Small loads (power wise) but high use

rabbit_39
rabbit_39 Solar Expert Posts: 43 ✭
in Solar Beginners Corner #1
So I have a usage of 6kwh/day (from counting power of devices and hours used/day). Even though the devices are small they're used for long periods (200W for 8 hrs, 40w for 24 hrs, etc).

So I can use an 800W Victron inverter (so I can use the ATS and auto gen start for backup), plus a midnite solar classic charger. Now, the question is, should I use 48V or 24V? the cable run from the batteries to charger, batteries to inverter are quite short (less than 10 feet).

I was thinking that 24kwh of batteries would be enough ( 40% DOD for 2 days when I account for losses) and 2400Wp of panels.

Hopefully I used the rules of thumb that have been used here correctly.

Thank you!
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #2
    Re: Small loads (power wise) but high use

    What is your peak loads.?

    Consider using two inverters, one for the peaks, and a different smaller one for ongoing loads. inverters are most efficient when used nearer their capacity than hen only being loaded 10% for example.

    but he way welcome to the forum.


    Tony
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,642 admin
    #3
    Re: Small loads (power wise) but high use

    You can also add the inverter losses (around 0.85 efficiency)--if you did not... 2 days of no sun and 50% maximum discharge:
    • 6kWH * 1/0.85 eff * 2 days * 1/0.50 maximum discharge = 28 kWH of battery storage.
    You did not say where the system would be installed/how much sun you would get... Assuming 4+ hours of sun per day for 9 months of the year (fairly sunny location) and 0.52 for solar panel/charge controller/battery losses/inverter efficiency system derating:
    • 6,000 watts * 1/0.52 system derating * 1/4 hours of sun = 2,885 Watts of solar panels
    Pretty close to your estimate.

    You have the choice of 12/24 or 48 volt battery bank... The size of charge controller for the various battery bank voltage (0.77 is the typical worst case derating for solar panels and charge controller losses):
    • 2,885 Watts * 0.77 typical derating * 1/14.5 volts charging = 153 amp controller
    • 2,885 Watts * 0.77 typical derating * 1/29 volts charging =76 amp
    • 2,885 Watts * 0.77 typical derating * 1/58 volts charging =38 amp
    So, if you go with 48 volt battery bank, you may be able to use a smaller charge controller (and hopefully less expensive).

    "Optimum" charge rate for the battery bank around 5% to 13% of bank capacity:
    • 28,000 WH battery bank * 0.05 rate of charge * 1/0.77 derating = 1,818 watt minimum
    • 28,000 WH battery bank * 0.10 rate of charge * 1/0.77 derating = 3,636 watt nice size array
    • 28,000 WH battery bank * 0.13 rate of charge * 1/0.77 derating = 4,747 watt maximum "cost effective array"
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • rabbit_39
    rabbit_39 Solar Expert Posts: 43 ✭
    #4
    Re: Small loads (power wise) but high use

    Luckily, my peak load is also not much more than nominal load which is 450W. The only motor/inductance loads are small fans (40 watts total) and 36 watts of CFL. Other than that the rest are electronics components (radio repeater, handheld radio charger, etc).

    The location is in Kalimantan, Indonesia. Less than 1 degree south of the equator. Average annual of 4.69PSH and minimum of 4.2PSH if I remember right from NASA's page.

    Thank you all in advance and for all the info I've been frantically trying to absorb. Hope you guys don't mind me posting system specs and asking questions about them. I see a lot of systems here and often not sure if they're designed correctly (don't even talk about safety or NEC compliance).

    Thanks!
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,642 admin
    #5
    Re: Small loads (power wise) but high use

    Not a problem... Everyone is here to help (and learn).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • rabbit_39
    rabbit_39 Solar Expert Posts: 43 ✭
    #6
    Re: Small loads (power wise) but high use

    Another quick question, does the 5-13% charge rate apply to OPzV batteries? Or do they have a different charge rate?

    I noticed that most people here are using either 12V batteries or 6V batteries rather than the 2V cells. Is there a reason for that?

    Thank you!
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #7
    Re: Small loads (power wise) but high use
    rabbit_39 wrote: »
    Another quick question, does the 5-13% charge rate apply to OPzV batteries? Or do they have a different charge rate?

    Best to ask the manufacturer that one. The 5-13% is a rule-of-thumb that works well with most FLA batteries. Even then there are adjustments that need to be made for particular models, such as the "tall case" L16's wanting more current than something like a T105.
    I noticed that most people here are using either 12V batteries or 6V batteries rather than the 2V cells. Is there a reason for that?

    Thank you!

    Simple economics: you can get the FLA's, particularly the "golf cart" type 6 Volts, readily and cheaply for the Amp hours provided. They are not the best in performance - until you factor in the costs. Easier to move about that forklift batteries too. :roll:
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  • rabbit_39
    rabbit_39 Solar Expert Posts: 43 ✭
    #8
    Re: Small loads (power wise) but high use
    Cariboocoot wrote: »
    Simple economics: you can get the FLA's, particularly the "golf cart" type 6 Volts, readily and cheaply for the Amp hours provided. They are not the best in performance - until you factor in the costs. Easier to move about that forklift batteries too. :roll:

    Got it. I asked about that too, especially since when calculated for averaged cost, a good AGM seems to be the best value. The answer I got was that since logistics for replacement is often VERY difficult, it's best to do it once and be done with it.

    Yep, almost 100lbs per cell on the 500Ah OPzVs if I remember right.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,642 admin
    #9
    Re: Small loads (power wise) but high use

    5% is the minimum because many batteries require at least that amount of current for proper mixing. Also, if you have less than 5% rate of charge, it takes a long time to recharge the battery bank (more sulfating issues) and self discharge of a bank becomes a larger factor (flooded cell batteries and especially forklift batteries have much higher self discharge as they get old). AGM's have lower self discharge and no mixing requirements--so you might go a bit lower than 5% rate of charge--but it will still take a long time to recharge the bank (and still sulfating the longer the battery spends below 75% state of charge).

    For the 13% rate of charge--Flooded cell batteries can overheat if forced >C/8 (12.5%) rate of charge for a long period of charging. Also, batteries after they are over ~80% state of charge tend to reduce charging current--so larger arrays tend to be a waste if most of the time your batteries are >80% state of charge.

    For AGM, they can take much higher charging current (for Concorde, they can take 4C (400% of rated capacity)--However, that tends to be a "waste" of money to recharge a battery in 15 minutes. And have the panels do nothing for the rest of the day (assuming you can put heavy enough copper cables and solar charge controller to run that much current).

    So the 13% rate of charge tends to be more of an "economic" limit. Some battery manufacturers recommend ~10% minimum rate of charge--so you will see that recommendation too--especially for "tall" flooded cell batteries.

    The larger your bank vs load (i.e., more towards 3+ days of "no sun" and 50% maximum planned discharge) many times means that the array is sized on the minimum charging current vs the actual amount of energy needed to power your loads every day.

    So--we tend to do the solar array calculations twice--First based on your daily power needs. And second based on the rate of charge needed for a "happy " battery bank.

    "Too Large" of battery bank tends to be an expensive problem. Need lots of solar power to keep properly charged, and potentially a lot of generator run time during winter use to keep the battery from spending too much time below 75% state of charge (where the batteries tend to sulfate and permanently lose capacity).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #10
    Re: Small loads (power wise) but high use

    Is there any reason you've chosen the OPzV batteries? They're gel batts and so will be more expensive than the flooded type OPzS and will be more sensitive to overcharging.

    OPzS really requires very little maintenance, they're also 2V cells with a clear cases and a huge amount of spare electrolyte above the plates. Typically they only need to be topped up with water once a year. You also get the added benefit that you can check the state of the battery with a hydrometer, which you can't do with the sealed OPzV batts.
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  • rabbit_39
    rabbit_39 Solar Expert Posts: 43 ✭
    #11
    Re: Small loads (power wise) but high use
    stephendv wrote: »
    Is there any reason you've chosen the OPzV batteries? They're gel batts and so will be more expensive than the flooded type OPzS and will be more sensitive to overcharging.

    OPzS really requires very little maintenance, they're also 2V cells with a clear cases and a huge amount of spare electrolyte above the plates. Typically they only need to be topped up with water once a year. You also get the added benefit that you can check the state of the battery with a hydrometer, which you can't do with the sealed OPzV batts.

    The application for these installations are very remote (one I was looking at recently required a 300 mile/12 hour drive and 2 hour speed boat to get to), so low maintenance is required. As in none, if possible.

    Also, availability of OPzS in Indonesia isn't too great. A lot of times the design is less than ideal based solely on the available components.

    Thank you for reminding of that though, I'll look into OPzS more and see if there are applications where we can use it.
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  • rabbit_39
    rabbit_39 Solar Expert Posts: 43 ✭
    #12
    Re: Small loads (power wise) but high use
    BB. wrote: »
    "Too Large" of battery bank tends to be an expensive problem. Need lots of solar power to keep properly charged, and potentially a lot of generator run time during winter use to keep the battery from spending too much time below 75% state of charge (where the batteries tend to sulfate and permanently lose capacity).

    -Bill

    Thank you all again for the great info. I think I'll be saying that a lot :-D

    There's another system I will ask about soon, speaking of Too Large. Currently 2 parallel banks of 48V/420Ahx2V cells OPzV batteries. He wants to add 2 more banks of 48V/500Ah x 2V cells OPzV. Anyway...will post details later.

    "Winters" in Indonesia tend to be short. And days without a good mid-day sun is very rare (in places I've observed, I haven't been to very many places in Indonesia nor have I stayed very long at many different places here). According to the NASA data, the places that has the solar panel installations that I know of gets 4+ hours as minimum average monthly insolation.

    Thanks.
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  • TheBackRoads
    TheBackRoads Solar Expert Posts: 274 ✭✭
    #13
    Re: Small loads (power wise) but high use

    You stated you were going to use an existing inverter.. that would be the decision on your battery voltage......:confused:
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  • rabbit_39
    rabbit_39 Solar Expert Posts: 43 ✭
    #14
    Re: Small loads (power wise) but high use
    TheBackRoads wrote: »
    You stated you were going to use an existing inverter.. that would be the decision on your battery voltage......:confused:

    I'm not sure where I said that. If I had said something that was taken as that, it was a mistake. I'm hoping to use a Victron inverter so I can use their built in ATS and use a genset back up with auto start. The Victron does have an auto gen start signal right?

    Thanks,
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  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #15
    Re: Small loads (power wise) but high use
    rabbit_39 wrote: »
    The Victron does have an auto gen start signal right?

    Yep auto start based on load or on battery voltage. It just provides an on off signal, your generator will have to have the facility to start and stop based just on this signal.

    The Victron multiplus look like very nice units, I'd recommend you buy the communication cable too as you can then program it from your PC using their free software.
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