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Education

NiMH 2000 Cycles

Comments recently asking how the NiMH chemistry fared beyond the first 1000 cycles in the battery life cycle test was a question i asked myself back in the day as well, so a second set of 1000 cycles were run but results were not posted til now.

After a second 1000 cycles the NiMH capacity faded an additional 5%, dropping to 58% of rated capacity from 63% after the first 1000 cycles.

The NiMH chemistry held up well in terms of cycle life when not abused or over-heated by the charger which in-practice can be a difficult thing to do with NiMH battery types due to difficulty at detecting and terminating a full charge correctly and preventing damage from over-heating, especially at faster charge rates such as C/2 and without the use of a temperature sensor.

The PCBA 5010-4 battery analyzer uses a custom ‘voltage only’ method of full-charge termination with multiple redundant detection methods for NiMH and NiCd chemistry types developed over ~15 years which now demonstrates its effectiveness in life cycle testing by not overheating the cells or causing venting of electrolyte and capacity loss other than by what seems to be normal loss as a function of normal use.

Categories
Education

Lithium ion failure

A good explanation of why lithium ion batteries lose capacity over their cycle life is that components of the electrolyte oxidize at the cathode under high voltage and high temperature conditions which then are reduced at the anode side and eventually block-off the porosity and ionic conduction deeper into the anode material which then results in lithium metal plating and shutting down of the cell.

This seems a more likely cause and effect than my explanation for the lithium ion lifecycle test below thinking the failure was due to pulverization of the cathode material since that would in turn cause oxygen release and gassing into a puffy cell which did not happen, so i think electrolyte reduction and coating on the anode would better explain the sudden loss of capacity.

A very interesting theory and technique for measurement of electrolyte degradation are explained in the video below by Jeff Dahn of Dalhousie University.

Why do Li-ion Batteries die ? and how to improve the situation?

Professor Jeff Dahn (Dalhousie University)
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