Re: [RE-wrenches] Depth of discharge

[Kent Osterberg]

Wrenches, I noticed an error in my reply below when Allan's second comments arrived. Correction: the battery is considered 100% discharged (not 80%) at 10.5 volts (1.75 vpc) under the conditions of use. Kent Kent Osterberg wrote: William, I'll try to add a little to Allan's and Ronald Parades' comments.  Nearly every battery manufacturer specifies a maximum 80% depth of discharge (DOD); that corresponds to about 10.5 volts for a nominal 12-volt battery. Notice how close that is to the default setting used by many inverter manufacturers for starting the generator right away. For deep-cycle batteries, a cycle life in the low hundreds should be expected for a battery that is routinely discharged to 80% DOD. Designing for a lower DOD will result in longer battery life. I've got a table from Ronald that shows the L16RE battery life expectancy is about 7 years for a 25% daily DOD. Since that is their warranty target, it seems like they should specify a 25% daily average DOD in the battery instructions. If charging primarily with a generator, 50% DOD before starting the generator is good compromise (but the generator shouldn't have to start every day, every other day maybe). Fuel economy really falls off the chart while charging above 90% SOC. So if you are starting a generator at 75% and charging to 100% it'll use a lot more fuel that using the generator to charge from 50% to 80% or 85%. Trouble is that routinely charging to only 85% SOC will lead to sulfation problems. That can shorten the battery life too. Kent Osterberg Blue Mountain Solar, Inc. Allan Sindelar 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. Wm Miller Solar Voice :805-438-5600 email: will...@millersolar.com http://millersolar.com License No. C-10-773985

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