How to Build a Battery Pack

This breakout session is a brief overview of how to do cell binning and building a balance battery pack.

http://americansolarchallenge.org/ASC/wp-content/uploads/2019/02/190202_How-to-Build-a-Pack.pdf

Balancing

  • Implementing active balancing is challenging and requires a dedicated team member
  • Passive balancing will waste precious energy. Pack-level balancing (i.e. force the voltage of all modules to be the same) can be done by connecting all modules in parallel. However, if there is a damaged cell, it will consume all the energy and heat up very quickly
  • Building well-balanced is more effective

Battery approval

  • It is necessary to acquire detailed technical documents from the manufacturers
  • It is encouraged to submit battery approval form early and revise it based on Steve’s comments

Binning or Matching

  • Group battery cells by impedance and/or capacity
  • Pick the more important binning metric
  • If the battery quantity permits, bin battery by both impedance and capacity

State of New Battery Cells

  • Fresh battery cells might be in a particular state as specified by the manufacturer
  • A new cell be in discharging or charging part of the hysteresis loop
  • A new cell might has some energy (usually nominal voltage) in it but it’s unlikely to be 0% or 100%

Controlling Battery Variance

  • Cells might varies in impedance, capacity, weight, number of cycles, and physical dimensions
  • The impedance and capacity of a batch of cells should follow a normal distribution. An non-normal distribution is an indication that the cells are not in the same manufacturing lot or pre-binned in the factory
  • Ordering cells in the same manufacturing lot may reduce the variation
  • Some manufacturers offer low-grade (i.e. low capacity) battery at a discounted rate. These cells are binned at the factory (separated from the high grade cells). They might have lower variations, which might perform better than a better but mismatched batter pack

Backup Battery

  • Prepare a set of good quality backup modules
    • Ideally we should have enough modules to build another pack
    • Recommended to buy 3 times of the number of cells in a pack
  • Cycle the backup modules to match the operating modules to have the same characteristics

Signs of Bad Battery Modules/Cells

  • High voltage drop under heavy current load
  • Heat up
  • High impedance

Module Voltage Measurement Bias

  • The voltage loss due to the bus bars and wires between the modules might be significant under high current
  • Use Kelvin sensing to mitigate the issue?

Module Testing

  • Measurement equipment should be 10 times better than the variations
  • Compare the battery module's actual performance with the predicted performance
  • Be careful of over-stressing the cells if the current load is too high (use step voltage/current)

Spot-Welding

  • Practice on at least 100 cells before production
  • Same person uses the same equipment and finishes on a single day if possible
  • Put the cells in the refrigerator before spot-welding 
  • Consider out sourcing to professional companies 

Terminals and Connectors

  • Consider mechanical vibration and stress
  • Anderson connectors' mechanical bonding between the contact and the housing might breakdown early than expected
  • Prevent short circuit due to foreign objects or human errors

Battery Selection 

  • Motor
  • Array
  • Regen
  • Higher discharge for MOVs