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Inverter Size vs Battery Size Tutorial.

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  • Inverter Size vs Battery Size Tutorial.

    OK hopefully I can shine some light on a serious subject that comes up frequently on this and other forums. It is serious because there is real danger of fire when relationship of inverter power, battery capacity, and cable size, and contact resistance are blown out of proportion. Especially at the extremely low voltages battery inverters use of 12, 24 and 48 volt batteries.

    The problem is the mathematical relationship of current, voltage, power, and resistance which is mathematically expressed by Ohm’s Law which you can search and see. What is important when operating at low voltages it take high current to obtain high power. High current means large expensive conductors and electrical components. It also means a considerable increase in risk of fire at higher current levels. When you operate at high current levels requires special attention to termination resistance and voltage losses. That requires very expensive tooling, training,equipment that the DIY cannot typically afford or knows anything about.

    Ever wonder why electric companies use extremely high voltages to transport power? It is because of the relationship of power, voltage and current. Power or Watts = Voltage x Current. Example of the extreme is to transport 1 million watts (Small potatoes for a POCO): At 1 million volts takes 1 amp of current or a conductor the size of a toothpick to transport a hundred miles. At 100 volts takes 10,000 amps a conductor the size of your fat asses. Get the picture? Now think about the expense of such madness at low voltages.

    Same logic applies to battery inverters. There are limits on what you the DIY types are completely unaware of. Those limits are battery size, cable/wire sizes, voltage/current relationship, heating, tooling, connectors, and most importantly FIRE. Ever see 2000 watt 12 volt inverters? Do you own one or want one? Well you are playing with FIRE.

    A 12 volt 1000 watt inverter requires roughly 90 amps of current at full power. If you push the limit of copper cable requires a minimum of a #6 AWG conductor, in reality a #4 AWG or larger conductor. Well unless you have a $1000 special tool and training to terminate conductors for #4 AWG and larger is out of reach for most DIY. So 6 AWG and 90 amps is a limit you do not want to mess with. Once you get above 100 amps, you had better know what you are doing and have specialized training with a several years of supervised experience. Otherwise you are playing with Fire

    You think I am making this stuff up? Think again. The largest amperage charge controller is 80 Amps! A 80 amp MPPT Charge Controller has the generic Power input limitations vs. Battery Voltage:
    • 1000 watts @ 12 volt
    • 2000 watts @ 24 volt
    • 4000 watts @ 48 volt


    Interesting relationship huh? The same relationships extend into batteries of how much current you push into them and take out of them in a given amount of time. Which brings up the question of TIME. Energy and Batteries measurements are expressed in units of Time of Watt Hours and Amp Hours respectively. Watt Hours = Watts x Hours and Amp Hours = Amps x Hours. Pretty simple math any 5th grader can understand.

    Batteries are rated in Amp Hour, usually specified at a 20 hour charge or discharge rate. So if you have a 100 AH battery means it can be discharged for 20 hours at 5 amps until exhausted. Mathematically expressed as C/20 where C = the battery AH rating and 20 is the number of hours. It is important to understand that going forward. 100 AH / 20 hours = 5 amps

    As stated batteries have maximum charge and discharge rates they can withstand. It is related to the batteries Internal Resistance. The maximum rate depends on the battery chemistry type like Lead, Nickel, Iron, or Lithium, and how they are constructed. For Renewable Energy applications we use secondary lead acid chemistry. Lead Acid Chemistry comes in two flavors of Flooded Leas Acid (FLA) and Sealed Lead Acid (SLA). In those two groups are more sub categories which I will not go into other than to say for RE we want true Deep Cycle Batteries of either FLA (preferable because they have longer life cycles, and AGM. Both have advantages and disadvantages over each other.

    OK as a general rule the maximum charge/discharge current for FLA batteries is C/8, and SLA/AGM is C/4. Some AGM’s can Charge/Discharge at higher rates of C/2 to 1C, but those are exceptions. Now lets tie it all together as it relates to Battery Inverters, cable size, and current.

    A FLA battery you can go as high as C/8. Got it? So how much current can a 12 volt 720 AH battery provide? C= 720 AH right? So C/8 = 720 AH / 8 H = 90 amps. Where have we heard 90 amps before? Using Ohm’s Law we know 12 volts x 90 amps = 1080 watts. Just about exactly what a 1000 watt Battery Inverter uses at full power. How many of you have a 1000 watt inverter with a 12 volt 720 AH battery? To clue you in that is about 7 car batteries.

    Go back to the MPPT Controller relationship again. A 80 Amp MPPT controller can only handle roughly a 1000 watt solar panel input. Have you put the dots together yet. Panel Wattage = Roughly Inverter wattage with a battery rated 8 time larger in current for a FLA battery?

    AGM we can use a 360 AH battery because they can handle higher Charge Discharge currents of about C4, and some up to C/2 or 1C. So with a 12 volt 1000 watt inverter we can go as low as 100 to 350 AH battery. But of course there is a catch, we only get 1/8 to ¼ run time as opposed to a properly sized FLA battery.

    So what can we conclude here? Well for one a good rule of thumb is inverter size should not exceed:
    • 1000 watts @ 12 volt
    • 2000 watts @ 24 volt
    • 4000 watts @ 48 volt


    Bet you do not like that idea huh? Also bet some of you are looking at your setup wondering where you went wrong. I can answer that.
    MSEE, PE

  • #2
    Inverter Size vs Battery Size Tutorial.
    Originally posted by Sunking View Post
    OK hopefully I can shine some light on a serious subject that comes up frequently on this and other forums. It is serious because there is real danger of fire when relationship of inverter power, battery capacity, and cable size, and contact resistance are blown out of proportion. Especially at the extremely low voltages battery inverters use of 12, 24 and 48 volt batteries.
    Great post, Dereck. Should be a sticky in this 'off grid' section.

    Comment


    • #3
      Originally posted by FloridaSun View Post
      Inverter Size vs Battery Size Tutorial.


      Great post, Dereck. Should be a sticky in this 'off grid' section.
      done and closed to keep it clean and not full of extraneous stuff
      NABCEP certified Technical Sales Professional

      [URL="http://www.solarpaneltalk.com/showthread.php?5334-Solar-Off-Grid-Battery-Design"]http://www.solarpaneltalk.com/showth...Battery-Design[/URL]

      [URL]http://www.calculator.net/voltage-drop-calculator.html[/URL] (Voltage drop Calculator among others)

      [URL="http://www.gaisma.com"]www.gaisma.com[/URL]

      Comment

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