Power-Sonic Sealed Lead Acid Batteries - Technical Manual - Manual (Page 6)

Oxygen Recombination
To produce a truly maintenance-free battery, it is necessary that gases generated during overcharge are recombined in
a so-called “oxygen cycle”. Should oxygen and hydrogen escape, a gradual drying out would occur, eventually affecting
capacity and battery life.
During charge, oxygen is generated at the positive and reacts with and partially discharges the sponge lead of the
negative. As charging continues the oxygen recombines with the hydrogen being generated by the negative, forming
water. The water content of the electrolyte thus remains unchanged unless the charging rate is too high.
In case of rapid generation of oxygen exceeding the absorbing capacity of the negative plate, the pressure relief valve
will open to release excessive gas.
Deep Discharge
Power-Sonic batteries are protected against cell shorting by the addition of a buffering agent that ensures the presence
of acid ions even in a fully discharged state.
Power-Sonic defines “deep discharge” as one that allows the battery voltage under load to go below the cut-off (or
“final”) voltage of a full discharge. The recommended cutoff voltage varies with the discharge rate. Table 1 shows the
final discharge voltages per cell.
It is important to note that deep discharging a battery at high rates for short periods is not nearly as severe as
discharging a battery at low rates for long periods of time. To clarify, let’s analyze two examples:
• Battery A – Discharged at the 1C rate to zero volts.
“C” for a 4 AH battery, for example, is 4 amps. Full discharge is reached after about 30 minutes when
the battery voltage drops to 1.5V/cell. At this point, only 50% of rated capacity has been discharged (1
C amps x 0.5 hrs = 0.5C Amp. Hrs). Continuing the discharge to zero volts will bring the total amount of
discharged ampere-hours to approximately 75% because the rapidly declining voltage quickly reduces
current flow to a trickle. The battery will recover easily from this type of deep discharge.
• Battery B – Discharged at the 0.01 C rate to zero volts.
0.0IC for a 4 AH battery is 40mA. Full discharge is reached after 100+ hours when the terminal voltage
drops to 1.75 V/cell. At this point, the battery has already delivered 100% of its rated capacity (0.01 x
100 hrs = 1C Amp. Hrs.). Continuing the discharge to zero volts will keep the battery under load for a
further period of time, squeezing out every bit of stored energy.
This type of “deep” discharge is severe and is likely to damage the battery. The sooner a severely
discharged battery is recharged, the better its chances to fully recover.
Theory of Operation
Discharge Current Final Discharge Voltage Per Cell
0.1C or below, or intermittent discharge 1.75
0.17C or current close to it 1.75
0.6C or current close to it 1.70
From 1C to 2C or current close to it 1.50
3C or current close to it and above 1.37
Table : Final discharge voltage per cell