Saturday, March 8, 2014

Lithium Ion Batteries and Cycles



My laptop batteries are pretty much dead and I use the power cord most of the time.   My cell phone batteries have decayed and I have spare battery pack.  Why have the batteries on these devices decayed and in pretty short order?  Three things helped speed up the decay of these batteries.   Rate of charging, rate of discharging, and the temperature the batteries operated under while charging and discharging

There are side reactions that cause crud to build up on the carbon anode of a lithium ion battery.   This crud eventually blocks the surface of the anode and eventually may render the battery dead.    Interestingly these side reactions are time dependent and temperature dependent just like all chemical reactions.   The more time taken during charging and discharging the greater the number of these occurring side reactions and the higher the temperature the faster these reactions occur.

My lap top and my phone operate under conditions where these side reactions are most favored.  The lap top is hot and the phone is hot.   The charging rates are slow.  The discharging rates are slow.  Viola these batteries are kaput after five or six hundred cycles.

One would think Mr. J in New Jersey is probably doing harm to his Tesla model S by charging slowly and driving slow.   He is lucky the winter has been cold.  But actually his Tesla has a good battery thermal management system and the batteries Panasonic makes for Tesla have some exotic additives in the electrolyte to minimize these side reactions.  Last week I opined that Elon Musk has the best data on how long these battery packs will last.  First in the car and then being used to store grid power.  Each day I become more convinced that the batteries in a Model S will enjoy many thousands of cycles. 

I think we will see Tesla offer a car for a small down payment and then a monthly charge of say 20 cents a mile.   The “owner” will buy the electricity and Tesla will get recurring revenues like Gillette selling razor blades.  I was correct in telling the world the initial price of lithium ion batteries will not come down, what I and many others missed is that batteries  made today in a state of the art manner with additives to the electrolyte and deployed in packs with good thermal management systems will last a long time.  The first cost is irrelevant in a world where the Federal Reserves has easy monitory policy and credit can be extended for capital goods that last a long time.

From a Green perspective this is actually wonderful.   Waste is the enemy of eco efficiency.  Longer lived batteries mean less waste.  There is a high initial amount of energy needed to mine and refine raw materials needed to fabricate the batteries, but if this is amortized over 5,000 cycles instead of 500 cycles this primary energy is well worth spending for later recapture.

Mr. Marsh the Leaf owner in Seattle who has driven 100,000 miles in his Leaf proved the Leaf is green in Seattle.  He lives in a climate with moderate temperature and charges and discharges his batteries rapidly.  Hence he enjoys the best possible battery life on the Leaf.   Motorists in Phoenix Arizona who drove their Leafs in hot weather and did small trips here and there have already destroyed their battery pack.   The Leaf had a poor thermal management system and Phoenix was simply too hot a climate for the car.


There is not much one can do about laptops and phones from a rate of charge or discharge perspective.  The temperature control is also very basic.  These batteries may improve as more vendors learn how to formulate the electrolyte with improved chemicals to minimize the side reactions.   Dr. P may have got a bargain on his model S.  He has high rates of charge and discharge and operates in a moderate climate.  We all hope he gets 300,000 miles on his battery pack that Elon has guaranteed for 8 years will not go below 70% of initial capacity.  Dr. P we have our fingers crossed for you that you win a green star.

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