My mind was wondering the other day (as it often does!) and I thought about how much more power we have available in RC cars now when compared to Sanyo Ni-Cad days. Much like today's internal combustion car engines, we are living with unprecedented power and efficiency.
One of the reasons for this increase in power is the reduced resistance to the power flow with the advent of LiPo battery packs. This resistance is what is referred to as 'IR'. Resistance is measured in 'ohms'. The resistance in LiPo backs is so low it is measure in thousand's of an ohm (.001 Ohm or mOHM). In this case, the lower the number, the less resistance is being introduced, and the LiPo pack has the potential to supply more power to the motor.
What is electrical resistance? We feel resistance to many things, such as air resistance when we hold our hand outside a window in a moving car. In an electrical circuit, resistance is seen as reduced power availability. While the electrical flow from battery to motor does not actually 'feel' resistance, we see the result when the power flow is inhibited (high resistance) by poor connections due to bad solder joints or loose connectors: heat. For example, we see performance reduced and localized heating, in some cases severe enough for melted connections and cases, many times at the pack itself.
Every component in a power system has some resistance due to its physical construction and materials. The LiPo pack's IR is due to four factors:
The cell's internal physical construction.
The pack's physical cell arrangement.
The solder joint where the cells' tabs are connected inside the case.
Age of the pack.
LiPo's IR, when new, ranges (in 2S packs) from 1.4 mOHM to 5mOHM at 25 degrees C (this resistance varies widely dependent on pack temperature). As you increase the power of the motor (for example, going from a 17.5 turn motor to a 7.5 turn motor), the more important it is to have low IR rating for the pack.
While there are several other factors affecting power to the motor, this is how the pack's IR will affect the cars performance. Reduce the pack IR, more power will be available to your motor.