Regardless of their chemistry, every battery cell is born differently. This is due to the random nature of the chemical reactions between each cell's anode, cathode and electrolyte materials. Currently, manufacturers compensate for this phenomenon by engineering battery cells to higher capacities which increases material usage by 60% - 200% making current practices highly inefficient and costly.
Manufacturers state variances of 4% - 12% in new cells however, as a battery ages this figure increases greatly. Problems arise when cells are assembled in a series configuration and these differences only allow the stack to be charged until the weakest cell is full, even if its neighbors would be happy to accept a deeper charge. The weakest cell also governs how much current the stack can deliver before it is depleted to a point where crystals called dendrites precipitate from the cell’s electrolyte, causing micro-shorts within its structure.
Internal resistance can be defined as any opposition to the flow of current within a battery. It is the gatekeeper of a battery's power delivery and is a key indicator in a battery's state of health. Imagine the current passing through each cell is like water flowing between connected tanks. Each tank contains a different sized filter for the water to pass through that creates varying resistances. Cells with a higher resistance limit the flow of current to neighbouring cells and therefore have a lagging effect on current during charging and discharging.
Cells with a higher resistance also increase the temperature of the battery. This can be particularily dangerous with the more reactive chemistries such as Lithium-ion where the chance of thermal runaway is much higher.
Current battery management systems deal with the problems of statistical variance and internal resistance by performing rudimentary load balancing. This is done by either bleeding out higher voltage cells or transferring energy between neighbouring cells. The problem with these methods is that they are highly wasteful, generate a lot of heat, expensive to manufacture. On top of this, the performance of the module is still limited by the BMS switching the module off when any cell reaches its capacity.
Battery technology has made huge advances in recent years with chemistries such as Li-ion and NiMH evolving into highly sophisticated and efficient energy solutions, however there are still many limiting factors restricting today's batteries.