Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Battery Sizing for AC system

  1. Jul 24, 2017 #1

    Interested in sizing some DC lithium-ion batteries (through a controller) to power the following system: 90KVA, 200/115 Volts AC, 400 Hz, 3-phase.

    The current approaches I've taken have been drastically oversized so I'm clearly missing something.

  2. jcsd
  3. Jul 24, 2017 #2


    User Avatar

    Staff: Mentor

    How long does it need to produce 90 kva?

    My understanding is that Tesla does 88kvah by paralleling thousands of small standard cells...
  4. Jul 24, 2017 #3


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Perhaps show us your calculations. They might be correct.
  5. Jul 24, 2017 #4
    It needs to produce for around two hours, yes I'm currently looking at utilizing Nissan Leaf battery packs which have a capacity of approximately 24 kWh.
  6. Jul 24, 2017 #5
    To elaborate, CWatters, I'll step you through my process back to front. To obtain the effective power derived from the AC system I calculated the rms current and voltage, which then using a power factor of 0.9 (assumption as cannot find specific to the system) and a added factor of sqrt(3) for the three-phase aspect provided the following:

    V_rms=V_max / sqrt(2) = 200 V/sqrt(2)=141.4 V
    I_rms=90 KVA / V_rms = 636.4 A
    P_eff = V_rms * I_rms * PF * sqrt(3) = 140 kW

    Which would indicate for two hours of operation I would need a capacity of at least 280 kWh.

    Any feedback would be appreciated, come from a mechanical background so am not confident with my methodology.
  7. Jul 24, 2017 #6


    User Avatar

    Staff: Mentor

    I would think you are given voltage in RMS (it almost always is), but I'm not familiar with a 200V system...

    In either case, since you already have kVa, you don't need to do anything other than correct it for power factor: 90*.9=81 kW....and for 2 hours, is 162 kWh.
  8. Jul 24, 2017 #7
    Thanks for the feedback russ_waters, still larger than I was expecting but it's good to know the maths is sound.
  9. Jul 27, 2017 #8
    Not enough info:

    What is the "system" - 90 KVA may be the rating but at what rate does it use real power, not just Watts vs LVA, but I doubt it runs at FULL power for 2 hours. That spec looks like a UPS? -- The UPS, if it is controlling the battery, needs to be connected to the same battery technology. So if it was built for Lead Acid - you can not just change it to LiPO. The issue is the charging of the LiPo - discharge not as much of a concern.

    The load - may use 90 KVA and use very little power - so the system can be running at maximum rating, but the battery supplying little power.

    In an AC system - KVA can be "no" real power, but a confusing point is that in a DC system ( battery) - VA is effectively the real power. Batteries are rated in Amp-Hours, but you have to look at the datasheet to see where this rating was developed, the RATE of discharge affects the total A-H delivered, and the rating is to what Voltage.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted