Battery max charge rate?

johnnysolarjohnnysolar Registered Users Posts: 14
Can someone help me to understand this better. Rolls s600 48v 450(c20) amp hr battery bank. 5000 watts of PV on XW mppt 80/600 charge controller. Should I be limiting the charge controller max charge rate to 10%-15% or 45- 60 amps (max is 80). That lowers the overall amount of power I can actually get from my pv down to about 3528(58.8v x 60a). To fully utilize a a 5000w array with this charge controller I would need around a 550 aH battery. Is this right or am I confused. I always thought I should size the PV to be able to fully charge my battery in a day and supply the watt hours needed by the avg daily load. It sounds like I have too much solar?

If I wanted to do a 24v 200aH battery with a max charge rate of 30 Adc x 29.4v then the maximum size array would be 889 watts not counting derating. This doesnt seem right? I want my systems to be able to allow the full power of the array to pass thru the busbar straight into the inverter, keeping the battery in float but utilizing full array power. Help please.

Comments

  • zonebluezoneblue Solar Expert Posts: 1,218 ✭✭✭✭
    Re: Battery max charge rate?

    5kW/48V is pushing the limit for the controller aswell. However unless you have very agressive oppurtunity loads, i doubt youll lose much power by limiting the top off, in kwH/day terms. Off grid systems tend not to run all day every day, and when it is super sunny, they float earlier in the day.

    The advantage of "too much solar" is rainy day support. Most people use generators for that, but if your sun hours stack up, oversized arrays also works.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


  • vtmapsvtmaps Solar Expert Posts: 3,739 ✭✭✭✭
    Re: Battery max charge rate?
    5000 watts of PV on XW mppt 80/600 charge controller.
    <snip>
    It sounds like I have too much solar?

    If I wanted to do a 24v 200aH battery with a max charge rate of 30 Adc x 29.4v then the maximum size array would be 889 watts not counting derating. This doesnt seem right?

    You don't have too much solar, but you don't have enough controller (at 24 volts) to handle that much solar, even if your battery could handle 5000 watts.

    Next issue is limiting the current into your batteries. First of all, its only a problem during bulk charge... during absorb the battery will limit its current no matter how much current the controller has to offer. Unfortunately, if you do program the controller to limit its current, that limit will also apply to powering your loads.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • johnnysolarjohnnysolar Registered Users Posts: 14
    Re: Battery max charge rate?
    vtmaps wrote: »
    You don't have too much solar, but you don't have enough controller (at 24 volts) to handle that much solar, even if your battery could handle 5000 watts.

    Next issue is limiting the current into your batteries. First of all, its only a problem during bulk charge... during absorb the battery will limit its current no matter how much current the controller has to offer. Unfortunately, if you do program the controller to limit its current, that limit will also apply to powering your loads.

    --vtMaps

    Oops sorry about the confusion. I'm talking about two different systems.

    I was going to put 3000 watts on the 24v 245 aH battery (DEKa 8A8DLTP 12v AGM) According to DEKA my max charge rate on battery is about 74 amps @28.8v or 2131 watts. Im talking classic 150's here. So if I want to cram 3000watts on there should I just go for it cause I know the 3 kw array x .77 = 2310 and thats close enough or should I split up the 3000 watt array over to controllers.

    Im coming from the big solar array/ smallest battery possible school of thought. Most of my customers dont have any idea how big of a system or battery they need, and are unwilling and incapable of filling out a loads worksheet. The most I can get from them is what they want to run when power is out and how long. I usually end up with the fridge, the freezer, some lights (I force the led on them), and misc plug in devices. Sometimes a well pump is involved. Obviously, I dont want to be oversizing/overselling solar arrays. Unless they specify otherwise I give them enough battery storage for 1 day of autonomy. I try to perpetuate the idea that when the sun is shining they will be able to float the battery and take advantage of the excess solar with some pass thru solar straight to the inverter. I imagine the solar generated electricity going right thru the bus bar into the inverter and kinda skipping across the top of the floated battery like a rock skipping on the water. Furthermore, I have observed this, or so I thought, numerous times when I test my systems with customers. Am I right or wrong. I don't want to be a solar charlatan.
  • BB.BB. Super Moderators, Administrators Posts: 32,008 admin
    Re: Battery max charge rate?

    You are probably helping your customers rather than hurting them. Solar panels are so much cheaper than a decade ago, and they last for 20+ years--It is hard not to have a larger array vs more generator run time in poor weather.

    In the end, if your customers are happy and sucessfully running their homes--And the batteries are lasting--I am not going to complain.

    With a larger array (greater than ~13% rate of charge), I would highly suggest installing a Remote Battery Temperature Sensor. Both optimizes charging (cold batteries use higher charging voltage) and better safety (hot batteries need lower charging temperatures--With high charging currents and a cold charge controller--Internal Batt Temp Sensor--Hotter battery, lower charging voltage, possible thermal run-away). Not all charge controller support a RBTS--So that will be part of the hardware selection process. With an RBTS, you can charge upwards of 20-25% rate of charge pretty easily/safely (as always, review battery mfg. manuals for details).

    In general, we have a set of rules of thumbs that help design a "balanced" solar system. 1-3 days of storage and 50% maximum discharge. 2 days + 50% discharge (4x daily load) seems to be a good compromise. There are lots of reasons that it ends up working out well--Including better surge capacity (starting well pumps), usually keeps daily cycling between 25% and 50% nominally for good battery life (cycling less than 10-20% or deep cycling below 50% can reduce battery life). Also, when the sun does shine, a larger battery bank can store the available good solar days more quickly (batteries that are less than 80% state of charge are more efficiently charged and can take higher charging current than 90% or higher SOC--more heat/hydrogen production).

    Also, when discharging batteries, C/20 works out pretty well for most installations. I.e., 5 hours an evening/morning discharge for 2 days to 50%. If you do a C/10 rate that is 5 hours of discharge to 50% capacity--C/10 rate battery is less efficient (apparent battery AH capacity is less at C/10 vs C/20 rates).

    For batteries, the rules of thumbs I like to use (based on flooded cell batteries, but still works well for AGM too):
    • C/20 = Battery AH capacity used for charging/disch. arging rules of thumbs
    • C/20 = "Average" discharge rate
    • C/10 = "Heavy" discharge rate
    • C/8 = "Max continuous load
    • C/5 = "Short Term" very heavy loads (well pump? for a few minutes at a time)
    • C/2.5 = "Max Surge" load (few seconds to minute or so)
    • C/20 (5%) = Minimum rate of charge (charge during day, use power at night)
    • C/10 (10%) = Nominal rate of charge--Works well for most people and some loads during day
    • C/8 (13%) = Max "cost effective" rate of charge--Batteries quickly recharged and back in float long before sun sets, supports more daytime loads, RBTS not needed (still helpful)
    • C/8-C/4 (13-25%) = Usually used for Generator chargers--Quickly recharge a battery bank to >80% SOC--Then let solar continue charge to float (RBTS highly recommended).
    • 100 amps -- Try to keep nominal DC current at or below 100 amps
    • 60-80 amps -- Generally the maximum charging current for most larger solar charge controllers
    For example--I use 3.3 kWH per day for a very energy efficient home with Energy Star Refrigerator, lights, well pump, washing machine, laptop, radio+TV. About the minimum size recommended for a "near normal" electric life. Here how the numbers work out:
    • 3,300 WH per day * 1/0.85 AC Inverter Eff * 1/24 volt battery bank * 2 days of storage * 1/0.50 max discharge = 647 AH @ 24 volt battery bank
    That works out nicely with 3 parallel strings of 6 volt ~220 AH golf cart batteries (3parallel * 4series = 12 batteries). This is great for a less expensive "starter" battery bank. Last 3-5 years and let people learn how to take care of the bank. Normally, I suggest one series string as ideal, and not more than 3 parallel strings (more strings, more connections, more cells to check/fills, more things to go wrong, harder to keep battery currents balanced, etc.).

    At a 10% rate of charge, that is ~65 amps of charging current--Just about the maximum a single 60-80 MPPT charge controller can manage... You can always go 48 volt battery bank to use a larger array and keep the costs/simplicity of one solar charge controller (and uses larger AH capacity cells to reduce number of parallel strings).

    Next is sizing the solar array for size of battery bank--A larger battery bank needs a larger solar array to properly charge. Generally 5% is the absolute minimum charging current (charge during day, use power at night)... But for a daily cycling off grid cabin, a 10%+ rate of charge is highly recommended:
    • 647 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 1,237 Watt array minimum
    • 647 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 2,474 Watt array nominal
    • 647 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 3,217 Watt array "cost effective" maximum
    • 647 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.20 rate of charge = 4,949 Watt array with RBTS
    • 647 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.25 rate of charge = 6,186 Watt array with RBTS not recommended to exceed

    And there is sizing of the array based on reasonable estimate of minimum sun... I typically use PV Watts or Solar Electric Handbook for "hours of sun" per day by month. Using PV Watts for Atlanta Georgia, fixed array, tilted 34 degrees from horizontal:
    Month    Solar Radiation (kWh/m 2/day)
    1      3.86     
    2      4.67     
    3      5.21     
    4      6.17     
    5      5.95     
    6      5.81     
    7      5.82     
    8      5.83     
    9      5.21     
    10      5.51     
    11      4.42     
    12      3.72     
    Year      5.18
    

    Typically, we use 4 hours of sun minimum. In much of the US, people get 4+ hours of sun for 9+ months of the year. Use genset for winter/poor weather backup. Atlanta seems to have lots of sun, so lest use 3.72 hours of sun (long term average) for December "break even" estimate array:
    • 3,300 WH per day * 1/0.52 end to end system eff * 1/3.72 hours of sun = 1,706 Watt array minimum
    So, based on the above, the array should be around 1,706 Watts to 3,217 Watts max--With 2,474 Watt array as a healthy nominal size. A nominal array in December would generate an average of:
    • 2,474 Watt array * 0.52 system eff * 3.72 hours of sun = 4,786 WH = 4.8 kWH per day
    Again--A pretty balanced system that is capable of running significant daytime loads too (irrigation, daytime laptop/at home job, etc.).

    And what would a 647 AH @ 24 volt battery bank battery bank "support"?
    • 647 AH * 24 volts * 0.85 inverter eff * 1/20 discharge rate = 660 Watt averge loads (over two days, 5 hours per day)
    • 647 AH * 24 volts * 0.85 inverter eff * 1/10 discharge rate = 1,320 Watt average heavier loads (microwave oven, irrigation pumping)
    • 647 AH * 24 volts * 0.85 inverter eff * 1/8 discharge rate = 1,650 Watt max continuous loads (10+ hours)
    • 647 AH * 24 volts * 0.85 inverter eff * 1/5 discharge rate = 2,640 Watt you really don't want to ever exceed steady state loads
    • 647 AH * 24 volts * 0.85 inverter eff * 1/2.5 discharge rate = 5,280 Watt maximum surge power
    You can see the above numbers also help you define the range of inverters that would work well with this battery bank... A 1,500 watt to ~2,500 Watt AC inverter would do well... 1,500 watt average maixmum load. Most good inverters can support 2x maximum surge or (5,280 watts / 2 =) 2,640 max supported AC inverter.

    Numbers are intended to be conservative and support loads both in first year of battery bank life and ~3-5+ years later too.

    Also, numbers for solar are within 10% accuracy... I carry out digits to avoid round off error, and so you can see/reproduce my math.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery max charge rate?

    Keep in mind that when we talk about a charge rate the number is net; almost always in an off-grid application there is some load detracting from that number. This is why people who go for the minimum 5% so often end up with shortened battery life.

    It is also a peak rate, not a constant. As a rule flooded cells can take a 20% peak rate without any trouble. This would be 90 Amps on a 450 Amp hour battery bank. No single controller will manage that.

    As for maxing out an 80 Amp controller on a 24 Volt system 2500 Watts would do it. Therefor a 5kW array is 50% wasted. If it were a 48 Volt system all would be fine.
  • Blackcherry04Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    Re: Battery max charge rate?

    The only issue I can see with your thinking is that you assume a battery will maintain it's capacity over time. A Deka/MK 8A8DLTP has a 24 month warranty with a 50 % capacity. Generally Batteries used in critical systems is considered unserviceable when they reach between 70 - 80% capacity, RE people will go to 50% . If you don't build in enough reserve you end up with a system that is incapable carrying their loads in a couple years.
  • BB.BB. Super Moderators, Administrators Posts: 32,008 admin
    Re: Battery max charge rate?

    And then there is charging with a Genset... Larger AC inverters tend to be available with integrated AC battery chargers. These inverter/chargers are, usually, much more programmable and have more features than "simple" AC Inverters or AC Battery Chargers.

    And while they are great when they work--There is always the question of backups... A full matched inverter/charger on the self, or a "simple/cheap" backup inverter + AC Battery charger to allow you to get by when things break (you are your own Utility when off grid).

    Most people tend to "oversize" the AC generator... Generators are "cheap", and the larger ones are frequently designed as "Prime Movers"--Which are better designed for running off grid for long hours at high loads...

    However, if your home/cabin is very efficient and you have a "balanced" design with a "just right" sized battery bank, frequently the 8+ kW/kVA gensets are really too large to efficiently recharge the battery bank... Meaning a person may use 2-4x (or more) fuel to recharge their battery bank than they really need too... Ideally, you want to run the genset around 50% to 80% of rated load. If you fall much below 50% of rated output, your fuel efficiency falls (gasoline/propane gensets run at ~50% fuel flow for 0%-50% of rated loads, and Diesel gensets can carbonize, glaze cylinders, "wet stack" if operated below ~50-40% of rated load).

    In many cases, it does make sense to get two gensets... One large genset for running the shop and as emergency backup for a smaller (usually non-prime mover rated) genset to fuel efficiently recharge the battery bank.

    You can recharge your battery bank at >13% rate of charge at less than 80% State of charge, but only at 5-2.5% rate of charge >90% SOC (typically during equalization--It is not recommended to recharge >90% SOC during normal operation--"Bubbling" batteries is hard on them; it generates lots of heat, erodes plates, causes positive grid corrosion, and uses lots of distilled water). You one equalize when needed and just long enough to accomplish your goals. And stop when done (or batteries get over >~115-120F).

    So, some quick numbers for a 647 AH @ 24 volt battery bank with a "typical" AC Battery charger (non-power factor corrected):
    • 647 AH * 29 volts charging * 1/0.80 charger eff * 1/0.67 PF * 1/0.80 max genset loading * 0.10 rate of charge = 3,501 VA rated genset @ 10% rate of charge
    • 647 AH * 29 volts charging * 1/0.80 charger eff * 1/0.67 PF * 1/0.80 max genset loading * 0.20 rate of charge = 7,001 VA rated genset @ 20% rate of charge
    For the "most efficient" AC battery charger (typically an Inverter/Charger):
    • 647 AH * 29 volts charging * 1/0.90 charger eff * 1/0.95 PF * 1/0.80 max genset loading * 0.10 rate of charge = 2,195 VA rated genset @ 10% rate of charge
    • 647 AH * 29 volts charging * 1/0.90 charger eff * 1/0.95 PF * 1/0.80 max genset loading * 0.20 rate of charge = 4,389 VA rated genset @ 20% rate of charge
    Another advantage of some Inverter/Chargers is the "Generator Support" Function... The AC inverter can charge from the Generator, and if there is a large load that exceeds the generator output, the inverter/charger will dial back on the charging current, or even start acting like an inverter and add battery+inverter power to the limited generator output.

    Inverter/Chargers are quite interesting and getting better all the time (at not too much more cost)... Something to think about when folks have the occasional large loads and still want to run a smaller/fuel efficient genset.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Super Moderators, Administrators Posts: 32,008 admin
    Re: Battery max charge rate?

    One thing I would suggest if you are gong to "over panel" an installation. Batteries really do well with long term/moderate charging currents. They really do not "like" 4-6 hours of maximum charging current.

    In the "olden days", you could stretch the hours a day of good charging current by using a one or two axis tracker. You do have the added expense of the hardware and maintenance issues.

    But, today, with the much lower costs of solar panels... There is the idea of virtual tracking. Basically facing 1/2 of the array south east and the other 1/2 of the array south west. This gives you several more hours a day of charging current without the costs/hassles of a mechanical tracking system.

    You can play with PV Watts and optimize the system (different tilts and compass headings) for your location to get you more hours of charging and, potentially, longer battery life.

    You can also look at the energy usage and adjust the array to better meet the needs--Is this a 3 season home/cabin? Do they use A/C and need more summer power? Do they need more power in the winter (more lighting, fans for moving warm air around, etc.)?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • vtmapsvtmaps Solar Expert Posts: 3,739 ✭✭✭✭
    Re: Battery max charge rate?
    BB. wrote: »
    It is not recommended to recharge >90% SOC during normal operation--"Bubbling" batteries is hard on them; it generates lots of heat, erodes plates, causes positive grid corrosion, and uses lots of distilled water

    I agree that gassing is hard on batteries, but isn't stratification also hard on batteries? I think that stratification increases with each partial charge cycle. I guess its just a matter of picking your [battery's] poison.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery max charge rate?
    vtmaps wrote: »
    I agree that gassing is hard on batteries, but isn't stratification also hard on batteries? I think that stratification increases with each partial charge cycle. I guess its just a matter of picking your [battery's] poison.

    --vtMaps

    Stratification is a problem that rarely occurs, and usually not to a battery-threatening level.
    However it's really easy and very common to 'cook' batteries at high Voltage for too long a time.
  • vtmapsvtmaps Solar Expert Posts: 3,739 ✭✭✭✭
    Re: Battery max charge rate?
    vtmaps wrote: »
    Unfortunately, if you do program the controller to limit its current, that limit will also apply to powering your loads.

    After I wrote this, I went over to Midnite's forum and asked if their new current measuring circuitry would make it possible to do what you want. The answer is not yet, but coming soon.
    Read the reply here: http://midniteforum.com/index.php?topic=1656.msg14455#msg14455

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • johnnysolarjohnnysolar Registered Users Posts: 14
    Re: Battery max charge rate?
    BB. wrote: »

    You can also look at the energy usage and adjust the array to better meet the needs--Is this a 3 season home/cabin? Do they use A/C and need more summer power? Do they need more power in the winter (more lighting, fans for moving warm air around, etc.)?

    -Bill
    I always use a RBTS.

    From what they were telling me they come up as often as the can in summer and winter. They are "preppers" and this is there lakehouse/bugout location. Their main concern is a pump in there septic tank, lights, fridge, occasional appliance. I'm realizing I'm gonna have to use two charge controllers with this 3000 watt array on a 2 24v 245 aH battery (2 DEKa 8A8DLTP 12v AGM). I hope the battery isn't too small to give them 1 day of autonomy. This is just a backup system. It will always have the grid for charging too, unless.....
    I would use a FLA battery but they aren't there enough and don't seem like the type to want to water them. This brings me to my problem that I already gave them the price and basically got the soft close they want to do it but now I'm thinking I need to change the gear anyone have any suggestions. Im thinking I need a bigger battery and two charge controllers. I could always go to them hat in hand but I'd hate to look stupid.
  • BB.BB. Super Moderators, Administrators Posts: 32,008 admin
    Re: Battery max charge rate?

    It is really tough to do a "cost effective" off grid solar power system that is subjected to random use.

    Generators + Propane store pretty well and only "cost money" when actually running.

    Batteries tend to age if used, tend to age faster if not treated well, and will age pretty fast if just floated and not cycled. The Phone Companies will address the issue by getting batteries designed to float for decades--But if they are cycled (hurricane, etc.), the batteries are replaced after the emergency is over.

    It is difficult.... I would suggest that they pick the minimum amount of loads they wish to use once the grid has failed and they are out of propane/stored fuel---I.e., something like 1 kWH per day (lights, laptop computer, cell phone charging, radio/tv, small water pump) or possibly 3.3 kWH per day (add an Energy Star refrigerator and well/short duty cycle pumps).

    An alternative would be to install the array and charge controllers for a larger battery bank--But only load batteries for the 1-3.3 kWH per day loads (I really like the battery bank 4x larger than the daily loads--It makes for a well balanced system). If events suggest more storage (civil unrest, string of storms, utilities become unreliable), then install the "full size battery bank...

    But even that is a tough call... Can you obtain batteries for a larger system on short notice--And in many cases, the answer is probably no (batteries do not store/ship well--So distribution is not going to have a lot of stock on hand--So you are pushing the supply chain back to the manufacturer+shipping).

    Your choices of AGM--Great for those that don't like to service battery banks (no water usage, much cleaner), but more expensive.

    You could also go with Fork Lift Batteries--They can last 15+ years--But should be cycled and really do need 10% or greater rate of charge for longer battery life. Plus they (typically) use more distilled water (need source of distilled/de-ionized water for battery bank). And they will need servicing.

    We tend to suggest "golf cart" or similar inexpensive batteries with a ~3-5 year life... They get good value for the money and can lean all about batteries/off grid/backup power. And in 3-5 years, they will be able to make informed choices based on their actual experiences (without spending an arm and a leg on the first bank). And if they kill the first bank--It is not the end of the world (aka "training batteries").

    The reason you cannot give an 'ideal answer' is because there is no ideal answer.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490mike95490 Solar Expert Posts: 9,361 ✭✭✭✭✭
    Re: Battery max charge rate?

    Better find out what kind of pump is in the septic and if the inverter can power it. May want to kick it up to a 48v system to get the battery capacity. And if this is backup for grid, I would NOT waste a large battery bank on it, the batteries are likely to die of age before they see 50 cycles!
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • johnnysolarjohnnysolar Registered Users Posts: 14
    Re: Battery max charge rate?

    I am going to up the system to 48 volts and use a bigger battery. Id rather have a bigger battery than 2 charge controllers on a small battery. From what I am hearing the bulk stage is the only part where the total current of max charge rate is available to battery so once I get into absorption I could bump it back up to 100% and let the excess power pass through to loads.
  • Blackcherry04Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    Re: Battery max charge rate?
    I am going to up the system to 48 volts and use a bigger battery. Id rather have a bigger battery than 2 charge controllers on a small battery. From what I am hearing the bulk stage is the only part where the total current of max charge rate is available to battery so once I get into absorption I could bump it back up to 100% and let the excess power pass through to loads.
    In Bulk, anything below gassing there is no heat and no stress on the battery. Some AGM can go to 500% of capacity. ( Lifeline) Anything below the number of amp hours you are returning is fine. If I was going to cut the current it would be in Absorb where a longer lower current and longer time is better and you are gassing. What you heard was backwards.

    Added : What High Current does is drive the Display Voltage ( Charger Over Voltage ) and the actual Battery voltage to a larger spread. That spread can skew the trigger voltages you have set. Example : If the display voltage is 13.8 and the actual battery is 13.2 you have a .6 spread when you go into Absorb. That spread takes time to catch up and equalize. If your Absorb time is set to low the battery will never fully charge. If you use ( Return / Ending Amps ) you'll have much better success using higher currents to obtain full charges.

    If your using time ( maybe the only option on some equipment ) the best way is when absorb ends is to take a hydrometer and get a SG reading of the SOC. That you have to balance against the temperature of the cell. This way you'll be able to adjust the absorb time for a full charge.
    .
  • westbranchwestbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Battery max charge rate?

    Have you considered gen use for that septic pump and battery for all else?

    Or Generator-support to handle the pump load and PV to support all other loads?

    http://forum.solar-electric.com/showthread.php?18459-Demonstration-of-Generator-Support&highlight=generator+support
     
    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,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
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