Battery Technology
- Pei Liang Guo
This breakout session briefly covers basic knowledge about battery and different battery technologies. The presenter, Steve McMullen, is responsible for for approving the battery cells. His email is steve_mcmullen@hamericansolarchallenge.org
Metrics for Battery
- Energy density
- Weight is important for a competitive solar car
- Cell capacity
- Amount of charge store in a battery cell
- Important for cell configuration
- Current capacity
- Related to cell capacity?
- Higher current capacity is necessary for frequent regento mitigate thermal issues
- Nominal voltage
- Correlated to reducing effect?
- Li-ion has the best and the highest nominal voltage
- Impedance (Cell and Module)
- The internal resistance of the battery
- Lower impedance correlates higher capacity. Impedance changes over multiple cycles
- Impedance matching is more important than aiming for low average impedance
- Weight
- Not recommended but weight has some degrees of correlation with the impedance. It is possible do batter binning based on weight if we run out of time
- Charge and discharge rate
- High charge and discharge rates are beneficial for regen
Battery Technologies
He mentioned technologies in his presentation, check it out if you are interested. Here are the two important technologies:
NMC - 18650
The most common lithium-ion battery technology. It currently owns around 70% of the battery market. It is possible and safe to use NMC in a parallel configuration. Well tested and widely used in many applications. Mass manufacturing has the benefit of lower variations and lower possibility of contain design and manufacturing defects.
NCA - 21700
Recently developed by Panasonic and Tesla. It uses less cobalt than NMC thus less likely to experience cobalt supply issue (in the distance future). It has the promise of lower cost and higher energy per kg than NMC. In the near term, it might be more expensive and heavier than NMC. There is a higher possibility of unknown design and manufacturing issues.
Battery Testing Methods
- Test it in real time
- Charge and discharge the cells for multiple cycles in real time
- Extrapolate the data
- Test the cells with a limited time and cycle and extrapolate the measurements
- Some battery defects may not show up in a short period of time
- Test it at an elevated temperature
- Charge and discharge the cells at a higher temperature (~40 C)
- The cells will degrade faster than usually
Failure Mechanisms
- Abuse Failures
- Mechanical
- Electrical
- Thermal
- Infantile Failures
- Manufacturing defects
- Hard to detect
- End of Life Failures
- The battery cells slowly degrade over time
- Usually harmless
Pack Imbalance
Over the duration of the road race, the voltage difference between modules can increase from 2% to 29% over 7 cycles
Pack Cycler
Teams use pack cycler to test the battery modules. Some teams cycle the backup battery modules to match the functioning battery modules in the car. Here are some ways of cycling the modules:
- Light bar
- Electric BBQ grill
- Motor