The greater the difference in capacity and internal resistance for the cells you are connecting the greater the difference in how much current each will supply when they are paralleled. This is also true as the power level of the discharge is increased.
A pack using the same model cell, all from the same batch and purchased at the same time from the same vendor, will only have slight differences between the current shared by each cell. You might not even need a BMS to keep them balanced until you have cycled them a lot.
A pack that mixes very different cells though can result in a big difference in how much current each cell has to supply. So while you can often mix cells that aren’t too different in a pack I recommend not doing so unless you are well below the current ratings for all the cells and aren’t discharging the pack to a low cutoff voltage (where the differences between cells can have the biggest effect).
Don’t do something like trying to add 5A-rated ultra-high capacity cells to a pack using 30A-rated low capacity cells to try to increase your range. You cannot assume that adding two 5A cells to a p-group using three 30A cells has a new total rating of 5A+5A+30A+30A+30A = 100A. The current rating for that p-group will probably have to be a lot lower in order to keep the 5A-rated cells from supplying more than 5A each.
Can paralleling different cells be dangerous? Yes, if you insist on not paying attention to current ratings, use old or damaged cells, build a crappy pack, or misuse or mishandle the cells or pack, then it can certainly be dangerous…like any pack would be.
But paralleling good “power” cells like the high current rated Molicel P42A’s with good “energy” cells, like the high capacity Samsung 50G or Molicel M50A (or even the so-so Samsung 50E or Tesla Model 3), shouldn’t result in a pack that is unsafe to use. It can definitely lead to reduced performance though.
Adding energy cells to a power cell pack means you won’t get the higher current rating you would have gotten from adding more of the same power cells. Adding power cells to an energy cell pack means you won’t get the extra run time you would have gotten from using more of the same energy cells.
In both cases though your range would be extended since adding any cells means more run time is available. You just have to be aware of the current ratings for the cells and how they might divide the pack current.
As always this assumes the packs are well constructed and the cells are discharged and charged within their ratings and not when hot or cold, etc. Never misuse or mishandle any cells or packs. We use these cells at our own risk and mixing cells in a pack increases the risks, be nice to them.
But if the cells are in good condition and not nearing the end of their life then there shouldn’t be any big danger in paralleling different cells if you do not exceed their current ratings.
The “power” cell for the simulations is a good performing, low internal resistance cell rated at about 25A. The “energy” cell is also good performing cell, with about 30% higher internal resistance and about 15% more capacity and rated at about 10A.
The power cell has just over 4200 units of energy storage available in these simulations. The energy cell has just over 5000 units of energy storage available.
The cell internal resistances and the differences in run times are roughly based on two popular cells used by the esk8 community. The voltage sag and cell interactions when paralleled should be pretty accurate but I have made no attempt to accurately model the actual run times so capacity numbers mean nothing.
These are not accurate simulations of actual cells for use in estimating what your packs will do. These simulations are only to demonstrate what can happen when different cells are paralleled, to show how they interact versus when operating alone.
I ran two sets of computer simulations to show what happens when two different cells are used separately and when paralleled at different power levels.
As a baseline for comparison I did three simulations for each cell when used alone at different power levels, 20W, 40W, and 60W, from 4.2V down to 3.0V. These wattages simulate use of a battery pack by someone who keeps bumping the throttle up as the pack voltage drops in order to keep the speed constant.
This is what is known as a “constant-power” discharge and results in more and more current being drawn from the pack as the voltage drops in order to make the same amount of power at the motors. It can result in a lot of heat being created in the pack at lower voltages since internal resistance goes up as the pack nears empty and the current level can increase significantly at lower voltages too.
I then paralleled the two cells and did three simulations at what would be the same power levels each if the cells divided the power equally, 40W, 80W, and 120W.
In the next two posts I describe what happens for each discharge above each graph. The power cell is always the red plot line and the energy cell is always the green plot line. The blue plot line in graphs #4-6 shows the voltage of the paralleled cells.