When we connect cells in series the resultant pack is only as strong as the weakest cell in the string.
- Discharging, first cell to hit 2.5V stops the discharge of the whole pack.
- Charging, first cell to hit 4.2V stops the charging of the whole pack
- If one cell hits 60°C then pack has to stop charge and discharge
Example
In the simplest string of three cells in series, let us assume they are LFP with a nominal 3.2V and 100Ah. However, in reality the middle cell has a capacity of just 95Ah.
This means that when we discharge the string of cells and we have extracted 95Ah the middle cell will be at the end of discharge and hence we will have to stop. Hence what was a pack with a nominal capacity of 3s x 3.2V x 100Ah = 960Wh will actually have a capacity of 3s x 3.2V x 95Ah = 912Wh.
We haven’t just lost 5% of the energy of one cell, but 5% of the total pack energy.
The impact of this is you need to ensure:
- All cells are made identical
- same capacity
- same internal resistance
- All connections between cells are identical
- Cooling and Heating of all cells is identical
This post has been built based on the support of and sponsorship from: Thermo Fisher Scientific, Eatron Technologies and About:Energy
There can be a number of reasons why cells age differently and fail [1] and the overall electrical/thermal design of the pack is important [2].
This means we need:
- Quality control in cell manufacturing
- Pack engineering that can design, model and validate all functions of the pack
- Battery management system (BMS) that can control the pack over it’s lifetime
There is just one cell in an iPhone, it is expected to last around 2 years, we keep it close to us and so it is kept at an optimum temperature and it discharges quite gently at C/24. The electric car might have 6000 cells, it is expected to last at least 10 years in all conditions and can be discharged in C/2. One cell can weaken this whole pack.
BatteryDesign.net
Example
Take the Tesla Model 3 21700 battery pack, this has an a cell configuration of 96s 46p.
total energy = 96s x 3.6V x 46p x 5Ah = 79,488Wh
If one cell is not connected anywhere in the pack it means that parallel string is 45p. You don’t just lose the 3.6V x 5Ah = 18Wh
total energy = 96s x 3.6V x 45p x 5Ah = 77,760Wh
References
- Why Batteries Fail and How to Improve Them: Understanding Degradation to Advance Lithium-Ion Battery Performance, Faraday Insights
- Paolo Cicconi, Pradeep Kumar, Design approaches for Li-ion battery packs: A review, Journal of Energy Storage, Volume 73, Part D, 2023
See also: Cell Balancing, Cell to Cell Variation