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