How not to kill your backup batteries

DOD is mainly known for the Department of Defense but it also means Depth of Discharge. DOD is the amount of reduction of a battery’s capacity from it’s fully charged state. It’s usually measured as a percentage.  So backup batteries that are at 50% DOD is half-empty (or half-full, if you are more of an optimist).

DOD is used to predict what a battery’s cycle life or lifespan will be. The harder backup batteries have to work, the sooner they will fail. As you can see in the chart below, the shallower the average discharge, the longer the life. This is why it’s important to size a battery system to deliver at least twice the average power required, to assure shallow discharges.

Depth of discharge vs. lifetime of a battery

Depth of discharge vs. lifetime of a battery.
* You may experience longer or shorter life based upon application, charging regimen, temperature, rest periods, type of equipment, age of battery, etc.

For instance, if a Sealed Lead Acid (SLA) or Value Regulated Lead Acid (VRLA) battery is made to last 1000 cycles at a DOD of 80% – or about 2 years and 9 months – when it would be considered “expired”.  All common battery technologies are affected by DOD cycling, but not all in the same way. The previously mentioned lead acid battery that lasts less than 3 years when cycled to 80% DOD can last up to 10 years or longer if only cycled to 20% DOD. Lithium chemistry batteries can cycle to 70 or 80% DOD on a daily basis and still attain a 10 year lifespan.

The solar UPS controller that is the “heart” in my one fridge off the grid campaign has the capability to monitor DOD and will only allow 10% DOD to extend battery live to a maximum.

Recommended also: Battery bank voltage, Energy storage in the basementBest Energy Storage, Manifesto

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