Is C/11 - C/12 ok for lead acid charging?

2twisty
2twisty Solar Expert Posts: 199 ✭✭✭
in Off Grid Solar & Battery Systems #1
When my panels are making peak power, I get about 37-39A into the batteries. By bank is 410Ah@48V. That comes to about C/11 for charging, but that's when my panels are making peak power (rated at 2170W, I call "peak" power anything over 2kW)

What's got me concerned is that I only make that rate at peak. Most of the rest of the time, I'm making closer to 1800W, so my charge rate goes down to C/12-C/14. Is this something I need to worry about?

I can occasionally (and more so lately due to cloudy days here) use my generator and let the Outback charge the batteries at a higher rate. Is this sufficient, or do I need to start planning another string (of 3) panels so that I can always be able to charge at C/10 or better?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Is C/11 - C/12 ok for lead acid charging?

    You may have seen me write a few hundred times "peak charge current". It is exactly that: the maximum current you will see. It is not necessary nor even likely to have that maximum throughout the whole Bulk charge stage. Panels start out producing lower in the morning and increase as the sun becomes more direct on them.

    What matter is: are the batteries getting charged? If so, the rate is not particularly relevant. It is a shortcut to assuring you can put enough energy back into them to complete a charge on a good, sunny day.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #3
    Re: Is C/11 - C/12 ok for lead acid charging?

    How is the specific gravity after the solar charging is completed? Are you reaching >90% state of charge a couple times a week?

    Using genset more than you want?

    How is water usage... Roughly if you are needing to add water every couple of months--You are probably doing fine.

    From what I can see, your panels are doing really well--Using our rule of thumb for typical array performance:

    2,170 Watt array * 0.77 panel+controller derating * 1/58 volts charging = 28.8 Amps "typical" maximum (all things being equal)

    More panels typically makes things easier... If you have the money and space available.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • 2twisty
    2twisty Solar Expert Posts: 199 ✭✭✭
    #4
    Re: Is C/11 - C/12 ok for lead acid charging?

    OK. Sounds like I'm OK then.

    I have had to add a little bit of water now and again, SG looks good after solar charge (>1.265 most of the time) and I routinely get past 90% I augment charging with the generator so that I don't fall below 65% SOC. Genny auto-starts and charges to 90%. If you've seen my previous posts about this, it used to be set at 60 and 85, respectively, but I decided to bump the window a bit, since I really don't want the batteries to fall below 70 regularly.

    I never let them fall that low without the genny recharging. As such, I only really have a day's worth of power available in my bank, since I generally get into the 70s by morning in the winter. I want these batteries to last as long as possible, so I try to make sure they get to float every day or every few days, even if that means using the generator.

    Am I being too paranoid about this?

    To charge the batteries from 65 to 90SOC, My generator runs at just below 50% capacity for about 3-4 hours (I've never timed it since it usually kicks on and off in the middle of the night if it's going to). That consumes a smidge over 2G of fuel, so right now it's costing me about $5 when I need to use the genny to charge.

    My goal is so that the batteries **NEVER** get anywhere close to 50% DOD/SOC. We get really hot here in the summer time, so my batteries are going to suffer during the summer months even with ventilation. So, since that will lower their life, I want to make sure I do as much as I can to extend their life....

    What is a "healthy" level of paranoia about this?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #5
    Re: Is C/11 - C/12 ok for lead acid charging?

    Paranoia isn't healthy. Concern is.

    When you come right down to it as long as the system works, it works.
    Too far off right and you have sudden failure, but even that isn't devastating on a one-time basis. Repeated undercharging or overcharging eats into lifespan. A once-in-a-while "oops" doesn't (at least not with FLA's).

    I check my batteries once a month when I'm there, leave 'em go all Winter on their own, have a theoretical >10% peak charge rate, and don't worry about things that haven't happened yet.

    Of course if something does happened I'm fairly well qualified to deal with it and devise a work-around if needed. ;)
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  • CALLD
    CALLD Solar Expert Posts: 230 ✭✭
    #6
    Re: Is C/11 - C/12 ok for lead acid charging?

    Hi 2Twisty, As coot said the main thing is your batteries need to get a full charge.

    My panels peak at around C/9 in summer C/15 in winter. The current I need to charge them basically depends on how discharged they are. If I've only used 10 or 20% of the batteries C/20 or even C/30 will do the job. I think occasionally you want to charge them "rapidly" to heat the plates a bit to help remove sulfate, but other than that you just want to put back what you've taken out within 10 hours or less.

    Another interesting thing is that charging at a lower rate increases efficiency, charging at higher rate gets the voltage up to absorb voltage at a lower SOC, which means the battery needs to stay there for longer to get to 100%. In general a healthy un-sulphated battery is fully charged once the current at absorption voltage falls below C/50 or 2A per 100Ah capacity. So if the current acceptance on your 416AH pack falls below 8 amps at the absorption voltage then they are getting enough charge. Note, if they are sulphated then current acceptance will be false...
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  • 706jim
    706jim Solar Expert Posts: 519 ✭✭✭✭
    #7
    Re: Is C/11 - C/12 ok for lead acid charging?

    My 660Ah 24 volt battery has been charged at what appears to be too low of a rate.

    And yes, I read the Trojan battery manual.

    But those batteries lasted 18 years/19 seasons before giving up the ghost.

    So I figure the low rate is ok as long as the batteries occasionally get equalized and aren't excessively discharged.
    Island cottage solar system with appriximately 2500 watts of panels, 1kw facing southeast 1.3kw facing southwest 170watt ancient Arco's facing due south. All panels in parallel for a 24 volt system. Trace DR1524 MSW inverter which has performed flawlessly since 1994. Outback Flexmax 80 MPPT charge controller four 467A-h AGM batteries. Insignia 11.5 cubic foot electric fridge 1/4hp GSW piston pump. My 31st year.
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #8
    Re: Is C/11 - C/12 ok for lead acid charging?
    706jim wrote: »
    My 660Ah 24 volt battery has been charged at what appears to be too low of a rate.

    And yes, I read the Trojan battery manual.

    But those batteries lasted 18 years/19 seasons before giving up the ghost.

    So I figure the low rate is ok as long as the batteries occasionally get equalized and aren't excessively discharged.

    IMHO the biggest drawback to regular slow charge would be stratification of the electrolyte. Over the long term this can lead to loss of battery capacity, but if you do an equalizing charge using generator or POCO power that will certainly stir the electrolyte. What is necessary is to get the cell voltage above the gassing level while still providing enough current to produce lots of bubbles to do the stirring.
    SMA SB 3000, old BP panels.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #9
    Re: Is C/11 - C/12 ok for lead acid charging?

    Need around 2.5 to 5% rate of charge to equalize and gas.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #10
    Re: Is C/11 - C/12 ok for lead acid charging?

    *sigh*

    Current is not directly relevant to gassing (re-mixing of electrolyte) and especially not equalizing (higher sustain Voltage to force more sulphur off plates).

    You need the current rate in order to charge the batteries fast enough so that there is enough sunlight left to provide the higher Absorb Voltage for long enough to mix the electrolyte. It is particularly problematic in 'tall case' batteries which really need the stirring because they are more prone to stratification due to the case design. Standard batteries hardly ever suffer this fate.

    Lower than 5% capacity current may not be enough to even affect a charge.

    Charging from an AC source changes the rules, except in so much as most people do not want to run a generator for a long time just to charge batteries as it uses a lot of fuel for a little effect.
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  • 706jim
    706jim Solar Expert Posts: 519 ✭✭✭✭
    #11
    Re: Is C/11 - C/12 ok for lead acid charging?
    inetdog wrote: »
    IMHO the biggest drawback to regular slow charge would be stratification of the electrolyte. Over the long term this can lead to loss of battery capacity, but if you do an equalizing charge using generator or POCO power that will certainly stir the electrolyte. What is necessary is to get the cell voltage above the gassing level while still providing enough current to produce lots of bubbles to do the stirring.

    I agree with you. On the advice of my battery supplier, I will be equalizing my new battery bank (8-370Ah L16's) using my 5k Honda generator. He suggested doing this at least once a summer and stated that a combination of high current as well as high voltage was required to do this properly.
    So that pretty much agrees with what most others have stated here.

    Slightly off topic; I participate in a couple of boating forums. Posters there can't understand why they can't run A/C on their boats overnight off of a couple of 12 volt deep cycles or 4 GC's.....
    Island cottage solar system with appriximately 2500 watts of panels, 1kw facing southeast 1.3kw facing southwest 170watt ancient Arco's facing due south. All panels in parallel for a 24 volt system. Trace DR1524 MSW inverter which has performed flawlessly since 1994. Outback Flexmax 80 MPPT charge controller four 467A-h AGM batteries. Insignia 11.5 cubic foot electric fridge 1/4hp GSW piston pump. My 31st year.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #12
    Re: Is C/11 - C/12 ok for lead acid charging?
    706jim wrote: »
    Slightly off topic; I participate in a couple of boating forums. Posters there can't understand why they can't run A/C on their boats overnight off of a couple of 12 volt deep cycles or 4 GC's.....

    'Cause math is hard. :p
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