Storage

Self Discharge

The self discharge rate of NP batteries is approximately 3% per month when stored at an ambient temperature of 20°C. The self discharge rate will vary as a function of ambient storage temperature. Figure 5 shows the relationship between storage times at various temperatures and the remaining capacity.

Fig 5

Shelf Life

In general, when lead acid batteries of any type are stored for extended periods of time, lead sulphate is formed on the negative plates of the batteries. This phenomenon is referred to as "sulphation" Since the lead sulphate acts as an insulator, it has a direct detrimental effect on charge acceptance. The more advanced the sulphation, the lower the charge acceptance.

Brief excursions i.e., a few days, at temperatures higher than the ranges recommended above will have no adverse effect on storage time or service life. However, should the higher ambient temperature persist for one month or more, the storage time must be determined by referring to the new ambient temperature. Ideally NP batteries should be stored in dry, cool conditions.

Table 8 below shows the normal storage time or shelf life at various ambient temperatures.

Table 8.Shelf Life at Various Temperatures

In general, to optimise performance and service life, it is recommended that NP batteries which are to be stored for extended periods of time be given a supplementary charge, commonly referred to as a "top charge", periodically.

Fig 6. Temperature/Life Characteristics of NP Batteries

Figure 6 shows extrapolated Service Life condition for NP batteries ambient temperatures. As can be seen from figure 6 higher ambient temperatures will reduce service life.

Available Capacity

Measured by open circuit voltage.

The approximate depth of discharge, or remaining capacity, in a Yuasa NP battery can be empirically determined by
referring to Figure 7.

Figure 7 Open Circuit voltage vs. remaining capacity

Impedance

The internal resistance (impedance) of a battery is lowest when the battery is in a fully charged state. The internal resistance increases gradually during discharge, Figure 8 shows the internal resistance of an NP6-12 measured through a 1000Hz AC bridge.

Fig 8. Internal Resistance of NP Battery

Impedance testing can be performed using the Yuasa YPI-2 Impedance/comparator test meter, this form of testing is non-intrusive and can be performed online with the battery still connected within its system. (Note: The YPI-2 meter cannot be used where a high AC ripple content exists ) By using this test method deterioration can be detected without removing the battery from its standby mode.