[RE-wrenches] Depth of discharge

[James Surrette]

Evening all, 

A quick note on DOD. 

Batteries / cells are often rated at voltages above and below 1.75VPC (1.7 - 1.85 are "common"), so ratings should be verified with the supplier / model.  However, as anyone that has ever run a discharge test knows, the time / available AH from 1.8 to 1.7VPC is not a lot and generally voltage drops quickly. 

The voltage at 50% DOD will be influenced by the discharge rate with faster discharge rates (6, 8, 10 hr) having lower voltages at various discharge rates than longer discharge rates (72 or 100 hr). 

Regards, 

Jamie 

>>> Allan Sindelar <allan at positiveenergysolar.com> 3/26/2011 5:50 PM >>>
Kent and list,
And I was half awake this morning and realized my error in yesterday's post to which you responded... I said "A battery is considered empty at 10.75V (12V), or 1.79 VPC."
No, it's at 10.5V, or 1.75 VPC, as you have correctly stated. Sorry.
Allan

On 3/26/2011 12:01 PM, Kent Osterberg wrote: 


 
Default Title 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: 


 
Default Title 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. 


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