Manual

745-680-B8-002, Rev. B
12
2.1 Float Charge, continued
Float Current
The normal oat current observed in fully charged SMU-F batteries at 2.27Vpc at a temperature of
68ºF (20ºC) is approximately 30mA per 100Ah. Because of the nature of recombination phenomena,
the oat current observed in SMU-F batteries is normally higher than that of vented batteries and is
not an indication of the state of charge of the batteries.
Thermal Runaway
Float current is primarily a function of voltage and temperature. As either voltage or temperature
increases, the oat current also increases exponentially. Much of the oat current is going into the
recombination reaction, which is exothermic. If the heat generated by recombination exceeds the rate
at which heat can be transferred out of the battery (based on conduction, convection, and black body
radiation), thermal runaway can occur. The battery will continue to take very large amounts of current
from the rectier and excessive gassing and overheating will result.
To minimize the risk of thermal runaway:
Use temperature compensated chargers1.
Never allow the batteries to exceed 131ºF (55ºC)2.
Make sure cabinets are properly ventilated3.
Provide spacing between batteries to enhance convective cooling4.
Visit sites annually to check for shorted cells, improperly set voltages, lter cleaning on ventilation 5.
systems, etc.
In the most severe cases of thermal runaway, equipment can be damaged by sulfuric acid mist that escapes
the battery, hydrogen can build up to dangerous levels, and battery cases can rupture because of weakening
and melting of the plastic. Ruptured cases can lead to ground faults.
2.2 Recharge Following Discharge
Recommended Charge
The recommended recharge method to maximize battery life is to charge with a constant voltage
equal to the oat charge voltage (2.27Vpc at 68ºF (20ºC)) (see Table 2-1) and a maximum charge
current of 0.25 C
8
amperes.
Fast Charge
If it is necessary to reduce the recharge time, charge with a maximum voltage of 2.4Vpc at 68ºF
(20ºC) and a maximum current of 0.25 C
8
(use the temperature adjustment formula in section 3.1 for
voltage adjustment). This recharge should be used no more than once per month to maximize the
service life of the battery.
Avoid situations where excess current is available to recharge the battery. This can occur when the DC load is
low relative to the charger or maximum rectier output, and the battery is fully discharged. If too much current
enters the battery, the battery can heat up excessively, be permanently damaged, or may cause an explosion.
WARNING!
WARNING!