Alpha Technologies AlphaCell SMU-HR - Manual (Page 12)

745-680-B2-002, Rev. B 12
2.1 Float Charge, continued
Float Current
The normal float current observed in fully charged SMU-HR batteries at 2.27Vpc at a
temperature of 68F (20C) is approximately 30mA per 100Ah. Because of the nature of
recombination phenomena, the float current observed in SMU-HR 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 float current also increases exponentially. Much of the float
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 rectifier and excessive gassing
and overheating will result.
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 float charge voltage (2.27Vpc at 68F (20C)) (see Table 2-1) and a
maximum charge current of 0.25 C8 amperes.
Fast Charge
If it is necessary to reduce the recharge time, charge with a maximum voltage of 2.4Vpc at
68F (20C) and a maximum current of 0.25 C8(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.
WARNING!
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.
To minimize the risk of thermal runaway:
1. Use temperature compensated chargers1.
2. Never allow the batteries to exceed 131F (55C)2.
3. Make sure cabinets are properly ventilated3.
4. Provide spacing between batteries to enhance convective cooling4.
5. Visit sites annually to check for shorted cells, improperly set voltages, filter cleaning on
ventilation 5. systems, etc.
WARNING!
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 rectifier 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.