Generator sizing

bentherebenthere Solar Expert Posts: 113 ✭✭✭
I've seen reference to sizing a generator to run efficiently at 5% to 13% of the battery bank capacity. Does that mean that my generator needs to be able to hit 13%, or just operate in the 5% to 13% window?

Thanks!

Comments

  • BB.BB. Super Moderators, Administrators Posts: 30,176 admin
    Re: Generator sizing

    The 5%-13% rule of thumb applies the battery rating (~20 hour rating in AH * 0.05-0.13). Not a hard and fast rule--but very good for sizing/pricing systems quickly. Can always adjust the details for actual needs.

    The 5% is, roughly, the minimum current to get a flooded cell battery to "boil" (hydrogen gas production) hard enough to mix the electrolyte--needed in taller/larger cells/batteries.

    The 13% maximum is again for a standard storage battery and represents the maximum current before the batteries start to significantly overheat.

    There are some battery constructions that support operation outside of this range... AGM is one type of battery that can. Does not require equalization, and can accept higher current charging (upwards of 20% or more).

    Also, below 5% takes a long time to recharge batteries--and does not support attached loads (battery oversized for load?). And above 13%, that is a lot of generation capacity that may not be used (waste of solar panels or oversized generator and wasted fuel as the charging current is reduced for the last 10-20% of the charging cycle).

    As always, read the operators manual for exact requirements.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • bentherebenthere Solar Expert Posts: 113 ✭✭✭
    Re: Generator sizing

    Are we talking about a charge in amps or amp-hours?

    Curious because the PV array sized by this online calculator would not reach the charge rate in amps: http://store.altenergystore.com/calculators/off_grid_calculator.php

    Thoughts?

    Thanks!
  • BB.BB. Super Moderators, Administrators Posts: 30,176 admin
    Re: Generator sizing

    Watt*Hours=Amp*Hours * Volts
    Amp*Hours= (Watts/volts) * Time (in hours) = ( Volts * Icurrent / Voltage ) * Time (in hours)

    Electrically, Amp*Hours is just Watt*Hour divide by the voltage--that is why the voltage needs to be stated when working out the numbers.

    Regarding a battery--It turns out that a battery (any type) is almost 100% efficient when measuring Amps*Hours... Watt*Hours (the voltage) changes over time and if charging discharging--so Watt*Hours has the ~10-20% losses (plus there are "losses" when equalizing a battery in both AH and WH).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • bentherebenthere Solar Expert Posts: 113 ✭✭✭
    Re: Generator sizing

    Bill,

    When you mentioned modules, I thought I'd run a double check on the web-based worksheet I have been working on. The numbers didn't work out.

    Let's say that my load is 4000 watt hours a day. Six times that, for a rough estimate, is 24000 watt hours. My battery bank will be 48 volts which leaves me looking for a 500 amp hour battery bank.

    I had taken the 5% to 13% charging rate to mean that I would want to be able to charge at a rate of 25 to 65 amps at 48 volts with my generator. (1200 to 3120 watts)

    The PV array suggested by the web calculator using these figures is only 866 watts.

    Where have I gone wrong?

    Can anyone point me to a good explanation of system sizing?

    Thanks!
  • BB.BB. Super Moderators, Administrators Posts: 30,176 admin
    Re: Generator sizing
    benthere wrote: »
    Bill,

    When you mentioned modules, I thought I'd run a double check on the web-based worksheet I have been working on. The numbers didn't work out.

    Let's say that my load is 4000 watt hours a day. Six times that, for a rough estimate, is 24000 watt hours. My battery bank will be 48 volts which leaves me looking for a 500 amp hour battery bank.

    I had taken the 5% to 13% charging rate to mean that I would want to be able to charge at a rate of 25 to 65 amps at 48 volts with my generator. (1200 to 3120 watts)

    The PV array suggested by the web calculator using these figures is only 866 watts.

    Where have I gone wrong?

    Can anyone point me to a good explanation of system sizing?

    Thanks!

    I used the link to the Alternative Energy Store' calculator, and I found that they recommended 1,130 watts of solar panels (for New York City).

    1,130 watts / 56 volts nominal charging = 20 amps

    A bit on the low side, but closer than you have.

    If I use "my" PV Calculator... and you want 4,000 Watt*Hours per day... Use: 1kW of solar panels (easy to scale), 0.52 derating for off-grid flooded cell battery with AC inverter--and I like to assume 9 months of the year it will produce your 4,000 Watt*Hours per day for NYC (*30 days * 1/1,000 = 90kWhrs per month minimum).

    The 4th lowest month's production is Febuary at 61 kWhrs per month. Or to scale:

    91 kWhrs needed min per month * 1/61kWhrspermonth * 1kW of panels = 1.49 kW of solar panels (1,490 Watts)

    1,490 watts / 60 volts battery bank charging voltage = 25 amps

    Or right at the 5% of the minimum charging current recommended by the "rule" of thumb.

    Remember that these rules of thumb make many assumptions which are modified by other assumptions (like where the panels are located, what the weather is like) and such.
    added by niel-the 5% minimum charge rate is not dependent on what type of charge source it is, but that the battery needs this and has nothing to do with if one would orient a pv upsidedown in a snowstorm or if it's from an oblique axis wind turbine through an electric chair switch. what one should know is that there are grey areas to the rule of thumb meaning the rule of thumb isn't that well defined that at a 4.99% rate of charge would become no good. this rule of thumb is for what the battery sees after all charge sources and loads are accounted for regardless of what they are or are from.

    Running your batteries at 4% max charge rate is probalbly not going to cause an issue--but running at 2.5% may.

    I like the rules of 2 and 10 that we use in engineering... If something is within a factor of 2 or 1/2 of something--they are close enough to be almost identical. If somthing is 10x larger or 1/10th smaller, then the larger "thing" will swamp the smaller "thing" and it can be ignored.

    That is why, what you did by checking the first results with other rules of thumbs/recommendations is a good idea. The first calculations just get you in the rough area of a system suggested design. Going back and looking at the details will make sure the system performs to your needs/requirements.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • MalasombraMalasombra Solar Expert Posts: 24
    Re: Generator sizing
    BB. wrote: »
    The 5%-13% rule of thumb applies the battery rating (~20 hour rating in AH * 0.05-0.13). Not a hard and fast rule--but very good for sizing/pricing systems quickly. Can always adjust the details for actual needs.

    And what about C100 which " rule of thumb " can be used? Just in case don't have C20 specs.

    Thank you
  • BB.BB. Super Moderators, Administrators Posts: 30,176 admin
    Re: Generator sizing

    The C100 rating make the battery look something like ~10% larger... It really is not going to make much difference. For example:

    C20=100 Amp*Hours
    10% of 100AH=10 amps

    C100=110 Amp*hours
    10% of 110 AH=11 amps

    Again, these are just rough rules of thumbs--If your battery Mfg. has other recommendations, you should make sure to follow them--or understand why not.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • VicVic Solar Expert Posts: 2,987 ✭✭✭✭
    Re: Generator sizing

    Will add my few cents-worth,

    Is is good to have some reserve in the genset to allow for downstream loads, and the effect of inverters (if used as chargers) taking huge gulps of current from the AC in. These large gulps of input current require additional current capability in the generator and its control system. Also if the batteries ar low, a long genset run time will required to charge them. Having some reserve capacity in the genset to accommodate this will give it a longer life. Furthermore, in the power range that you are noting (about 3500 watts), the most common generators in this size range are the portable/inexpensive units, which may not do well on long charge intervals -- just not rated for long duration loads near their maximum. And allowing some room for growth in loads is an additional consideration.

    Some battery manufacturers recommend C/10 (minimum) charge rate capability for at least some recharge cycles (as in Surrette Bulletin #614) It does depend upon the exact battery, tho.

    A further note, My Surrette batteries are rated at 1350 [email protected] hours, and 1900 [email protected] hr rate -- about 40 % difference.

    You have not specified much about your system in this post. Assume that you are off-grid. Most off-gridders need to pump water, and this is often the largest single load on a system. SSOOOO be certain to consider this aspect in the system sizing if there is a chance ou will need water pumping.

    Too many guesses on my part. Good Luck. Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 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.
  • blackswan555blackswan555 Solar Expert Posts: 246 ✭✭
    Re: Generator sizing

    The 5 - 13 % relates to the battery charge current, your batteries are 48 v / 500ah, [email protected] around 50v required, assuming inverter/gen is 230v, 5 - 14 amps AC required, I would add about 20% for losses to be on the safe side,
    Then look at your house loads that will be running at the same time and factor those in on top,
    Things to also take into consideration with your generator:
    which fuel ? you have to de-rate for gas (lpg etc) are you at altitude, again you have to de-rate, how many hours do you expect it to run ? there is the choice of prime power or stand-by set`s, If you are using diesel, once you get to around 25% of it`s rated output, It`s fuel consumption is fairly linear to current demand, which works well, Or if you have a 30kw + constant load, A microturbine is the dog`s dangley`s
    Have a good one
    Tim

    Edited to add, Also depending on your inverter, If it has generator support, you could also factor this in, eg inverter will stop charging and kick in to give the gen a hand if a high load is detected
  • hillbillyhillbilly Solar Expert Posts: 334 ✭✭
    Re: Generator sizing

    All this talk of charge rates is good; in general it seems that there are a WIDE range of ideas, and "rules of thumbs" out there. I would recommend speaking very directly and thoroughly with both the dealer of the batteries AND the manufacturer. I've seen a lot of differing opinions out there, and in the case of my surrette's the manufacturers website bullitin's and the specific recommendations of their tech support were quite different.

    The tech guys (had to call back to get a second opinion) were all adamant that for "ideal" charging a 15% rate was minimal and that rates up to 20-25% were fine.
    Some dealers out there don't seem to be terribly up on battery information either, if you have the choice to deal with a shop that has helpful and informative sales staff that would be well worth the trouble. Given that they are a big investment, and that they will hopefully be a long term investment I feel that it's worth it (even if it costs more upfront) to deal with a shop who will help steer you through any pitfalls or troubles. Ask a lot of questions at the shop, and if they don't seem to understand the details of the batteries they sell (or how to charge and maintain them), then I would move along to the next shop.

    If you try to size a PV array to fit this type of aggressive charge rates you'll find it to be very cost prohibitive, but as long as you have a generator and are not scared to use it you could probably get by with sizing the PV array closer to your actual needs (and size the generator to the needs of the batteries). This came from tech support at Surrette, and may or may not work for your batteries or cycling duty. The advice I was given was that normal daily cycling, and "mostly" charging with a PV array would be ok as long as the batteries got a FULL 100% charge (at a higher charge rate) at least once a month.

    Another really good piece of advice, once you do get your battery bank connected, monitor and record how they behave and make adjustments to your charging parameters based on what the batteries seem to need. For example, pay attention to the SG levels, and water consumption (for flooded); those will tell you a lot as far as weather or not you are indeed charging them ALL the way, and weather or not you are overcharging them. Pay attention to the fact that as they age, batteries will tend to behave a bit differently too.

    Good luck,
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