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Batteries Are Chess, not Checkers: A Primer for Web Developers

Kumar Thangudu

July 04, 2015

Batteries Are Chess, not Checkers: A Primer for Web Developers

tldr: There are many ways to measure and look at battery storage technology. I outline some of the ways in this article.

I’m going to follow the outline below to define batteries, describe their characteristics, and get a grasp of all the claims being made in the media about battery technology.

  • How do batteries work?
  • Cell Chemistry
  • Temperature Profile
  • Self Discharge
  • Internal Impedance.
  • Battery Equivalent Circuit
  • Effects of Internal Impedance
  • Discharge Rates
  • Battery Loads
  • Duty Cycle
  • Peukeurt Equation
  • Ragone Plot
  • Pulse Performance
  • Cycle Life
  • Deep Discharge
  • Charging Characteristics

How do Batteries Work?

 

Important Notes:

  • Galvanic chemicals are the fodder for batteries.
  • Galvanic elements include Lithium Ion(Li-ion), Nickel-metal Hydride(NiMH), and Nickel-Cadmium(NiCd)
  • Electrochemical/Galvanic batteries derive electrical energy from redox reactions that take place inside the battery. It consists of 2 different metals connected by a salt bridge, or individual half-cell separated by a porous membrane.

Cell Chemistry

The nominal or known voltage of a galvanic battery is determined by its chemistry. The known voltage or nominal voltage is the voltage without variation.

Each and every cell’s chemistry is defined by its own nominal voltage and discharge curve.

The area under the discharge curve is the Total Battery Capacity. The bottom axis can also be thought of as time.

 

 

Notice, that these are the discharge curves at a 0.2C rate. 

Keep in mind, the actual w/ variance voltage depends on multiple factors.

  • Load current
  • Internal Impedance
  • Temperature
  • State of Charge
  • Cell’s Age

Important Notes: 

  • A flat discharge curve is desirable because it offers effective utilization stored energy.

Temperature Characteristics

Battery performance varies drastically with temperature. Let’s take a look at discharge curves for one type of battery under different temperature profiles.

Important Notes

  • We can understand a lot about a battery by looking at a discharge curve.
  • Generally, the lower the temperature the less desirable the discharge curve.

Self Discharge

Self discharge describes how quickly batteries lose their energy sitting on the shelf because of undesirable inevitable reactions inside the cell. I hate using rudimentary descriptions for the risk of misunderstanding, but I often think about this the same way I think about radioactive decay. The same pattern with different metrics apply.

Primary Cell Shelf Life

  • Zinc Carbon (Leclanché) 2 to 3 years
  • Alkaline 5 years
  • Lithium 10 years or more

Discharge Rates

  • Lead Acid 4% to 6% per month
  • Nickel Cadmium 15% to 20% per month
  • Nickel Metal Hydride 30% per month
  • Lithium 2% to 3% per month

How Do We Look at Self Discharge Rates

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