Sunday, July 29, 2012

A Lifetime of Power? (Part 2)

Ok, so the question, "How long do these Lithium ion batteries last?" was partially answered in Part 1 a few days ago.

But there's a second aspect to a battery's longevity...  Cycle life.

Cycle life is independent of calendar life.  You can leave a cell on a shelf and never use it once and it will die.  That's its calendar life... The time it takes to die of old age from the day it was "born".

Cycle life is how many cycles of discharge and recharge the cell can do before it wears out from working hard.  Work a cell harder and it wears out faster.  Treat it to an easy life, and it will die of old age.

Batteries are somewhat akin to people in that respect.

So...  People often ask then, "Well how many cycles can this battery do?". That's also a question that has the answer, "It depends...".

All cells are quoted as living for so many cycles if you treat them right.

The biggest impact on cycle life is from how much you discharge them in each cycle.  If you discharge good gel Lead acid batteries to 50% of their capacity (50% DoD), they will last for maybe 850 cycles.  Push them harder, by discharging to 80% DoD, and the same battery may only last 500 cycles.  In each case the battery does not suddenly die, but it's ability to hold and deliver power is eroded.  When the battery capacity has dropped to 80% of it's original rating, it is considered "near dead".  This is the case for Lead acid batteries because they then rapidly get worse after that level of damage.

Lithium ion cells vary in their quoted cycle life depending on the particular chemistry.  The Winston Battery (LiFeYPO4) cells are supposed to be the longest living, with the Yttrium as the added ingredient that extends their life, even beyond the generally long life of general Lithium Iron Phosphate (LiFePO4) cells.  Claimed life for the Winston cells is up to 8,000 cycles at 70% DoD.

If used in a solar system that naturally has a daily charge and discharge behaviour, that would suggest over 22 years of daily use.  This may be longer than the calendar life of the cells though.  But it certainly suggests that, unlike Lead acid batteries, the Lithium ion cells I'm using now should die of old age before they expend their cycle life from over work.

Tests done by the department of control and telematics at the Czech Technical University in Prague have demonstrated over 13,000 actual cycles on an automated test rig that charged and discharged these cells to 10% DoD and 1.5C discharge and charge rates with no degradation of performance.

See the report here.


  1. Replies
    1. "13k cycles? That's amazing"

      Why ? at 10 % DOD one can expect similar cycles from flooded lead acid OPzS Batteries

    2. Yes, you could expect maybe that many cycles from OPzV lead acid cells costing double the price of a comparable LiFeYPO4 battery and with only 30% of the capacity available in daily cycling and a requirement to keep the lead acid cell at 100% SoC all the time.

      The test did not say the cells lasted for 13k cycles, but that they could not detect any degradation after 13k cycles. But of more interest is that the cells are rated for 8,000 cycles at 70% DoD.

      For solar applications, the LiFeYPO4 cells affinity for partial charging means it is vastly superior to any normal lead acid battery (all of which are destroyed by partial charging).

      I am keeping an eye on the hybrid lead-carbon cell from Axion Power. It uses a traditional positive plate and then a nano-carbon sandwich ultra capacitor negative plate. This cell has the potential to be cheaper than lithium-ion and not suffer from sulphation of the negative plate (as no lead sulphate is formed on the negative plate). This would allow partial charge operation.

      But the PbC cells are a way off commercial availability and in the meantime LiFeYPO4 cells are available 'off the shelf' and offer unmatched advantages in compactness, light weight, and freedom from gassing that lead acid cells (especially cheap flooded ones) can't compete against.