Re: [RE-wrenches] Depth of discharge

[Allan Sindelar]

William, Here I am repeating the RE equivalent of GAAP, Generally Accepted Accounting Principles, as applied to battery sizing... A battery is considered empty at 10.75V (12V), or 1.79 VPC. When you test a battery's capacity, it's generally down to this point. How many amp-hours that is, and what actual percentage that represents, depends on several factors: C/rate (Peukert's Exponent), temperature, programmed charge/discharge efficiency, etc. It's at best a fuzzy number. The 10.75V is the historic rule of thumb that represents "empty" - I suppose that would be 100% DoD. I don't know then what 80% DoD would be - perhaps 11.4V (1.9 VPC) or so? I believe that Windy followed these principles in designing his sizing spreadsheet in the 1980s: the historic rule was, and still is, "don't ever discharge below 20% SoC (80% DoD) or some permanent damage will result". This rule translates on the spreadsheet to [(calculated amp-hours needed)/0.80 max DoD = rated capacity of battery bank in amp-hours (at the C/20 rate)]. Does that clarify anything? Allan Allan Sindelar al...@positiveenergysolar.com NABCEP Certified Photovoltaic Installer NABCEP Certified Technical Sales Professional New Mexico EE98J Journeyman Electrician Positive Energy, Inc. 3201 Calle Marie Santa Fe, New Mexico 87507 505 424-1112 www.positiveenergysolar.com On 3/25/2011 5:40 PM, William Miller wrote: Allan: Is it possible to discharge a battery to 80%.  What is the battery voltage one would expect with that DOD? William At 02:28 PM 3/25/2011, you wrote: William, I use 80%, as the intent is that this is maximum, not typical. It's directly used to calculate the battery capacity on the spreadsheet. 50% is more of a target occasional DoD, but it's not a maximum. In the sunny Southwest where Windy and I are planted, that's pretty safe, as multiple days without sun happen, but not frequently. It may be less so in Atascadero. Allan Allan Sindelar al...@positiveenergysolar.com NABCEP Certified Photovoltaic Installer NABCEP Certified Technical Sales Professional New Mexico EE98J Journeyman Electrician Positive Energy, Inc. 3201 Calle Marie Santa Fe, New Mexico 87507 505 424-1112 www.positiveenergysolar.com On 3/25/2011 1:48 PM, William Miller wrote: Friends: I am using a modified version of the Dankoff load chart (with credit given) to calculate off-grid loads.  There is a section where you enter the maximum depth of discharge (DOD) for a battery bank.  It is my understanding that the maximum DOD for a lead acid battery bank is 50%. Is this correct in concept? Is this a correct value to enter in this cell? Thanks, William Miller PS:  I called Ronald Parades at Trojan before this e-mail was sent and here is what he said:  He recommends 20%-30% DOD.  Any greater DOD results in shorter battery life.  He noted that in many approaches a design criteria of multi-day autonomy which is rarely or never used actually calculates into the final AH result as a virtual decrease in DOD.

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