Battery Deterioration

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"Rated" Cycles

Remember! Manufacturer "rated cycles" are attainable only if you follow the "recommended" factors. If you do read the manufactures specs you will likely be dismayed at the difference between the maximum and, in small print, the recommended.

Perhaps most noticeable is the disparity in high drain RC airplane Li-Po ... the 2-3 hundred "rated" cycles providing 2-3 dozen usable cycles!

More common is SLA (Sealed Lead Acid) batteries. Rated at say 10Ah and 3-4 hundred cycles. Actual eBike use will take advantage of barely 5-6Ah and, if lucky, a few dozen good quality cycles. See - Batteries LeadAcid


Baseline Cycle Life

Battery Deterioration is due, or assignable, to various factors.

  • The omnipresent is cycle life.

Every other factor is a contributory factor, increasing or decreasing the baseline number of "usable cycles".

Increasing vs Decreasing "Usable Cycles" - Factors

Charge level

    • Li-ion - Charging to 4.1V instead of 4.2V will double cycle life with an approximate 90% initial capacity. Charging to 4.3V instead of 4.2V will cut usable cycles in half with a <110% initial capacity. 4.3V might provide 200 usable cycles while 4.1V might provide 800! Most notable! ...

'How to Prolong Lithium-based Batteries - Battery University' - batteryuniversity com learn article how to prolong lithium based batteries.jpg

After ~100 cycles, 4.1V might be providing more usable capacity than the 4.3V charged cells!!!

Lithium2.jpg

Discharge Rate

Higher discharge & charge rates greatly degrade battery capacity and usable life

  • Representative LiCo (typical for laptops etc)

Cycle life and rate.gif

  • Deterioration to 80% of original capacity:
    • 3C produces < 50 cycles
    • 2C produces ~ 250 cycles
    • 1C produces ~ 500 cycles
  • High C rate discharges (and charges) are progressively damaging and as actual capacity declines, deterioration accelerates rapidly!
  • Higher discharge rates produces damaging heat and amplifies the possibility-danger of "thermal runaway" etc.


Discharge rate as sole variable

  • High discharge capable 18650 shows rapid deterioration at higher discharge rates

Cycle Life vs Discharge Rate.jpg

  • Deterioration to 80% of original capacity:
    • 10C produces ~ 300 cycles
    • 7.5C produces - 525 cycles
    • 5C produces ~ 1000 cycles
    • 1C produces ~ 2000+ cycles

Panasonic large 20700 cells at 2C vs 3C discharge rate

Panasonic NCR20700A 2c vs 3c.png

  • Higher discharge rates produces damaging heat and amplifies the possibility-danger of "thermal runaway" etc.

Depth of discharge

    • SLA - Sulfates faster at lower voltages
    • Li-ion - is effectively "empty" at ~3.6-3.7V, discharging further is analogous to wringing the last few drops of water from a water bottle.
  • Degree of discharge - lowest used voltage, seems similar to "Charge Level", in that deeper discharges decreases usable cycles.

Worse! - while charging .1V higher might give 10% more capacity ...

Discharging .1V lower might contribute a meager 2-3% additional capacity!

Capacity Map2.jpg

Logically ... using that additional 2-3% of discharge might cut usable cycles by half ... ?

So, optimally, best performance would be attained by limiting charge-discharge to the central ,capacity bulge, region of battery capacity.

Above graph is for a specific formulation-build of lithium cobalt cell. It should not be considered universally representative.

Manufactures description indicates DOD (Depth Of Discharge) as a major factor in cycle life. Discharging to 10% capacity doubles cycle life compared to discharging to 0%.

Swing 5300.jpg

See - 4.2V vs 4.15V vs 4.1V Li-ion Peak Charge

Capacity Profile - Why?

Lithiums - mAh/100th V - Discharge Tests

Low Tech Capacity Mapping

Temperature

  • Might be the most important factor!
    • Most every destructive factor seems to involve increased heat production!
    • Temperature itself is a factor. 78F-25C seems to be near the universal optimal temperature for batteries. Lower reduces capacity and discharge rate, higher seems to incur excessive damage. Charging most type batteries at lower or higher temperatures is almost universally warned against!

Charge rate

Higher charge rates produce additional heat and amplifies the possibility-danger of overcharge etc.

  • LiCo recommended at ≤0.5C
  • SLA recommended at 0.08-0.125C charge rate
  • Promptness of recharge - time left in discharged condition and at what depth
    • SLA - should be re-charged immediately, irregardless of degree of discharge!
    • Lithiums - Might be different ... recommended storage voltage is near the median capacity point, ~3.9V for LiCo. Logically, as long as additional capacity will not be needed, not re-charging until below this point might be advisable?

Accelerating Battery Deterioration - 18650 laptop type LiCo cells exampled

  • An important observation is that discharge and charge rates should be determined by actual capacity, not by original rated capacity.

Cell capabilities, discharge-charge rates are closely tied to actual capacity, rather than new rated capacity.

  • Most battery users notice that when a battery begins to deteriorate, it's deterioration accelerates.
  • Basic understanding and simple mathematics explain this.
  • High charge-discharge rates are damaging to batteries.
  • Batteries are typically rated with maximum and recommended discharge and charge rates.

The recommended noted as for best-maximum life.

  • Now, if we factor actual capacity rather than the new rated capacity ...

*40A actual capacity battery

  • A 40Ah pack, with 40A controller, might surge 1C and sustain .5C. = near to recommended
  • A 10A charger charges at .25C, (safely below the .5C charge limit for common 18650 cells).

*When capacity degrades to 30A

  • A 40Ah pack@30A actual, with 40A controller, might surge 1.33C and sustain .75C. = beyond optimal - damage accelerates
  • A 10A charger charges at .33C, (safely below the .5C charge limit for common 18650 cells).

*When capacity degrades to 20A

  • A 40Ah pack@20A actual, with 40A controller, might surge 2C and sustain 1C. = pushing maximum safety limits - rapid damage
  • A 10A charger charges at .5C, (right at the .5C charge limit for common 18650 cells).

*When capacity degrades to 10A

  • A 40Ah pack@10A actual, with 40A controller, might surge 4C and sustain 2C. = dangerous heat and catastrophic damage
  • A 10A charger charges at 1C, (dangerously beyond the .5C charge limit for common 18650 cells).

Coping?

  • As noted battery abuse-damage is geometrically progressive.
  • So ... It might be a good idea to build larger than initially "needed" packs or "retire" packs to less demanding use, while still of reasonable condition!