When deciding on what components to use for an EV conversion my first step is always to choose the motor. After selecting the motor that will go into the car some key parameters such as voltage range and required discharge current are defined.
Battery pack voltage and size
Next step after selecting the motor is deciding on battery pack size. This is where the importance of deciding series vs parallel connections.
In this blogpost I’ll focus on Li-ion chemistry and will leave LiFePO4 (LPF) out though it works according the same principals just at different nominal voltages (and different OCV curves). The most used voltage in production EV’s nowadays is 355V nominal (at 3,7V per cell this is 96S (aka 96 cell in series). Let’s assume that’s also what we need for our motor setup.
Cells in parallel
Depending on the cell type used they can supply a different (peak) current and have a different capacity. Cylindrical cells like for example the 18650 Tesla uses have a relatively small capacity so you need a lot of them in parallel. Pouch or prismatic cells often have a higher capacity. So you need less of them in parallel. In this example I’ll used the LG E78 pouch cell found in various Volkswagen MEB battery modules.
8s3p versus 12s2p
The Volkswagen modules we now see most are 8s3p and 12s2p. Both modules have the same capacity of about 6,85 kWh and the same number of LG pouch cells: 24 of them.
12s2p
In a 12s2p configuration two pouches are permanently joined together (+ on + and – on -). Each pair is then connected to the next pair (+ to -).
The nominal voltage is 12x 3,7v = 44,4 volt.
8s3p
Now three pouches are permanently connected together and a group of three cells is then connected in series.
At 29,6 volt the nominal voltage is lower and at the same time the current the module can provide is higher. The load is now distributed over three cells instead of two.
So to get to the same system voltage of 355V nominal (96s) one would get:
- 8x a 12s2p module, capacity = 54,8 kWh
- 12x a 8s3p module, capacity = 82,2 kWh
Free cell equalization
The cells permanently connected in parallel at the lowest level (so inside) a module will always self equalize.
This happens without interaction with the other cells in the module. In a well balanced and healthy pack these cells are equal and no energy flow. However, if there are internal resistance differenced under heavy load there could be a (temporary) difference. Also if one cell fails and leaks energy then its voltage could become lower. As a result the voltage of all the cells together in parallel on average will be lower and the BMS will notice.
Below this is visualized in a hypothetical 2s2p module.
Healthy and balanced pack.
One cell fails in a module in one series string,
The voltage of this failing cell and the permanently connected one equalize. the other string in the module is unaffected.
Double MEB pack size?
But what is someone wants more range and wants to use these modules? We cannot just add more modules in series as that would result in a higher (nominal) voltage which in this case the motor.
So the obvious solution would be to just use two of the 54,8 kWh or even two of the 82,2 kWh packs and combined them to a 109,6 or even 164,4 kWh pack. On paper it might even look like a good idea like visualized below with in total 16 of those 12s2p modules:
Modules in parallel = No Go
However, in my view putting modules in parallel like that is an absolute No Go in my view. I’ll revert back to my 2s2p module example to elaborate on why I think that is the case.
Pack with 2s2p modules in parallel
Two 2s2p modules wired in parallel and charged fully up to 4,18V per cell.
Cell failure of one cell
Then as visualized above a failure happens in one cell which starts to equalize with it’s permanently attached parallel neighbor.
Then modules equalize
As a result the voltage in that left module gets lower. And since the modules are also connected permanently in parallel these will equalize as well.
Cell overcharge!
Probably due to the faulty cell there will no be equilibrium (which makes it even worse) but lets assume there is and the modules equalize at 7,32V.
Now you can see the problem! The module on the right charged the module on the left. But not only the faulty string but also the healthy one which was already full. As a result these cells will overcharge and likely catch fire.
The cell voltages indicated above are not exact but are intended to help illustrate how permanently in parallel connected modules can lead to an overcharge situation and thus very likely a battery fire.
The same logic applies when connecting packs in parallel permanently!
Role of the battery management system
Doesn’t the battery management system detect the lower cell voltage?
Yes, the BMS will detect the lower cell / string voltage but due to the permanent connection (either module to module or pack to pack) there is not anything the BMS can do to prevent equalization.
Choosing the perfect pack?
From the above we can conclude that cells in parallel at the lowest level is the easiest way forward. Therefore I always look for the donor batteries that:
- Result in the required voltage range
- Match the capacity / range requirement.
This means in the 96s section a 83 kWh MEB pack is a convenient one to choose. If one insists on a bigger one then maybe a 90 or 100 kWh pack from a Tesla Model S.
Or, if that is still to small for a particular vehicle then maybe move to a parallel setup after all.
How to safely implement a parallel setup?
In my view is is possible to implement a parallel setup while mitigating the overcharge risk as indicated above. In my view the way forward would then be:
- Two battery management systems, each with its own precharge etcetera
- Each BMS dedicated for one string
- On top of that and a master deciding if they both can safely stay/go ‘online’.
So in case of a (sudden) cell failure the BMS of that string would notice and alert the master. In turn the master can then disconnect the strings to the one pack cannot equalize to the pack with the faulty cell.
Furthermore when parked the parallel string are then also disconnected so as safe as one string.
Recap serie vs. parallel cells and modules
If relying on cells in parallel to achieve the required capacity and/or current rating in my view this should ideally be solved ‘at the lowest level’. In that way the cells balance on cell level and no other cells are affected.
Adding more capacity later while preserving system voltage therefore is tricky. If you want to be able to do so, then it is better to start with less modules (as far as the system voltage bandwidth allows).
Having said that: A bigger battery in my view is certainly not always better. In my view very often the overall result is better when selecting a smaller battery and add CCS fast charging and active cooling. In doing so will result in a lighter car that often handles better and can charge very fast.