A "battery" is defined as a group of two or more cells wired together. Electric car batteries are usually 4-7 cells wired in series. This means that the voltage of the pack is the sum of the voltages of all the cells added together. If one or more cells become discharged before the others, the voltage of the battery decreases and your car will slow down.
The object is to have all of the cells run down at the same rate. This will allow the car to run the fastest up until the end of the race. It also allows us to fast charge the battery more efficiently. The key is that in a series wired battery pack, the same current flows through each cell regardless if the battery is being charged or discharged. By choosing cells of similar capacity and impedance (resistance to current), all of the cells will reach their maximum charge at the same time. Likewise they will run out of charge at the same time.
When charging a mismatched battery, some cells will reach their full charge state before the others. To get all the cells charged, we must keep the charge current flowing. When a cell is charged past its full charge state, it can no longer convert chemicals internally to store electrical energy. Instead the excess charge is converted to heat energy. This heats up the cell. If the current is low enough, as in trickle charging, this heat is dissipated as fast as it is generated. But our car chargers are fast chargers. Their charge rate is too high for this. The cell is heated quickly which can cause damage to the cell, cause burns, or even rupture the cell. If the cells were matched, they would all reach full charge at the same time and we can discontinue the charge process. When we are running the car with a mismatched battery, some cells will discharged before the others. But, current continues to flow. The cars slows and the discharged cell's voltage eventually drops to zero. It doesn't stop there! The voltage of the battery can be forced below zero (reversed). This is very hard on the cell as chemical reactions are forced to take place that were not intended. These are not reversible and represent permanent damage to the cell.
Either over-charging or over-discharging damages the cell. This causes the characteristics of the cell to change and the battery becomes even more mismatched!
Not all NiCd cells have exactly equal characteristics even when manufactured with the same process. Slight variations occur. Since we are using high currents with our cars, these variations are more important to us than someone using the battery to power a flashlight or a portable radio. Manufacturers us sophisticated battery analyzers to group cells into categories. This allows them to sell batteries of cells with closely matched characteristics. These are called matched batteries. They are usually more expensive than unmatched packs since the matching process is more labor intensive at the factory.
Regardless if you are using a matched battery pack or a sport pack, it is a good idea to trickle charge your cells overnight to insure that all of the cells reach full charge at the same time. This is sometimes called an equalization charge.