Batteries never charge 100%

experimenter
experimenter Registered Users Posts: 20 ✭✭
It is a problem if my batteries never charge 100%? With my small (9 panel) system, the battery usage ranges from 96% charged to 65% charged. They get to the low end of that range overnight, then the next day with the sun begin recharging but never reach 100% before the day ends.

I don't think I can add more panels to fix this, because the problem seems to be that the charge controller tapers off the charge as the batteries get into the 90%+ range. In other words, I still have plenty of voltage being generated by the panels, but the charge controller restricts this to a trickle charge towards the end of the recharge cycle and then the sun sets before reaching 100%.

So, just wondering if this erodes the life of my batteries. The batteries are 12V AGM Concorde 104AH batteries.
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Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    What is the Absorb voltage you measure at the battery with a good DMM?

    Is the voltage being held at 14.40 volts (or whatever), or is the voltage dropping too?

    Normally, I would expect the charge controller to hold 14.40 volts for at least two hours once the absorb voltage is reached (from bulk to absorb).

    AGM's are very sensitive to overcharging.... So, I am not sure I would panic at this point. Reaching >90% every day (or even just a few times a week) is usually "good enough" for flooded cell batteries.

    And how are you measuring the xx% SOC? Batteries rest for a few hours and you measure the resting voltage? Or are you using a battery monitor or something else?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • experimenter
    experimenter Registered Users Posts: 20 ✭✭
    I am relying on my Bogart Battery Monitor to read the charge level and voltages. Yes, the voltage stays high, and in the early morning hours the controller is feeding enough charging current ot the batteries so that the charge level rises by a percentage point or two every half hour or so. But at some point around 92-93% charged the controller tapers off the charge current. Even when the controller shows the panels are generating 18 amps the charge into the batteries is only 1-2 amps or less (with no other load on the system). I end up almost every day with about 96% charge as displayed by the Bogart meter.

    I can also reconnect the grid power to the system and then will reach 100% charge on the batteries, but I don't want to do that as the whole purpose of the system was to be standalone. As long as never reaching 100% charge is not a problem for battery life I am fine with 96% maximum charge.
  • Vic
    Vic Solar Expert Posts: 3,208 ✭✭✭✭
    Hi experimenter,

    First, it is usually NOT the Charge Controller (CC) that is limiting charge current during Absorption, it is the battery that is doing this, as it can only Accept a certain amount of current at any given voltage, and this Acceptance tapers.

    Also, battery Monitors can mislead, as they can only be as accurate as are the settings the user enters into the Monitor..

    Hope that you are using Remote Temperature Sensors on both the Bogart, and all charge sources that you use. AGMs are very sensitive, as Bill mentioned, and accurate charge voltage is very important.

    FWIW, Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    I have not read the details about programming/using the Bogart Monitor... Battery Monitors measure the Amp*Hours into/out of the battery and can drift over time. Usually there is some sort of settings that you can adjust to make the meters more accurate:

    First is some sort of charging efficiency setting. Flooded Lead Acid batteries are (in general) less efficient than AGM batteries for charging. Something like 80-90% efficient for flooded cell and ~90-98% efficient for AGM.

    Both type of batteries, the efficiency drops as the battery reach near 100% SOC. Gassing is "zero" percent efficient charging. Flooded Cell, you can gas them some and simply add distilled water every month or 3.

    For AGM, they can take a little bit of gassing (catalyst will recombine hydrogen and oxygen back to water)... But the catalyst can over heat (and it does wear out over time--the more gassing the faster it wears out). And when the catalyst is gone and/or the battery vents (from over charging or catalyst gone), the AGMs are not long for this world.

    Second setting is something like when the battery "sees" >14.2 volts for 2+ hours, the meter resets back to 100% SOC. Reaching this reset point at least once per week is not only good for the battery, it resets/recalibrates the Battery Monitor to be "in sync" with the batteries "true state of charge".

    Those are some of the questions I would have.

    Remember, if the charge controller is holding 14.4 volts (~75F battery temperature), the current will slowly taper down as the battery reaches near 100% SOC. You can watch the charging current (tail of the absorb charging curve)--You should see the current slowly flatten out. When the absorb current is "steady state", then the battery is 100% full. The steady state current may be in the 0.1% to 1.0% battery AH capacity range (i.e., a 200 AH battery will see 0.2 to 2 amp steady state charging current). A good AGM should be way smaller than 1% steady state current (probably 0.1% or less, but I am guessing).

    If/When you see a battery reach ~2% steady state current (battery near end of life)--That is a sign that the batteries need replacing soon. Pumping too much steady state current into the battery will "boil" (gas) it dry and cause meltdown/fire.

    Note that battery temprature is critical too... When AGMs exceed something like 40C or greater, they can thermally run away--The charging current does not drop but may rise, the charging voltage may drop 9hot batteries need lower Absorb voltage set point)--"Confusing the charge controller" into thinking the battery need more charging current/voltage.

    Do you have a remote battery temperature sensor for your charge controller? Does your controller have a RBTS option?

    AGM batteries are a tough customer to understand/monitor. For lead acid chemistry, the specific gravity of the electrolyte is the "gold standard" for understanding your battery. With AGM's, being sealed, you cannot measure the SG--So you are left with looking for secondary signs (resting voltage, looking for swelling battery case, excessive battery temperature, battery hissing/venting, etc.).

    Battery Monitors are almost a requirement for keeping track of the SOC for AGM batteries. However--They can/do drift if not setup correctly, or if your charge controller is not reaching/holding the proper absorb voltage/time.

    Hopefully, some others with more Bogart/AGM experience can chime in here too.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • experimenter
    experimenter Registered Users Posts: 20 ✭✭
    My Bogart and charge controller both have temperature probes connected to the battery. I can't recall changing any settings (or even seeing any setting to change) to calibrate the temperature probe, so I just hope it is doing its job correctly. Also both probes are connected to the same battery in the set (if that makes any difference).
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Sounds good. The lead acid default setting for the probes should be fine.

    The other charger and monitor settings, you probably need to study in detail.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • zoneblue
    zoneblue Solar Expert Posts: 1,220 ✭✭✭✭
    You MUST fully charge lead acid banks regularly. Not doing this will accumulate hard lead sulfate, which leads to capacity loss. However just because a battery monitor says X perent doesnt mean that it is X percent. For now, ignore everything it says.

    The procedure that most people use for VRLA batteries is to first try to find the best absorb voltage setpoint you can. If its a reputable battery use their data. If not youll have do some research to figure it out. Second factor is absorb time. This relates to a number of factors, cheif among them, is the charge acceptance decay throughout absorb. Ideally you want a charge controller that can terminate absord based on end amps. Failing that you have to make an assessment based on your SOC use profile, and teh battery technology. Take the battery at a "typical" discharge SOC, and watch it as it charges during late absorb when the battery is no longer drawing much current, when that current drops below 0.5% of the rated capacity of the bank (ie 1A per 200Ah), thats when the battery is considered 100% charged. THEN look at the battery monitor, what does it say. If less than 100% adjust its settings to increase its columbic efficiency etc. IF more reduce them. This will line up the battery with the monitor.

    This all assumes you have a charge controller that has adjustable parameters. If your doesnt, if you value your batterys, get one that does.

    And... just remember the monitor is not the battery. The important thing is that the charge controller completes the charge cycle most days.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


  • lkruper
    lkruper Solar Expert Posts: 115 ✭✭
    It is a problem if my batteries never charge 100%? With my small (9 panel) system, the battery usage ranges from 96% charged to 65% charged. They get to the low end of that range overnight, then the next day with the sun begin recharging but never reach 100% before the day ends.

    I don't think I can add more panels to fix this, because the problem seems to be that the charge controller tapers off the charge as the batteries get into the 90%+ range. In other words, I still have plenty of voltage being generated by the panels, but the charge controller restricts this to a trickle charge towards the end of the recharge cycle and then the sun sets before reaching 100%.

    So, just wondering if this erodes the life of my batteries. The batteries are 12V AGM Concorde 104AH batteries.

    What is the charge rate for your batteries? The Concorde manual says that these batteries should be charged at least 0.2C and up to 5C.

    http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&sqi=2&ved=0CB4QFjAA&url=http%3A%2F%2Fwww.lifelinebatteries.com%2Fmanual.pdf&ei=SjaGVdigGYqbyQS5lYSQCA&usg=AFQjCNFDlo0yvIgMNaddQr4ehNJbn_n3Zw&sig2=pW38q1oTR5RjbPcwpjL--w&bvm=bv.96339352,d.aWw
  • zoneblue
    zoneblue Solar Expert Posts: 1,220 ✭✭✭✭
    Some people do feel that AGMs benefit from the stimulation of a decent charge rate ie 0.2C It helps stir things up, preventing any stratification type issues. However while 5C may techically be possible, it will reduce charge efficiency and will not ensure long cell life, Worse still that even the smallest overcharge at that rate and the cells will explode. So its certainly not recomended. Thats just AGM makers trying not to feel so irrelevent in the face of looming competion from lithium.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Those particular Concorde batteries are far from typical AGM too.
    5C requires accurate temperature and voltage monitoring, and is more appropriate to generator based charging than to PV based charging.
    (minimize run time while using close to full potential generator output, especially for a diesel prime mover.)
    SMA SB 3000, old BP panels.
  • lkruper
    lkruper Solar Expert Posts: 115 ✭✭
    inetdog wrote: »
    Those particular Concorde batteries are far from typical AGM too.
    5C requires accurate temperature and voltage monitoring, and is more appropriate to generator based charging than to PV based charging.
    (minimize run time while using close to full potential generator output, especially for a diesel prime mover.)

    That's good to know. Perhaps going to 4C at the upper limit would give some buffer for temperature? The reason I asked about his charge rate is that if he does not have enough hours of full sun to finish charging by solar, upping the rate within the limits of the manufacturers specs would get the job done quicker.
  • lkruper
    lkruper Solar Expert Posts: 115 ✭✭
    zoneblue wrote: »
    Some people do feel that AGMs benefit from the stimulation of a decent charge rate ie 0.2C It helps stir things up, preventing any stratification type issues. However while 5C may techically be possible, it will reduce charge efficiency and will not ensure long cell life, Worse still that even the smallest overcharge at that rate and the cells will explode. So its certainly not recomended. Thats just AGM makers trying not to feel so irrelevent in the face of looming competion from lithium.

    What do you mean, reduce charge efficiency? That it will take more electrons to fully charge to 100% at the higher rate? Also, how would one overcharge the battery if they are using a 3-stage smart charger as the manufacturer recommends?

  • zoneblue
    zoneblue Solar Expert Posts: 1,220 ✭✭✭✭
    Lots of amps generates lots of heat, and that heat is energy lost, and exacerbates (most of) the secondary chemical reactions in the cell, that in turn reduce cell life. Overcharging a battery isnt that hard. Wrong settings, glitch in unit, failed unit. Um, unlisted cheap / nasty unit... Plenty of examples to be seen here.
    lkruper wrote: »
    he does not have enough hours of full sun to finish charging by solar, upping the rate within the limits of the manufacturers specs would get the job done quicker.

    Well thats odd, because if you have the current to fast charge the bank, you have by definition the pv to charge it at the generally recomended 0.1C, which for teh genreally recomended 20%DOD will by definition take about 1/0.1 * 0.2 = 2 hours. Plus a bit for charge efficiency, plus a bit more for absorb. Thats the neat thing about solar and FLA combo, the charge rate works about right for daylights hours.

    If theres no solar, then yes a genset changes things. In that case you do want to run it hard and fast to reduce fuel consumption and noise. Hey go get yourself some LiFePo4s and call it good. You can charge the snot out of those things.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


  • lkruper
    lkruper Solar Expert Posts: 115 ✭✭
    zoneblue wrote: »
    Lots of amps generates lots of heat, and that heat is energy lost, and exacerbates (most of) the secondary chemical reactions in the cell, that in turn reduce cell life. Overcharging a battery isnt that hard. Wrong settings, glitch in unit, failed unit. Um, unlisted cheap / nasty unit... Plenty of examples to be seen here.

    lkruper:
    Interesting. The same amount of amps would be used to charge the battery at 0.2C and 5C. If this is similar to a resistive heater, the total amount of heat would be the same. What I do see is that under certain circumstances the burst of heat could exceed the limits Concorde sets for their batteries. I also know that heat ages batteries and at higher temperatures the battery ages faster. However the manufacturer does warranty their batteries for the same length of time regardless of whether or not the battery is charged at 0.2C or 5C. In reality, in the scenarios I have considered, most of the time no one would likely have a charger that allows for 5C on a good sized Concorde battery. What do you consider the sweet spot for charging between the range Concorde sets ... 0.2C to 5C?

    Edit: Forgot to ask about the overcharging with a three stage charger. I thought the context was accidently overcharging because of charging at 5C. It does not seem that any of those ways a battery could be over-charged are more or less likely to happen at 5C or 0.1C. If the controller fails or one makes a mistake has no bearing on the charge rate one wants to use, does it?


    Well thats odd, because if you have the current to fast charge the bank, you have by definition the pv to charge it at the generally recomended 0.1C, which for teh genreally recomended 20%DOD will by definition take about 1/0.1 * 0.2 = 2 hours. Plus a bit for charge efficiency, plus a bit more for absorb. Thats the neat thing about solar and FLA combo, the charge rate works about right for daylights hours.

    lkruper:
    I should have read this first, but I am too lazy to go back and edit my first comment. Evidently you disagree with Concorde when they say that one should not charge their batteries below 0.2C. However there is a caveat... that this is especially a problem if the batteries are discharged deeply.

    The reason my question was not odd (to me!) is that I did not see anything in the post which informed me that the poster had sufficient amps to charge the batteries to completion while the sun was providing sufficient amps.

    If theres no solar, then yes a genset changes things. In that case you do want to run it hard and fast to reduce fuel consumption and noise. Hey go get yourself some LiFePo4s and call it good. You can charge the snot out of those things.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    With AGMs, you have to be very careful of thermal runaway... They get hot, the charging voltage falls, the charge controller thinks more current is needed... Catalyst has lots of Hydrogen+Oxygen to recombine--Batteries get hotter, cells vent, and more heat.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • zoneblue
    zoneblue Solar Expert Posts: 1,220 ✭✭✭✭
    lkruper wrote: »
    I thought the context was accidently overcharging because of charging at 5C. It does not seem that any of those ways a battery could be over-charged are more or less likely to happen at 5C or 0.1C. If the controller fails or one makes a mistake has no bearing on the charge rate one wants to use, does it?

    What i was saying is that the consequences of overcharging (whatever the cause) increase dramatically with charge rate. AGM batteries becasue of their oxygen recomnination cycle go into thermal runway much more readily than FLA. Although a quality catylyst does help with this, by enhacing the recombination, ironically the catalyst itself contributes to battery ruptures due the fact that the more oxygen and hyrdrogen in the upper battery area (such as would occur in overcharge conditions, the more active the catylyst becomes, teh hotter the catylyst gets. Those things have been known to reach 300*C, which is hot enough to melt the battery container obviously. To try to mitigate this, quality cells coat the catylyst with ceramic.

    Hey, if you want to charge your lead acid batteries at 5C go ahead, and report back in a couple years. See how it goes! But in general folk are better off sticking to the 0.1C-0.2C range to minimize heating. Remember, power is proportional to current squared.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


  • lkruper
    lkruper Solar Expert Posts: 115 ✭✭
    zoneblue wrote: »

    What i was saying is that the consequences of overcharging (whatever the cause) increase dramatically with charge rate. AGM batteries becasue of their oxygen recomnination cycle go into thermal runway much more readily than FLA. Although a quality catylyst does help with this, by enhacing the recombination, ironically the catalyst itself contributes to battery ruptures due the fact that the more oxygen and hyrdrogen in the upper battery area (such as would occur in overcharge conditions, the more active the catylyst becomes, teh hotter the catylyst gets. Those things have been known to reach 300*C, which is hot enough to melt the battery container obviously. To try to mitigate this, quality cells coat the catylyst with ceramic.

    Hey, if you want to charge your lead acid batteries at 5C go ahead, and report back in a couple years. See how it goes! But in general folk are better off sticking to the 0.1C-0.2C range to minimize heating. Remember, power is proportional to current squared.

    Thanks for the explanation. I don't have any batteries yet, but by my nature I tend to immerse myself into a subject before I do anything. My current project is for my cabin. I have excellent grid power and have not had major outages in 5 years, but I think El Nino this year may cause this to be a very high precipitation year. So if I get caught up here when that happens I may be stuck with no way out and no power. So my plan is this:

    Phase 1
    ========
    1) Add a manual transfer switch (coming in 10 days, getting quotes for install now)
    2) Keep a portable generator on hand (in place and tested at this elevation)
    3) Add UPS for DSL modem and entertainment center/PC - One UPS in place, one more to go.

    Phase 2
    ======
    4) Add FLA batteries in basement, charger and inverter - Keep charged with grid. Plug into transfer switch
    if I get tired of listening to the generator. Charge batteries with generator if power is out.

    Phase 3
    ======
    5) Add solar panel with charge controller - If an emergency was extended, my 5-10 gallons of gas
    would last only 5-10 days. Solar would extend power indefinitely.


    I think you said that Sam's Club batteries were well regarded here. If I can get them in California they
    would be my training batteries. Later on I might consider real RE FLA batteries.

    I am also interested in a conversion van. That is farther off. If that happens I may be interested in AGMs.






  • zoneblue
    zoneblue Solar Expert Posts: 1,220 ✭✭✭✭
    Well also remember its about the system design. A 0.1C charge rate matches available sun hours, and thats a major part of off grid specification.

    By well regarded i mean that GC2s have a low cost of entry which makes them good for learner banks. However even given their relatively short life people here have shown pretty reasonable cost per Wh delivered figures for them. They do tend to become a hassle when the number of strings exceeds 1 or 2.

    After that, well lithium in its various forms are gaining ground rapidly, and many of us have already demonstrated positive cost beneit models using them in RE, depsite their higher up front cost.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    lkruper wrote: »

    lkruper:
    Interesting. The same amount of amps would be used to charge the battery at 0.2C and 5C. If this is similar to a resistive heater, the total amount of heat would be the same.
    No. The same number of Amp-Hours would be required, but the total time it takes would be different. And charging at a higher rate will
    1. dissipate more power across the internal series resistance, increasing the battery temperature more over a short time, and
    2. the excess voltage over that required to regenerate the plate materials (which does NOT produce heat) can cause gassing. The rate of gassing will affect the rate of recombination, which will also produce heat as well as straining the capability of the catalyst and raising the pressure, increasing the chances of venting.

    If you run a 1000W heater at 1000W for one hour, you will get the same amount of heat energy as if you ran it at reduced voltage to produce 100W and let it run for 10 hours. But at 1000W the element will get a LOT hotter.
    SMA SB 3000, old BP panels.
  • lkruper
    lkruper Solar Expert Posts: 115 ✭✭
    inetdog wrote: »
    No. The same number of Amp-Hours would be required, but the total time it takes would be different. And charging at a higher rate will
    1. dissipate more power across the internal series resistance, increasing the battery temperature more over a short time, and
    2. the excess voltage over that required to regenerate the plate materials (which does NOT produce heat) can cause gassing. The rate of gassing will affect the rate of recombination, which will also produce heat as well as straining the capability of the catalyst and raising the pressure, increasing the chances of venting.

    If you run a 1000W heater at 1000W for one hour, you will get the same amount of heat energy as if you ran it at reduced voltage to produce 100W and let it run for 10 hours. But at 1000W the element will get a LOT hotter.

    Thanks for the correction re amps and amp hours. Also, I had considered that the heat would be higher, but to what effect?

    For example, the charts show the effect on the battery at storage temps. The battery lasts longer at 77 than it does at 82. But what if the battery gets to 82 for 1 hour per day? Surely the effect gets averaged in, doesn't it? Concorde also lists the temperature range it wants to see during charging. So if that range is not exceeded, then would there be any risk?

    I don't have any plans to get AGM batteries at this time, however once I do my analysis of the spot I think I could put a solar panel I am afraid I might end up with a few hours of intense sun bracketed by shade in the early morning and an early "sunset" as the sun dips below the canyon. If that happens, then I would need to bulk charge as fast as possible and get the batteries topped off in 3 hours or so (est).

    I would not consider anything but FLA batteries at this fixed location unless I could not get the job done with them. Does this risk-based analysis make sense, or is there a batter way to solve this sort of problem?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    A very handy engineering rule of thumb is that the life (of almost anything from electronics to batteries to food in tin cans) is for every 10C (18F) increase in temperature there is a 1/2 reduction in life. 20C higher is 1/2 * 1/2 = 1/4 the life.

    Also, thermal cycling is the bane of every engineering design. I used to test disk drives (decades ago) and would cycle them from 5C to 45C two cycles per day (all within specifications). The "bad designs" would fail inside of 2 weeks, the "good designs" would not fail in months.

    You are correct that 1 hour at +10C is the equivalent of 2 hours at room temperature... So a battery that runs 25C 23 hours per day and 35C 1 hour per day has will "age" 25 hours per 24 hour day or have a ~4% shorter life. (of course, real life battery temperature does not usually respond quite that way).

    Lead Acid batteries can take pretty high charging current below ~80% state of charge (almost all energy goes into charging, little wasted at heat/gassing). The problem with Lead Acid is that last 10-20% charging takes time--Something like 2-6 hours of "absorb" cycle (being held at 14.X volts for 2-6 hours).

    If you have a short time frame (shading) to recharge the battery bank--You may be better off with a Lithium type battery. They do not have limitation of charging time like lead acid require. I don't think AGM would "solve" your short charging period.

    If this was for emergency backup system... Actively cycling Lead Acid batteries can have a pretty good life cycling from 50% to 80% state of charge (and you can use high charging current). You only need to get >90% state of charge once a week (one battery vendor said once every 4 weeks was OK too). That is pushing the Lead Acid design envelope--But it does appear to be a "valid" way to run a deep cycle lead acid battery. Use a genset/utility power to "finish charge/absorb" the batteries once a week (or use utility power when it returns). You do not need a large genset--5% rate of charge would be OK (i.e., 5% rate of charge for 4-5 hours once a week--assuming no other AC/DC loads--Use larger genset+charger if you have loads during charging--i.e., weekly water pumping, running the washing machine, etc.)..

    If you where going to start "small" with (2-4x 6 volt @ 220 AH) golf cart batteries and a "large enough" solar array for your daily power needs--Experimenting over 2-5 years will show you if this is a good setup for your application or not.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • zoneblue
    zoneblue Solar Expert Posts: 1,220 ✭✭✭✭
    The thinking that the, you can do a days worth of charging in a short sun window is the same idea as ovepanelling in general.Take a bunch of expensive panels then double it and maybe double it again. Net effect, you just made a RE system that was barely (actually negative) cost effective that much less cost-effective.
    lkruper wrote: »
    Thanks for the correction re amps and amp hours. Also, I had considered that the heat would be higher, but to what effect?

    Well you have to factor both voltage and current. Above the gassing voltage, the amount of amp hours put into the battery is less than what you get out. Below the gassing voltage you pretty much get out what you put in. However where the heat energy is "lost" is seen in the extra voltage dropped.

    As an aside our design rule of operating lead banks in the 100-75% SOC range for daily cycling, tends to mean that overall Wh charge efficiency suffers from higher charge efficiency losses as a result of spending a large proportion of the time in absorb.
    BB. wrote: »
    The problem with Lead Acid is that last 10-20% charging takes time--Something like 2-6 hours of "absorb" cycle (being held at 14.X volts for 2-6 hours).

    Yes, thats a problem with the whole 'fast charge' idea. During absorb, its the battery itslef that dicates the current, and nothing you can do can rush it appreciably. While opportunity bulk charging using sun windows, or gensets or whatever, and not completing charge each time can work, (common on boats), its still important to get to that all important 100% periodically, to prevent capacity walkdown. And that takes time, in the winter if dont complete bulk by 1pm, absorb may not complete when the sun dissappears at 4 or so. The good news is that you dont need much sun (dull overcast is enough) to hold absorb once its well progressed, and this also frees up a lot of energy for loads.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    The limited sun window is why many users who have to resort to generators to keep their batteries at high SOC do it by running the genny in the morning to do the Bulk charge and then let the panels complete the Absorb through the remainder of the day. This uses the generator nearer to full load for best efficiency and keeps you from needing to run the generator many hours at low load for Absorb.
    SMA SB 3000, old BP panels.
  • lkruper
    lkruper Solar Expert Posts: 115 ✭✭
    zoneblue wrote: »
    The thinking that the, you can do a days worth of charging in a short sun window is the same idea as ovepanelling in general.Take a bunch of expensive panels then double it and maybe double it again. Net effect, you just made a RE system that was barely (actually negative) cost effective that much less cost-effective.



    Well you have to factor both voltage and current. Above the gassing voltage, the amount of amp hours put into the battery is less than what you get out. Below the gassing voltage you pretty much get out what you put in. However where the heat energy is "lost" is seen in the extra voltage dropped.

    As an aside our design rule of operating lead banks in the 100-75% SOC range for daily cycling, tends to mean that overall Wh charge efficiency suffers from higher charge efficiency losses as a result of spending a large proportion of the time in absorb.



    Yes, thats a problem with the whole 'fast charge' idea. During absorb, its the battery itslef that dicates the current, and nothing you can do can rush it appreciably. While opportunity bulk charging using sun windows, or gensets or whatever, and not completing charge each time can work, (common on boats), its still important to get to that all important 100% periodically, to prevent capacity walkdown. And that takes time, in the winter if dont complete bulk by 1pm, absorb may not complete when the sun dissappears at 4 or so. The good news is that you dont need much sun (dull overcast is enough) to hold absorb once its well progressed, and this also frees up a lot of energy for loads.

    Quoting from memory will get me into trouble, but it seems to me that Concorde gives the time for charging to be:

    Battery AH Rating * %DOD / Charge rate in AH + 2 hours.
    So lets take a 225 battery bank that is kept for backup and if used, will be discharged to 50% DOD.
    [Concorde says to charge no less than 0.2C for deep discharge, rolls AGM 0.25 [URL="http://support.rollsbattery.com/support/solutions/articles/4345-agm-charging]"]http://support.rollsbattery.com/supp...-agm-charging][/URL]
    Next two 300w panel (~39v 8amps output) with an MPPT charge controller that puts out 50 amps at full output.
    The AGM will charge at the full output and with Concorde's equation will take [changed -4 to] 2.5 hours.

    Now lets take the same scenario with FLA batteries.
    They can be charged up to 15% for bulk and then [changed float to] absorb at 0.42C. (Rolls site, I think)
    Bulk = [changed 6 to ] 2 hours, absorb another [changed 2.5 to] 2 = [changed -8.5 to ] 5 hours (did I do that right?)

    So if I can get [changed 4-5] 2.5 hours of sun, I can charge the AGMs but it takes [changed two days] to 4 hours to charge FLAs.

    Is that really wrong?


    Edit:
    My calcs were off, hopefully my edits are not too confusing, and hopefully my edits are now correct :)


  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    lkruper wrote: »
    Now lets take the same scenario with FLA batteries.
    They can be charged up to 15% for bulk and then float at 0.42C. (Rolls site, I think)
    Bulk = 6 hours, absorb another 2.5 = 8.5 hours (did I do that right?)

    I think there is some confusion there... Bulk from 50% to 80% State of charge would be ~2 hours (at 15% rate of charge), then another 4-6 hours of absorb charging (all for flooded cell batteries).

    And I have no idea what 0.42C for "float" means.

    Float current is usually somewhere in the range of 0.001C to 0.02 C for float current (depending on battery type, age, bank temperature, chosen float voltage, etc.).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    I think that if you apply the float voltage immediately to a discharged battery (say 50% SOC), then you would need to limit the current to a reasonable fraction of C or else risk damage to your battery bank.
    SMA SB 3000, old BP panels.
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    inetdog wrote: »
    I think that if you apply the float voltage immediately to a discharged battery (say 50% SOC), then you would need to limit the current to a reasonable fraction of C or else risk damage to your battery bank.

    For that matter, if you apply absorb voltage immediately to a discharged battery (say 50% SOC), then you would need to limit the current to a reasonable fraction of C or else risk damage to your battery bank.

    Most systems are designed so that the charging source (solar, generator, etc) does not have the capacity to over-current the batteries, or alternatively, parameters are set to limit the charge current.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • lkruper
    lkruper Solar Expert Posts: 115 ✭✭
    BB. wrote: »

    I think there is some confusion there... Bulk from 50% to 80% State of charge would be ~2 hours (at 15% rate of charge), then another 4-6 hours of absorb charging (all for flooded cell batteries).

    And I have no idea what 0.42C for "float" means.

    Float current is usually somewhere in the range of 0.001C to 0.02 C for float current (depending on battery type, age, bank temperature, chosen float voltage, etc.).

    -Bill

    I did make a mistake. I calculated bulk from 50% to 100% instead of from 50% to 80%. That is 2 hours. The 0.42 is from the user guide for Rolls FLA batteries (http://support.rollsbattery.com/supp...acid-batteries), see below between ==== .

    I also used the term "float" for the 0.42 when it is absorb.

    However I also made the same mistake with my AGM example :) So instead of two hours to charge at 50 ah it would be 36 minutes plus absorption.
    I don't know yet how many hours of good sun I will get at my location under the trees but the difference between 2.5 and 4 hours is significant.

    I really don't want the expense of AGMs. However, if a particular installation allowed for full charge with AGMs but my generator was needed every time to finish the charge with FLAs, then I would be tempted.

    BTW, I am finding that every manufacture seems to cover aspects of their charging differently, with one giving great detail in one area where another is silent.
    Question: With Flooded lead acid batteries, how much of what Rolls says applies to Trojan and visa versa?
    =======================================
    Absorption Time:

    The most important part of the charge cycle is the absorption charge. Since the bulk charge only recharges the battery bank to an 80% level, the absorption charge completes the charging cycle. Most chargers on the market have a timer that allows the user to adjust the duration for the required time to return the battery to full charge. In order to set the correct time, a simple calculation is required. With the help of the 20 AH capacity, you can figure out the remaining charge required for the battery bank using the following equation:

    t = 0.42 x (C/I)

    Where:
    t = Absorption Charge Time (Hours)
    C = 20 hr Rated Capacity (AH) [ex: 2 strings x S-530 models (400 AH) = 800 AH rated capacity]
    I = Charging Current (Amps) [charger output min 5% up to max 10% of 20 hr rate]

    EXAMPLE:

    2 strings of S-530 batteries

    20 hr rate = 400 AH x (2 strings) = 800 AH

    I =10% of 800 AH = 80 amps OR If charger is only 60 amps max then 60 is used

    T = 0.42 x 800/80 = 4.2 hrs OR T = 0.42 x 800/60 = 5.6 hrs
    ============================================================ =================================
  • lkruper
    lkruper Solar Expert Posts: 115 ✭✭
    inetdog wrote: »
    I think that if you apply the float voltage immediately to a discharged battery (say 50% SOC), then you would need to limit the current to a reasonable fraction of C or else risk damage to your battery bank.

    I made a mistake in my terminology. When I used the term "float" with respect to the 0.42 factor from the Rolls FLA user manual I should have said "absorb." Now it looks like in my somewhat contrived example (but possibly close to my needs at the cabin) it would take me 4 hours to charge two 6v 225AH FLA batteries discharged to 50% and about 2.5 hours if they were AGMs. So the question would be, is the extra expense of the AGMs worth it to save having to use the generator to charge, a generator I will have anyway,
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    OK, Rolls is saying that the absorb time is 0.42 * Capacity / charging current.

    Just to be clear, this is for their Flooded Cell Lead Acid Batteries.

    So, a 50% SOC battery at 20% rate of charge would be:
    • (0.80 - 0.50 state of charge) / 0.10 rate of charge = 3 hours of bulk charge
    • 0.42 * 800 AH / 80 Amp charging = 4.2 hours Absorb
    • Total charge time = 3+4.2=7.2 hours of total charge time
    At the end of an estimated 7.2 hours, the charge controller should return to float charging state (i.e., just high enough voltage from the charge controller to keep the battery from charging or discharging).

    AGM are still a form of Lead Acid Battery... So, I am not sure that the 0.42C/I factor is any shorter.

    Here is Rolls' AGM charging FAQ:

    http://support.rollsbattery.com/supp...5-agm-charging
    The charge time in the final phase should not exceed 8 hours and the total charge time should not exceed 20 hours.

    And they list total charging time (bulk+absorb) as <=16 hours.

    Hopefully somebody here with AGMs can give some experience about their Absorb Time with AGMs.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset