Inverter/Charger Battery Charge Rate

cdre
cdre Solar Expert Posts: 78 ✭✭
in Off Grid Solar & Battery Systems #1
Looking for a little guidance from the experts. I'm constantly tweaking my rough plan for the off grid home/office I'll be building in Belize in 2 years.

Dialing down to equipment needs, and wondering if I need to step up my inverter/converter size based on battery charge capability.

Home is completely off grid in Belize with large total loads due to office/AC requirements. Looking at total daily consumption of 22kW. I know.... try to cut it down and conserve... I'm planning for split unit ACs, but relatively heavy IT demands during the day + regular household loads lead to a high conservative estimate. Planning my battery bank for a total of 1 day and 2 nights with no charge to reach 50%. That converts to 33kW (681Ah at 48v). Doubled, means a 1,363 AH battery. GB sells one at 1,340 AH. Leaning heavily toward forklift which will require a higher charge rate - I'm assuming 177A at 13%.

With gen support and reasonable loads (no well pumps or anything), I don't need a gigantic inverter. I was originally leaning toward an Outback Radian at 8,000w, but battery charge capacity is listed at 115ADC. Assuming I'll need to better match the inverter/converter charge capacity to the bank. Then I know I have to match my bulk charging to the inverter's needs. Plan is to run a smaller (3kw) in generator support mode, but to have the larger one for heavy bulk charging when needed.

PV array is another topic, but I'm looking at matching the charge rate at 177A adjusted for 77% derate and 58v = 13kW, then pulsing that up by another 1.6kW to cover the overhead of expected daytime office loads.

Real question is on the choice/sizing of the charger/inverter to the large battery bank. Any other observations/comments are of course welcome.

Dropping the battery size and running the small gen more is an option, but diesel in Belize hovers around US$6/gal, so I'd like to leverage the sun as much as possible.
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Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,601 admin
    #2
    Re: Inverter/Charger Battery Charge Rate

    For solar, the rate of charge is Solar Current - concurrent loads... So, in the case when you have AC running during the day. A 100 Amp solar charging current - 50 amp A/C = 50 amp battery charging current available.

    So, can you estimate your daytime "average" AC (and DC) loads?

    What AH size battery bank (assuming 48 volt?).

    Note that the AC inverter does not need to support AC loads with its charging current, so you do not need to "up size" the AC inverter-charger for charging from the genset.

    Now though, you have the question--Is the genset going to be used very often for AC charging + AC loads (like air conditioning or "A/C"), or can you keep the genset a little smaller (just sized for charging and emergency backup--I.e., if the AC inverter is down for repairs, will you have a second "cheap" AC charger too, or will there be signifcant times when running genset + A/C -- Hot muggy/overcast days???).

    And then back to the first question--Sizing the battery bank to the loads--How does the math look for that? XX,XXX Watt*Hours per day, 2 days backup, 50% maximum discharge? Do you size for non-A/C days, and assume that when you run the A/C, you will have lots of "excess sun"?

    For the solar array sizing, what city will you be near?:

    http://solarelectricityhandbook.com/solar-irradiance.html
    Belize City
    Belmopan
    Orange Walk
    San Ignacio

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • cdre
    cdre Solar Expert Posts: 78 ✭✭
    #3
    Re: Inverter/Charger Battery Charge Rate

    Bill, thanks for the quick response.

    Estimated daytime loads 1.6kW. Thinking of a 1,300 AH battery bank at 48v.

    Hoping to run a smaller genset on autostart in generator support just to keep the battery above 50% SOC when needed. Then run a larger genset for bulk charging when I have a string of cloudy days. Pretty hot in Belize most of the year, so AC will be pretty consistent to keep the office as a reasonable working environment. Will work less hard in winter - less sun for PV then as well. I figure I'll use the large genset to ensure I get the battery back up to close to 100% once a week during the rainy season (Jul - Nov) when cloud cover will reduce my daily production.

    I'm not calculating a full 2 day's backup - just calculating 1 average workday use and 2 nights. That helps me keep the battery more reasonable. That's where I calculate a need for a 1,340 AH battery. My consumption is based on every day being an AC day - on weekends when not running the AC I'll have excess production (when the clouds cooperate). Now that I think of it, that will actually help with achieving my once a week full charge.

    Array will be about 12 miles northwest of Belize City.
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  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    #4
    Re: Inverter/Charger Battery Charge Rate

    For / at the present cost of PV panels, you might consider getting another 2 to 5 KW for replacement and possible increase of array size if those clouds do not co-operate as you hope...
    Since you re going to do Opportunity Loading you should study closely your loads, timing in particular, so that even in lesser sunny days the PV can handle say 75% of your loads then the bank will only need to supply the other 25%... Im assuming you get thise pesky afternoon showers often seen in the Caribbean...

    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,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,601 admin
    #5
    Re: Inverter/Charger Battery Charge Rate

    That is a lot of power... How many kWH per day?

    What is your "sun up" kWH vs "sun down" kWH usage (planned)?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • cdre
    cdre Solar Expert Posts: 78 ✭✭
    #6
    Re: Inverter/Charger Battery Charge Rate

    Expecting around 15kWh during the day (good thing is that office hours are roughly the same as hours of sunlight). Remaining 7kWh in the dark.
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  • cdre
    cdre Solar Expert Posts: 78 ✭✭
    #7
    Re: Inverter/Charger Battery Charge Rate

    westbranch, I agree - definitely a cost advantage to adding additional PV.
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #8
    Re: Inverter/Charger Battery Charge Rate

    Have you looked into water cooling for the data center ? And I think Google (or somebody) was looking into just clean filtered air for data centers.
    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 ,

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  • cdre
    cdre Solar Expert Posts: 78 ✭✭
    #9
    Re: Inverter/Charger Battery Charge Rate

    Mike, by no means is it a data center - IT loads will be a few laptops and LED monitors as well as router, modem and network attached drive.

    As I think through sizing the generator and inverter to the battery bank, I remember having set up my current system - an RV in the same location. My original converter (120v to 12v) burned out and I bought a new one capable of 80 amps. I realized after the fact that upsizing was overkill, as the batteries (4 golf carts) were rarely capable of taking that much amperage and then only for a short time. Might be the same case here where pursuit of max charging amperage may result in excessive unused capacity of both the inverter/converter and generator.
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  • cdre
    cdre Solar Expert Posts: 78 ✭✭
    #10
    Re: Inverter/Charger Battery Charge Rate

    Any further feedback on how important it will be to meet the battery's needed charge rate with the sizing of the inverter and generator. Thinking my focus will be on charging the bank with PV rather than generator, so likely not a huge issue should I not be able to throw max amperage at them. Can anyone confirm this logic?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #11
    Re: Inverter/Charger Battery Charge Rate
    cdre wrote: »
    Any further feedback on how important it will be to meet the battery's needed charge rate with the sizing of the inverter and generator. Thinking my focus will be on charging the bank with PV rather than generator, so likely not a huge issue should I not be able to throw max amperage at them. Can anyone confirm this logic?

    Well you definitely need to meet a decent charge rate from the PV because you only have 'so many' hours to charge in.

    From the generator you need to meet a minimum amount to have any 'respectable' amount of charging, but usually you want a high rate from the gen so you don't have to run it so long. If the batteries' minimum (net) current is met and the generator is fairly well loaded (>50% capacity at maximum charge rate) it should be fine.
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  • cdre
    cdre Solar Expert Posts: 78 ✭✭
    #12
    Re: Inverter/Charger Battery Charge Rate

    I expect I will "over-panel" the system to cover my daytime loads. Not having those loads over the weekend, I can probably plan to leverage PV + Gen to get the batteries back up to 100% periodically (assuming the weather cooperates).
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