amp hour 48 Volt battery bank @ 20 Amp (5% minimum) peak rate is 20 * 57 Volts (roughly) = 1140 Watts minus derating: 1480 Watts. At 10% it is of course double.

But different batteries have different preferred charge rates!

My personal reasoning is to try for 10% nominal, knowing that loads are going out too. But I also wouldn't size a battery bank for "three days no sun" which is a typical recommendation. The reason being that gets expensive in batteries and in enough PV to recharge them. Meanwhile, you have "over-capacity" on all those days when the sun does shine.

A good compromise is to try for 25% DOD, knowing you can take it down to 50% if needed, and then use the generator for "day three". On the whole, generators & fuel are a lot less expensive than batteries & PV.

Being able to leave things off and conserve power when the sun doesn't co-operate is essential. And of course reducing loads as much as possible to begin with (like eliminating any electric heating devices and paying close attention to motor use) is Step 1.

BTW, there is never a time when all the sulphur is off the plates. That little problem starts Day 1, and just gets worse over time. "Time" to a battery is not only a matter of chronology, but also of cycles and depth.

twistedtree · December 2010

Re: Charging large Surrette/Rolls batteries

Cariboocoot wrote: »

If you had the same Amp hour capacity in a different battery design, the 10% rate would not be so critical.

What would that "different battery design" be? More, smaller capacity cells? AGM? Something else? Trojan's owner manual show bulk current at 20% of C for their AGM and Gel cells, so that's going in the wrong direction.

BB. · December 2010

Re: Charging large Surrette/Rolls batteries

My guess, AGM's or less tall flooded cell would both work better with lesser maximum charging current that the tall cased batteries (from what I have read)...

-Bill

twistedtree · December 2010

Re: Charging large Surrette/Rolls batteries

That's kinda how I'm leaning next time around. Buy cheap batteries, then churn 'em and burn 'em. My last set of L16s were charged at around 2% of C and they worked great for about 7 years which is just what you'd expect.

niel · December 2010

Re: Charging large Surrette/Rolls batteries

twistedtree wrote: »

That's kinda how I'm leaning next time around. Buy cheap batteries, then churn 'em and burn 'em. My last set of L16s were charged at around 2% of C and they worked great for about 7 years which is just what you'd expect.

i suspect they would've lasted allot longer than 7 years if they were at a better charge rate. 2% is nearly a float charge and you must not have pushed them very much and that allowed them to eventually recharge. i have no doubt the cause of death in your case was sulfation. i wouldn't recommend a 2% rate even for an agm. by the way, trojan and surrette aren't the only battery manufacturers for high capacity. now i don't have a huge battery bank with 1 sunxtender, but i speak from my experience with concorde agms who do have high capacity agms and there are others that make heavy duty agms too.

Dave Angelini · December 2010

Re: Charging large Surrette/Rolls batteries

I am still puzzeled by your saying that one cell drops voltage very quickly when you load the bank. I say this because you have a great battery, and appear to be not far off of what i expect if you are completing charge near noon. Stay away from AGM's and solve this problem. What is Surrette telling you? It sounds like an intermittent. You can for test purposes run the voltage slightly lower and not use the "bad battery".

Charging large Surrette/Rolls batteries

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