How should you connect battery cells together: Parallel then Series or Series then Parallel? What are the benefits and what are the issues with each approach?
The operating voltage of the pack is fundamentally determined by the cell chemistry and the number of cells joined in series.
The ampere-hour capacity of the pack is determined by the capacity of a cell and the number of cells in parallel.
Parallel then Series
This is the approach used in most passenger car electric vehicles and smaller battery pack designs.
All of the cells working in parallel are joined together in groups and then these are joined in series.
Series then Parallel
This approach gives more flexibility for very large packs.
The cells are wire together in series to get to the full system voltage, these series strings are then connected in parallel.
- Simplest control
- Lower cost
- capacity of pack determined in design and manufacture
- no redundancy
- Flexible sizing
- Economies of scale
- high cost
- BMS required for each series string
- switch gear and fusing required for each series string
- out of balance string needs to be left open circuit
- overall master required to control total pack
In the case of the parallel and then series arrangement there will be balancing currents flowing within the parallel groups of cells. These occur due to small differences in the cells, cooling, connection resistance and how they age. These currents will be there rebalancing the cells.
The current flows between the series strings will flow when the strings are brough together in parallel. Hence it is important to measure the voltage of each string and set limits on the differences.
The difficulty occurs if the differences are too great. Then a decision has to be made as to whether you isolate parts or you charge/discharge to bring them into alignment. You could use a pre-charge resistor, but this might take a long time to align the strings.
One BMS can manage the battery pack as a whole.
One BMS is required to manage each series string, each string is a battery pack in it’s own right.
A master BMS then has to sit over the top managing the total system and having to make decisions on how these are connected together and under what conditions.
Both of these designs have strengths and weaknesses. Hence both have places where they are optimal.
Parallel and then series will be the lowest cost, but least flexible. Series and then parallel gives flexibility and redundancy and hence is often found in large battery packs.