Unlike primary batteries, rechargeable batteries, once discharged, can be returned to their fully charged state and repeatedly discharged for up to hundreds of cycles. The engineering of a rechargeable system is complex; the incubation period is often decades, not years. The key problem lies in the repeatability and safety issues related to the highly energetic materials.
In the field, a battery that suddenly goes dead is unacceptable, and therefore an indication of the state-of-charge at all times is a critical issue. Determination of the state-of-charge of a battery having a flat discharge curve and rapid voltage decay towards the end of discharge is a difficult task. In the future, environmental and cost considerations are likely to make secondary batteries more attractive than primary "throwaway" types. The widespread use of secondary batteries will require the support of fast chargers, and the logistics of providing these will present a serious challenge. Therefore, smart chargers are now being developed.
Secondary cells can be charged and discharged many times, making it economic to use a more costly construction. Most rechargeable batteries use an aqueous electrolyte but, nevertheless, the electrolyte solutions such as concentrated KOH or H2SO4, are still liquid at -40oC. Batteries do not have blocking diodes, otherwise one would not be able to charge them.