Battery Life Cycle: Explaining DoD vs Life Cycles

[QwertyXP]

Hello,

I have seen graphs of Battery's DoD (depth of discharge) vs Life Cycles. But at the same time, "life cycle" is defined as the no. of COMPLETE charge-discharge cycles that a battery can perform before its nominal capacity falls below 80% of its initial rated capacity.

Here's the contradiction:
From the definition, the term "life cycle" should only be valid for a DoD of 100% (or maybe 80%, whatever is the max. possible value). How can the battery's life be rated in terms of life cycle? In those graphs, does "life cycle" simply mean discharging to a specified amount (DoD) and then charging rather than COMPLETELY dscharging and charging the battery?

 

[DaLincerGuy]

Yes, you discerned it correctly, the graph measures the life of a battery [defined as having worn to the point of 50% of nameplate capacity] vs the DoD [Depth of Discharge]

However, out in the real world, with scooter and power wheelchair batteries I see lifes much shorter than I would expect based on these graphs.

I see commonly AGM batteries lasting 2-3 years. At one cycle per day, that is ~600-900 cycles. At a mere 10% DoD AGM batteries are expect to last {ON Average} over 2000 and Gel batters over 6000 over 10 years! This does not happen out in the real installations.

Open Questions to me:

1. Why not? Is it something to do with the moving / shaking causing the premature battery failures. Is perhaps the quality / size of charger? Maybe the load current is higher in the scooters? Do not know.

2. The graphs of DoD all start at approximately 10% what happens at 5%?

 

[davetrow]

There's an interesting article http://www.bruceschwab.com/pdf/Li-vs-LA.pdfthat might illuminate what's going on. Apparently, lead-acid batteries, regardless of the type, have a fairly narrow charge-discharge window, and going outside it stresses them. That's why car makers are so interested in Li-ion, which has its own set of problems, of course.