Greetings, All~ Mark Frye had asked: "...with a 4000w Outback Radian, AC coupled with 2000w of PV, what is the smallest Li battery I would want for stable operation?"
Mick's reply: In the lithium arena, one often finds the ratings in kilowatt-hours rather than in amp-hours. This is an easy conversion for us energy nerds to make, so below I'm mainly using watts & watt-hours with occasional conversions to amp-hours. I suggest that you start with the spec sheet (of whichever brand lithium battery is on your radar), & see what the manufacturer says about "allowable rate of recharge". You may find that C/2 is an acceptable rate with several manufacturers, Mark--much faster than what we're accustomed to with typical lead-acid batteries. Let's assume: * ...that your AC coupled PV array crests around 1800 watts to the battery when there are no loads * ...that the battery is below 100% state of charge, and * ...that the solar conditions are optimal. Let's further assume that those conditions represent your battery's highest energy rate in either direction. 1800w X two hours = 3,600 watt-hours dividing by 48v nominal = 75 amp-hours in the above hypothetical situation. ++++++++++++++++++++++++++++++ The rate at which a battery can receive energy is (usually) also the rate at which it can comfortably deplete, so let's briefly look at the flip side: Assuming the battery mfr. wants the watt-hours to be no less than two times the maximum wattage, a 3600 watt-hour (C/2) battery should not be depleted faster than 1800 watts. Your 4kW Radian, Mark, could exceed that if the loads demand it--& maybe your loads don't. Motor starting surges should probably be part of the thinking & it wouldn't hurt to ask the mfr.'s opinion about short term surges which briefly deplete the battery faster than the hypothetical two hour rate. By approaching this question based on the battery manufacturer do's & don'ts, one improves the chance of getting warranty coverage should it later be needed. "What the large print giveth, the fine print taketh away." The Wrench List is the Bomb! Mick Abraham, Proprietor www.abrahamsolar.com Landline: 970-731-4675 Cell phone or for text messaging: 970-946-6584 ᐧ
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