When nickel-cadmium batteries are being charged, the chemical in the battery's cells is being changed from one type of compound to another. This is how the electrical energy is stored. When the battery is used to run an R/C car, the chemicals revert back to the original state releasing energy in the form of electrical current.
But, what happens when the battery reaches full charge? The resistance of the chemical compound changes at this point. This creates a small decrease in the voltage of the battery. This decrease is said to have happened just after the peak charge point. A sensitive voltmeter can detect this small voltage drop.
In this case, "peak" is short for "peak detection". As a battery pack is charged by a peak charger, the voltage of the pack slowly rises. When the pack reaches its fully charged point, the voltage peak is detected as a small voltage drop. The charger's voltmeter circuit detects this and either shuts the charger off or reduces the charge rate to a trickle charge level. There are two advantages to this type of charger. First, it is convenient since their is no timer and the process is totally automatic. Second, the battery is not exposed to excess heating by charging it past its full charge state. This increases battery life and reduces the chance of battery pack explosion.
Before you get the idea that this is a perfect system, it should be pointed out that there are situations where the peak detection circuitry in a charger can be fooled. Remember that the typical car battery pack has 4-7 cells. These cells will probably not reach their fully charged states at the same time. This means that the charger may shut off when the first cell reaches its peak instead of the last. If the charge levels are out of sync then an incomplete charge of the complete battery of cells will occur. For this reason, we suggest that matched battery packs be used with peak chargers and that the packs be discharged between races and trickle charged to re-balance the charge levels of the cells.