Battery bank capacity

Mike at Energy CommissionMike at Energy Commission Posts: 50Solar Expert ✭✭✭✭
I just read the sticky "Battery-System-Voltages-and-equivalent-power" http://forum.solar-electric.com/showthread.php?15989-Battery-System-Voltages-and-equivalent-power I understand the charging considerations can be equal, what about the capacity to power loads?
Using the examples from the sticky, can a 48v, 220 Amp hr BB run a load for the same length of time as a 12v 880 Amp hr BB, solely from the storage capacity?

Comments

  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: Battery bank capacity

    Actually it will run slightly longer.

    The higher Voltage systems are more efficient because the energy is transferred more as Voltage than Amperage so less of it goes to heat.
  • BB.BB. Posts: 27,897Super Moderators, Administrators admin
    Re: Battery bank capacity

    More or less yes...

    More--The batteries are capable of the current.

    Less--The wiring to support high current loads (and charging) is much more expensive as you have 4x the current at 12 volts vs 48 volts, and around 0.5 to 1.0 volt drop on 12 volt wiring vs 2-4 volt drop on 48 volt wiring (for heavy loads/surge current).

    The other issues about lots of parallel batteries in a bank... More cells to check electrolyte levels, more wiring (cost), you should have a fuse/breaker per battery string (more than two parallel strings), instability of charging currents (some strings may have more current charging/discharge than others due to wire length and other issues), a shorted/open cell can take out other batteries and be difficult to notice, temperature differences between cells cause current flow differences, etc...

    My personal recommendation is to try for one string, with two to three parallel strings as a maximum--Use larger AH batteries, and even 6/4/2 volt battery blocks (to keep batteries light enough to move around without a crane/pallet jack). However, there are people that cannot get large AH "cells" and/or only have "standard" batteries available, or just prefer two or more parallel strings (in theory, back up string if one fails), etc... And they are very happy with their batteries.

    I believe that parallel strings require closer attention--But with a hydrometer and these pretty cheap DC Current Clamp DMM's (Digital Multi Meters), you take your time, it is possible to have parallel strings of batteries and be very happy with them.

    More or less, going higher in voltage allows you to keep the peak currents lower--And if you use ~100 amps as your maximum design current for the DC side, that means a rough maximum power for a 12 volt bank would be 1,200 watts; 24 volt bank ~2,400 watts, and so on... When you do the math for even a 1,200 watt inverter running on 12 volts, the actual circuit design is much higher current:

    1,200 watts * 1/0.85 inverter eff * 1/10.5 battery cutoff * 1.25 NEC wiring/breaker derating = 168 Amp minimum circuit/breaker branch circuit design

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Mike at Energy CommissionMike at Energy Commission Posts: 50Solar Expert ✭✭✭✭
    Re: Battery bank capacity

    ah ha! I think I am getting this! it all converts into watt hrs equally and it's the watt hrs that will determine how long the loads can run. I was getting hung up on Amp hr capacity thinking it was Amp hrs *only* that determined load run times. At least I think this is right.
  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: Battery bank capacity
    ah ha! I think I am getting this! it all converts into watt hrs equally and it's the watt hrs that will determine how long the loads can run. I was getting hung up on Amp hr capacity thinking it was Amp hrs *only* that determined load run times. At least I think this is right.

    That's right: Watt hours is a quantitative measurement of electrical power.
  • vtmapsvtmaps Posts: 3,738Solar Expert ✭✭✭✭
    Re: Battery bank capacity
    That's right: Watt hours is a quantitative measurement of electrical power.
    Since we're having a lesson on units....

    'Watts' is a quantitative measurement of electrical power
    'Watt hours' is a quantitative measurement of electrical energy

    BTW, I am equally guilty of this very common misspeak :blush: --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: Battery bank capacity
    vtmaps wrote: »
    Since we're having a lesson on units....

    'Watts' is a quantitative measurement of electrical power
    'Watt hours' is a quantitative measurement of electrical energy

    BTW, I am equally guilty of this very common misspeak :blush: --vtMaps

    Uh, no.
    But then I'm not being paid to teach English here so I won't bother. Otherwise I'd be spending all my time correcting spelling, punctuation, grammar, and usage. Trust me.
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