User manual

www.enersys.com
Temperature
Shelf Life
0
0
C ( 32
0
F) to 20
0
C ( 68
0
F)
21
0
C ( 70
0
F) to 30
0
C ( 86
0
F)
31
0
C ( 88
0
F) to 40
0
C (104
0
F)
41
0
C (106
0
F) to 50
0
C (122
0
F)
12 months
9 months
5 months
2.5 months
Figure 5. Self Discharge Characteristics
STORAGE TIME (MONTHS)
REMAINING CAPACITY
(%)
20
0
C
Discharge Current Final Discharge (V/Cell)
0.1C or below, or Intermittent discharge 1.75
0.17C or current close to it 1.70
0.26C or current close to it 1.67
0.6C or current close to it 1.60
From 0.6C to 3C 1.45
Current in excess of 3C 1.30
Storage, Self-Discharge and Shelf Life
Ohmic Readings
Figure 6. Open Circuit Voltage vs. State of Charge
Publication No: US-NP-AM-003 • January 2011
9
 Over-Discharge (Deep Discharge)
The dotted line in Figure 3 indicates the lowest
recommended voltage under load, or cut-off voltage, at
various discharge rates. In general, lead acid batteries are
damaged in terms of capacity and service life if discharged
below the recommended cut-off voltages. It is generally
recognized that all lead calcium alloy grid batteries are subject
to over-discharge damage. For example, if a lead acid battery
were discharged to zero and left in either open or closed
circuit for a long period of time, severe sulfation and shorting
would occur, thus raising the internal resistance abnormally
high. In such an extreme case, the battery may not accept a
charge.
Genesis
®
NP Series batteries however, have been designed to
withstand such occasional over discharge. While it is not
recommended, Genesis NP batteries can recover their full
capacity under normal charging conditions, even when they
have been subjected to extreme over discharge.
Final discharge voltage is as shown in Table 4.
Table 4. Final Discharge Voltage
When considering discharge currents exceeding 6C, consult
with an EnerSys Application Engineer.
 Self-Discharge
The self-discharge rate of Genesis NP batteries is approxi-
mately 3% per month when the storage temperature is
maintained at 20
0
C (68
0
F). The self-discharge rate will vary
with storage temperature and the remaining capacity. The
relationship between storage times at various temperatures
and remaining capacity is shown in Figure 5.
 Shelf Life
In general, when lead acid batteries of any type are stored in a
discharged condition for extended periods of time, lead
sulfate is formed on the negative plates of the batteries. This
phenomenon is referred to as “sulfation”. Since the lead
sulfate acts as an insulator, it has a direct detrimental effect
on charge acceptance. The more advanced the sulfation, the
lower the charge acceptance. “Brief storage”, ie., a few days,
at temperatures higher than the ranges recommended, will
have no adverse effect on storage time or service life.
However, if such use continues for more than one month, the
storage time must be determined according to the new
ambient temperature.
Table 5 below shows the normal storage time or shelf life at
various ambient temperatures. Figure 6 shows open circuit
voltage vs. state of charge.
Table 5. Shelf Life at Various Temperatures
 Recharging Stored Batteries
In general, to optimize performance and service life, it is
recommended that Genesis NP batteries which are to be
stored for extended periods of time be given a supplementary
charge, commonly referred to as a “refresh charge”,
periodically. Please refer to the recommendations listed
under REFRESH CHARGING in this manual.
Instruments exist from various manufacturing companies to
determine internal Ohmic measurements of cells such as
internal impedance and conductance that could be used to
assess the health of VRLA batteries. The internal impedance
(resistance) of a battery is lowest when the battery is in a fully
charged state. The internal impedance increases gradually
during discharge. Conductance is the inverse of impedance
which is measured in MHOS, also known as Siemens. The
internal Ohmic measurements of a battery consists of a
number of factors, including, but not limited to, the
temperature and state of charge of the battery, the physical
connection resistances, the ionic conductivity of the
electrolyte, and the activity of electrochemical processes
occurring at the plate surfaces. It should be understood that
neither conductance nor impedance are perfect predictors of
battery capacity.
The correct way to use ohmic readings is as a trending tool
over time to detect potentially weak or troublesome cells of a
VRLA battery string in float service. For ohmic measures that
are trended over time, insight can be provided into the
expected life of a cell. The user should establish a baseline
value for each block at the time of installation. Throughout
the battery's life, the ohmic readings should be compared
against this baseline. The most accurate health indicator is to
establish a baseline for each individual block at the time of
installation and periodically monitor ohmic readings.
54153_US_NP_AM_003.qxp:NP AppManual 1/21/11 1:45 PM Page 9