Daily charging parameters

Hi all,
I've recently commissioned our new solar system (details in sig) and I'm a bit unsure about the results I'm getting from daily charging. When I first turned everything on a couple of weeks ago, I gave the bank a really good boil, and got all my SGs to between 1270 and 1280 - so I'm happy that the batteries started in a reasonable state.
Since then, I've been trying to tweak the settings to get things running reasonably smoothly, and I'm finding that I'm not getting my SGs high enough with any reasonable "return amps" value. Today I even tried putting the value right down to 1% of the C20 capacity - i.e. 4.7A, and my charge still only finished at an SG of 1250-60.
Up until today I've been running with return amps set to more like 2% - i.e. about 9A, and my "charged" SGs appear to have drifted down to 1240-50 across the bank, which seems on the low side.
My charging voltages are all configured as specified by Rolls - i.e. a 60V absorb (which generally translates to about 59.6 at the moment after temperature compensation)
Am I being unrealistic in my expectations? What sort of SG should I be aiming for from a daily charge? I don't want to shorten the life of the batteries by routinely overcharging them, but I also don't want to end up sulphating them by undercharging them either. What reasons might there be for the battery current dropping off before they are charged? I thought that was generally a sign of sulphation, but I'm not quite sure how these batteries could have ended up particularly sulphated given where they started from a couple of weeks ago and the level of use/charging they've had. At worst they really only should have been a tiny bit undercharged - but perhaps I'm missing something here. Should I be thinking about increasing my absorb voltage?
Cheers,
I've recently commissioned our new solar system (details in sig) and I'm a bit unsure about the results I'm getting from daily charging. When I first turned everything on a couple of weeks ago, I gave the bank a really good boil, and got all my SGs to between 1270 and 1280 - so I'm happy that the batteries started in a reasonable state.
Since then, I've been trying to tweak the settings to get things running reasonably smoothly, and I'm finding that I'm not getting my SGs high enough with any reasonable "return amps" value. Today I even tried putting the value right down to 1% of the C20 capacity - i.e. 4.7A, and my charge still only finished at an SG of 1250-60.
Up until today I've been running with return amps set to more like 2% - i.e. about 9A, and my "charged" SGs appear to have drifted down to 1240-50 across the bank, which seems on the low side.
My charging voltages are all configured as specified by Rolls - i.e. a 60V absorb (which generally translates to about 59.6 at the moment after temperature compensation)
Am I being unrealistic in my expectations? What sort of SG should I be aiming for from a daily charge? I don't want to shorten the life of the batteries by routinely overcharging them, but I also don't want to end up sulphating them by undercharging them either. What reasons might there be for the battery current dropping off before they are charged? I thought that was generally a sign of sulphation, but I'm not quite sure how these batteries could have ended up particularly sulphated given where they started from a couple of weeks ago and the level of use/charging they've had. At worst they really only should have been a tiny bit undercharged - but perhaps I'm missing something here. Should I be thinking about increasing my absorb voltage?
Cheers,
Outback VFX3048E, Outback FM80, Outback FNDC, Outback Mate3, Outback Hub4, 15x Munchen 250W panels (5x3), 8 x Rolls S605 (48V system)
0
Comments
Next--You really only need to be >90% state of charge once or twice a week. And avoid going below 50% state of charge very often.
Do not try to hit 100% SOC every day (or even several times a week). That is not needed and can actually be a bit hard on the battery bank (you are, more or less, starting to equalize the battery bank--Which erodes plates, oxidizes positive plates/grid, uses water, and heats batteries up).
For equalization, once per month is usually often enough... Some battery vendors suggest equalization only if 0.015 to 0.030 difference between high and low SG cells in your bank. Getting a little bubbling/equalization helps for "tall cased" batteries to mix electrolyte (high SG electrolyte tends to sink, low tends to rise--Stratification layers are "mixed" by gassing during final charging/equalization).
There is an alternative method of running a battery bank... Some folks will run 50% to 80% state of charge and reach >90% State of Charge once per week. The batteries will not sulfate if they are being actively cycled (charged/discharged) even if below ~75% State of Charge.
-Bill
Thanks for the response. In ideal world, I was hoping to get the system running without me needing to intervene beyond a monthly checkup/maintenance session, where I review the charging data, record SGs, run an EQ if necessary. As such, I don't really want to be manually fiddling with settings each week to adjust how the batteries are charging.
At the moment, our house is only partially set up and we're waiting on permissions to go further. As a result, we're using a lot less power than the system is designed for. It being summer, we're going into float by mid morning most days. I guess the only way I can achieve what you're suggesting is to manually change either the end amps or the absorb timer once a week - which seems to slightly defeat the point of having spent $$$ on all the fancy outback monitoring gear, which I'd hoped would help me automate the system more...
Rolls serie 4000 should be 1.265SG when at 100% SOC, 1.260 with 1% end amp is not that bad.
End current can be as low as 0.5% (and below) on new Rolls batteries, set your absorb time to 3-4hrs and check your end current before it goes to float. I would set them to 59.2V for 3hrs as you're a bit overpaneled with 70/75A to the batteries.
EQ every 2-3 monthes and you should be ok. Don't know much about the FNDC, I'm on Midnite.
A+,
Erik
Do not fret that you are not reaching >99% SoC daily/weekly. Just a once a month full charge + equalize is usually enough.
Monitoring one cell SG daily until you understand that all is working well is usually a good double check on your battery bank.
Taking a battery bank--accidentally--And under charging, is probably the number one cause of early battery bank death... But over charging is probably the number two cause for early death.
You may want to try a voltage based monitor too--Just a belt and suspender solution (easy for everyone to read, and can give you information if other parts of your system are not working/programmed correctly):
MidNite Solar MNBCM Battery Capacity Meter
http://smartgauge.co.uk/smartgauge.html
Note: I have not used either of the above meters (I am grid tied), and have only read here, a few threads, about the Midnite meter being used on the forums (and happy with the results). Just some links to look at. The Midnite is not much more than a DMM.
Be careful of "full automation"... The more automation, the more wild and wonderful ways things can fail. I am not knocking the Outback FNDC--I just do not know anything about it.
Shunt based systems have the capabilities of being more accurate and giving you more data (actual current/power/energy usage/recharging)--However, they can also "drift" in normal use (usually, they "reset" to 100% full when the battery voltage is held to Absorb for 2+ hours or so). Voltage based systems are less accurate--But can let you know if basic stuff is not happening (for example, the Midnite Meter will warn you if the battery has not been fully charged in the last 7/14 days).
Do not trust one piece of your kit's battery monitoring with off grid solar... Reading S.G. is the "gold standard" with flooded cell batteries--But even hydrometers stick/crack/fail (and nobody likes reading/logging 24+ cells on a daily cycle).
-Bill
... DANG, just lost another Post, by erroneously trying to add a Link by Right-Clicking, instead of Ctrl Ving ...
A number of FNDC users are disappointed that it ends Absorb too soon, in error ... read this Topic on the OB Forum:
http://www.outbackpower.com/forum/viewforum.php?f=28
FWIW, Vic
You have 3750 watts of array. With a 0.77 derate you should expect to see 2887 watts.
Repeated shallow discharges are actually bad for the battery... lead dioxide clumps form on the positive plates. Maybe you should only charge the battery every other day.
--vtMaps
we are in a similar situation of construction plus being currently over-pamslle, and at 1680W STC.and a 24v 900Ah AGM battery, with the fridge and \net connection running 24/7 March to Nov are only using 1.8Kwh / ~10% a day and power tools only add another 500-1000Whr of consumption
so I redirect 1/3 of my panels to a standby (redundant) battery to keep it happy... More PV to put up too...
hth...
KID #51B 4s 140W to 24V 900Ah C&D AGM
CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM
Cotek ST1500W 24V Inverter,OmniCharge 3024,
2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge, Hughes1100 Sat Modem
Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
West Chilcotin, BC, Canada
-Bill
If it ever happens I can always spy on things using OpticsRE anyway/write my own remote monitor and stick it on a Raspberry PI.
Does anyone have any insight into how deep a daily discharge is enough (on average) to keep the batteries healthy? If I get it down to 85% SOC most days, and then maybe once every few weeks I turn the panels off and let it drop to 50%, is that good enough?
westbranch said: When you say "redirect", how are you achieving that? Do you have another charge controller?
I do have each string of the array on a separate breaker, so I guess I could just turn off 1/3rd of the array, but I'm not sure how helpful that would really be. The problem is not really that we're over-panelled, but that the whole system is too big. If I turn off part of the array I'm not really addressing the problem that the batteries aren't getting cycled deeply enough by our usage, I'm just slowing the recharge rate.
Probably the best thing is for me to just flip the main panel breaker every couple of days to take the batteries a bit lower... or to get on with buying some electric water heating to dump more of that free energy somewhere useful!
Cheers,
Yeah. And the 0.77 derate is obviously just an average for use in specifying a system. At the moment I'm hunting around for explanations of where *specifically* I'm losing all of that extra right now. I can account for some of it with temperature (although lacking an IR thermometer I'm not sure exactly how much I should really expect to be losing there), panel tilt (I'm at 40 degrees and solar normal is about 13 here at this time of year) and a bit of cable loss (~1.5%), but unless my cell temperatures are ~75C (nominal is 47C at 20C ambient with a light breeze) there's something else going on... but that's another question that I'm working on separately!
As Westbranch said, new batteries have a break-in period and you need to be patient for the first monthes of use. I would be curious to know where your real end current is. A common mistake is to set your controller to 1% EA with new batteries, if you take the time to check your end current, you will note that it could be far below when new. My Rolls bank is at 0.3% right now and is 6 monthes old (my old interstate bank was around 1% before it began to die). To me, end current is still the best way to avoid overcharging with low/daily discharges (90% SOC) in summer but it needs to be really accurate and I'm in doubt that the Outback can do it well. 60V is in the high range and you can have the same result with lower charge voltages and longer absorb time which would be better for your batteries in time. Anyway they're yours.
3) I'm in Canada and this 0.77 rule does apply to me only 2 monthes a year, I can see frequently 3kw output from my old 3.2kw array (roof mounted) in summer and more than 3.2kw for the rest of the year. It's all depend on your location and system design. One of my clients is doing 3.5kw regularly with his 3kw pole mounted array.
A+
Erik
--vtMaps
- 0.81 * 0.95 = 0.77 overall "average" maximum power
In cold weather, you can see more. At higher elevations you can get more power too (less haze). If you have below freezing daytime temperatures during winter, better output too. But, for a quick set of calculations, we use this numbers to get a system that will (for most people) perform to predicted output over most of the year.-Bill
I'm not sure why the Outback system would be particularly inaccurate in this regard? It's measuring using two shunts (one on the inverter and one on the CC) and a FlexNet DC (dedicated battery monitor); as far as I am aware, the resolution of the voltage measurement is pretty high. There are various known problems with the FNDC but as far as I am aware the accuracy of its current measurements isn't one of them.
I chose 60V because that's what Rolls specify for off-grid systems; are you suggesting a lower voltage because I'm not cycling them very deeply at the moment?
I'm losing about a volt on the round trip from the panels to the CC which puts the panels at 71V, but that's still 22V lower than the ideal. Accounting for that with temperature alone would put the cells at nearly 90C which seems unlikely. Are there any other factors that would reduce Vmpp? I had a cursory poke around with a voltmeter the other day and couldn't see any evidence of any obvious losses across connections/cabling within the array (it's all put together properly with MC4s), but this is my main target for further investigation at the moment.
Interestingly, Voc is 97.2 vs a nominal of 111. The thermal coefficient of Voc is 0.33% / degree, which would put the cells at 62.5C, which is a bit high but a lot more believable then 90C. In the absence of any other possible reasons (what other reasons are there for Vmpp to move dramatically other than temperature?), it *sounds* like I'm looking for a stray resistance somewhere - e.g. bad connection. But so far I haven't found anything untoward.
I think that a 3hrs/59.2V absorb time should be enough to get your SG to 1.265 (you don't want this SG to go higher for longevity), no matter how deep they are discharged.
I would keep this 60V for really hard cycled systems and/or for winter when you'll need to charge as much as you can in short days.
A+
Erik
In practice, when charging a lead acid battery bank and gassing--Oxygen gas is produced by electrolysis on the positive plates--Causing oxidation of the plates and support grid. In addition, gassing can cause plate erosion too. Lastly, heat is produced during electrolysis--A good rule of thumb is for every 10C (18F) above room temperature, the battery will age 2x faster. Even a 2% rate of charge (basically equalization voltage) can cause the battery bank to run hot--So, overall, I would humbly suggest that folks avoid trying to reach >>90% state of charge as a matter.
What ratings of the Vmp/Imp ratings of the panels and what is the series/parallel configuration of the array?
It sort of sounds like you have a bad panel??? The array voltages do not make sense. It is possible that the MPPT controller is miss-programed or failed--But I don't think that is the problem.
-Bill
I also have a marine battery selection switch and some other rewiring that allows me to move it to either battery in less than a minute
KID #51B 4s 140W to 24V 900Ah C&D AGM
CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM
Cotek ST1500W 24V Inverter,OmniCharge 3024,
2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge, Hughes1100 Sat Modem
Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
West Chilcotin, BC, Canada
And with the 250 Watt panels (your link):
Use your current clamp meter to measure per string current for each 3x panel string... Nice thing about having three identical strings of panels, it usually is very easy to see if one string is under performing the other two (You can also do a similar test, if you have 3x circuit breakers (one per string), just turn on one string at a time and see what the Vmp-array and Imp-array readings are).
-Bill
Herodotus,
You mentioned in the first Post, " ... My charging voltages are all configured as specified by Rolls - i.e. a 60V absorb (which generally translates to about 59.6 at the moment after temperature compensation) ... "
Unless Surrette just changed the recommended Absorb voltage (Vabs), it should be 58.8 (2.45 VPC).
It is good that you know that the FNDC is not causing the issue, as many seem disappointed with the FNDC, generally having lower than expected SGs
The Surrette banks here are 11 years, or so old. Had previously been running at about 0.93 % of actual 20-hour Capacity, with Vabs at about 57.5-ish volts. In the past nine months or so, have needed to increase the Vabs to about 58.4 V on one bank, and, now up to 58.8 V on the other largish bank. EA is now up to 1.4% of C. (am using a 5% reduced C, as C is speced for 1.280 SG, not the actual SG of 1.265).
So, you have actually watched or looked at log charges to see that you are actually getting down to the EA that you have set in the system??
The Classic CC does have in the latest FW versions, the ability to Skip Charge days, where the number of days to be skipped is entered by the user. On Skip days, the CC begins in FloatMPPT, and stays in Float all day, until the day after the Skip number has been satisfied. We Skip three days on both of the larger systems here, and recharge on the fourth day ... FWIW. Vic
Rolls has changed its absorb voltage for 60V at 25°C, not a small step!
Maybe, too much warranty issues at lower voltages.
http://support.rollsbattery.com/support/solutions/articles/428-state-of-charge-charging-flooded-lead-acid-batteries
A+
Erik
Herodotus, I may have missed it in this thread, but just how discharged is your battery at solar noon when you find lower than expected voltage from your array?
--vtMaps
So I've tested this two different ways - a couple of times just running a bunch of appliances while in float to see how much I can pull in from the array, and on another occasion turning off the array for a day or so to get the bank down to a level where it needed a decent bulk charge (I'm afraid I'm not sure what the exact SOC % was) and monitoring then. Obviously the voltage I'm talking about is the input voltage to the CC rather than the battery voltage...
Either way, the results are the same - I can't seem to pull more than about 2.5k max from the panels.
Cheers,
Hmm, that's interesting. I didn't think of discounting the value of C in my calculations according to the target SG. That makes sense.
Yeah, I've watched it get down to the return amps value that I've configured for the FNDC (CC End Amps is set to 0 to avoid conflicts) and then terminate the charge. In fact, I've fiddled with the value and bumped it back into absorb and then watched it charge a bit more until it gets down to the new value. All seems to be working fine
Interesting, I don't think that feature exists on the Outback FM80, sadly. That would be super useful. I wonder if it's possible for me to program that behaviour from a remote PC (I know the OpticsRE platform can control various behaviours of the system, so perhaps it's possible...)
And, hero..,
Good that EA is actually working on your system, when you want it to ... perhaps OB has actually fixed a few of those annoying FNDC bugs, let's hope.
And am sorry that I misstated the Surrette recommendations for Vabs ... we do not do real PSOC, but sure have never needed anything close to 60 V Absorb.
And also agree with Erik (believe it was), that 30 - 50, or even more cycles are needed for these batteries to build full Capacity and stabilize. These cycles probably need to be down to about 75% SOC, or even a bit lower (as measured with your Hydrometer).
Forget, you DO have a Hydrometer, or Refractometer don't you ??
Have Fun with the new system, Vic
Yeah, I have a hydrovolt hydrometer that I have been using to measure SGs. I did the whole bank a couple of times at the start and now I'm testing a couple of test cells each time. I've got my panels switched off for the moment so I can get a good deep cycle on the batteries. The FNDC battery monitor will probably wind up reading about 85% SOC, but I suspect the SGs will say otherwise - presumably because my batteries are actually lower capacity right now than they will be once they are "broken in".
Cheers,,