Self-discharge is a phenomenon in batteries in which internal chemical reactions reduce the stored charge of the battery without any connection between the electrodes. Self-discharge decreases the shelf-life of batteries and causes them to initially have less than a full charge when actually put to use. How fast self-discharge in a battery occurs is dependent on the type of battery, state of charge, charging current, ambient temperature and other factors. Typically, among rechargeable batteries, Li-ion absorb the least amount of self-discharge (around 2–3% discharge per month,) then lead-acid at 4-6%, while nickel-based batteries are more seriously affected by the phenomenon (NiCad, 15–20%; NiMH, 30%,) with the exception of Low self-discharge NiMH batteries (2-3%.) Primary batteries, which aren’t designed for recharging between manufacturing and use, have much lower self-discharge rates, with shelf lives of 2–3 years for zinc–carbon batteries, 5 for alkaline, and over 10 for lithium. Self-discharge is a chemical reaction, just as closed-circuit discharge is, and tends to occur more quickly at higher temperatures. Storing batteries at lower temperatures thus reduces the rate of self-discharge and preserves the initial energy stored in the battery. Self-discharge is also thought to be reduced over time as a passivation layer develops on the electrodes. The detailed chemical causes of self-discharge depend on the particular battery and are not well understood.