Battery bank sizing

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  • stephendvstephendv Solar Expert Posts: 1,571 ✭✭
    Re: Battery bank sizing

    Bill,

    Thanks for the explanation on plate sizes, makes perfect sense.
  • WindsunWindsun Solar Expert Posts: 1,164 ✭✭
    Re: Battery bank sizing
    bryanl wrote: »
    ...Quantity of lead with a proportionate amount of electrolyte determines capacity. Surface area of lead exposed to electrolyte determines peak current (this gets into surface charge)....

    No, the quantity of lead has nothing to do with it. If you take two cubic 500 pound lead blocks and make a battery with it, it will have very low capacity. However the cycle life would probably be measured in decades.

    As BB noted, surface charge is due to a temporary imbalance in the electrolyte specific gravity in the cell.

    Surface area is not the same as just multiplying plate size dimensions. SLI batteries have a very large surface area because the plates actually look kind of like lead wool or lead sponge pressed into flat plates when new. (in fact, that is how some of the pre-1930's batteries were made). Whle that huge surface area gives a very large instantaneous current, it also wears out very fast if deep cycled.
  • BB.BB. Super Moderators, Administrators Posts: 32,027 admin
    Re: Battery bank sizing

    Here is a quick description of some variations of lead acid batteries:
    Lead Calcium Batteries
    Lead acid batteries with electrodes modified by the addition of Calcium providing the following advantages:
    • More resistant to corrosion, overcharging, gassing, water usage, and self-discharge, all of which shorten battery life.
    • Larger electrolyte reserve area above the plates.
    • Higher Cold Cranking Amp ratings.
    • Little or No maintenance.
    Lead Antimony Batteries
    Lead acid batteries with electrodes modified by the addition of Antimony providing the following advantages:
    • Improved mechanical strength of electrodes - important for EV and deep discharge applications
    • Reduced internal heat and water loss.
    • Longer service life than Calcium batteries.
    • Easier to recharge when completely discharged.
    • Lower cost.
    Lead Antimony batteries have a higher self discharge rate of 2% to 10% per week compared with the 1% to 5% per month for Lead Calcium batteries.

    ...

    SLI Batteries (Starting Lighting and Ignition)

    This is the typical automotive battery application. Automotive batteries are designed to be fully charged when starting the car; after starting the vehicle, the lost charge, typically 2% to 5% of the charge, is replaced by the alternator and the battery remains fully charged. These batteries are not designed to be discharged below 50% Depth of Discharge (DOD) and discharging below these levels can damage the plates and shorten battery life.

    Deep Cycle Batteries
    Marine applications, golf buggies, fork lift trucks and electric vehicles use deep cycle batteries which are designed to be completely discharged before recharging. Because charging causes excessive heat which can warp the plates, thicker and stronger or solid plate grids are used for deep cycling applications. Normal automotive batteries are not designed for repeated deep cycling and use thinner plates with a greater surface area to achieve high current carrying capacity.

    Automotive batteries will generally fail after 30-150 deep cycles if deep cycled, while they may last for thousands of cycles in normal starting use (2-5% discharge).

    If batteries designed for deep cycling are used for automotive applications they must be "oversized" by about 20% to compensate for their lower current carrying capacity.
    Each additive/change to a battery's construction brings some positives and negatives to the table...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • GreenPowerManiacGreenPowerManiac Solar Expert Posts: 453 ✭✭✭
    Re: Battery bank sizing

    While everyone was super educating me on the subject, I decided to rewire both battery banks. Now BOTH banks are set up with +/- Bus bars.

    So far I don't see any difference, however I'm letting the system charge up over a couple days to maximum where the diversion control kicks in. Anyone wishes to see my setup then I'll post a picture or two on my site.

    The SLA's are true deep-cycles, at least that's what they say. Float voltage runs around 12.7 to 12.9 charge. When charging it gets over 14 volts if allowed by the controller. Since the dip switch setting is at 14 volts, it rarely gets over that. As soon as the sun goes down, the power falls back to 12.7 ish.

    Any pictures I post will be the Gels, not the SLA's.
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  • RCinFLARCinFLA Solar Expert Posts: 1,399 ✭✭✭✭
    Re: Battery bank sizing

    I am surprised someone has not jumped on you for putting 15 batteries in parallel.

    I have 40 batteries hooked up on a 48v inverter system as ten, 4 series batteries.

    I get good battery life in my application which is primary UPS to the whole house.

    One thing I can recommend is periodic monitoring of the battery balance. Suggest you invest in a clip-on amp meter (D.C. capable). This allows a quick check on each battery to see how the charge and discharge currents are distributed on each battery. You may see a 30-50% difference at different points in the recharge cycle but this is okay if it balances out over the whole cycle. I have 100 amp current measurement shunts (1 milli-ohm) on each of my ten series legs.

    The reason to keep the net wire length (or specifically resistance, including connectors) equal to all batteries is to help keep the charge and discharge currents balanced avoiding taking or giving more current from a battery with less resistance in its path.

    Another thing you might consider is breaking up the batteries in several groups (like 3 groups of 5 parallel, or 5 groups of 3 parallel) and use Anderson connectors to allow unpluging a battery group for testing and maintanance while keeping inverter on line. I have five Anderson 350 amp connectors on my system. http://store.solar-electric.com/anhicupoco.html
  • bryanlbryanl Solar Expert Posts: 175 ✭✭✭✭✭
    Re: Battery bank sizing

    I think some folks are reacting and not reading.

    Thanks for the examples of the doping actually used and for the 'reduce to the absurd' of the 'deep cycle; thick plates' out of date model for batteries. Good stuff for critical thinkers.

    But the reactions support getting back to some relevant measure for a good base. When someone can cite some relevant measure to support the 'thick plates' and 'deep cycle battery' marketing hype then we will be getting somewhere. Meanwhile, sticking to outdated battery models in this context just doesn't help anyone IMHO.

    Look for actual, objective, relevant measure for what you want to do. All else is just noise.
  • dwhdwh Solar Expert Posts: 1,341 ✭✭✭
    Re: Battery bank sizing
    Why is it that DWH is the one speaking my language ?

    40 amp fuse rated for 8 awg wire. I'm assuming that I want the fuse to pop before the wire heats up. Knowing that, I could use say, 30a inline fuses. I'll give it a shot.

    It's not necessary to downsize the fuse - #8 wire is rated to carry 40a continuously (sometimes even 50a). If all you need is 30a, you could use #10 instead.
  • GreenPowerManiacGreenPowerManiac Solar Expert Posts: 453 ✭✭✭
    Re: Battery bank sizing

    My system is already set up for 12 volts. Converting to a 24 or 48 volt system is costly, isn't it ?

    How would I get my wind turbines up to 48v when each is rated at 12v ? Even in series, I'd need 4 turbines.
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  • BB.BB. Super Moderators, Administrators Posts: 32,027 admin
    Re: Battery bank sizing

    You would need a new inverter and probably a new solar charge controller (the MPPT type usually work on any bank voltage between 12 and 48 VDC).

    And many wind turbines will produce much less useful power on higher voltage battery banks.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • GreenPowerManiacGreenPowerManiac Solar Expert Posts: 453 ✭✭✭
    Re: Battery bank sizing

    Is there a way to increase the voltage without raising rpms on the turbines ?
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  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery bank sizing

    AC output through a step-up transformer.
    DC through a "DC to DC converter".

    Of course if the Voltage goes up, the Amperage goes down because the Wattage is the same. And the conversion will use a bit of power itself.

    Perhaps (and this is a bit zany, okay more than a bit) you could feed two DC turbines into a couple of large, 'stacked' capacitors to even out the fluctuations. This used to be done sometimes when converting AC to DC:

    AC Leg 1 feeds center of serial-connected capacitors
    AC Leg 2 is rectified through two diodes, one feeding (+) pulses to the (+) end of the capacitors and one feeding (-) pulses to the (-) end. For turbines, it'd have to be some really large capacitors. You're not dealing with milliwatts! :D
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