This type of charge continually monitors
and maintains a pre-set battery voltage, regardless of charge conditions.
Lead
acid charging uses a voltage-based algorithm that is similar to lithium-ion.
The charge time of a sealed lead acid battery is 12–16 hours, up to 36–48 hours
for large stationary batteries. With higher charge currents and multi-stage
charge methods, the charge time can be reduced to 10 hours or less. Lead acid
is sluggish and cannot be charged as quickly as other battery systems.
Lead
acid batteries should be charged in three stages, which are
·
constant-current charge
·
topping charge and
·
float charge
The constant-current charge applies the bulk of the charge and takes
up roughly half of the required charge time; the topping charge continues at a lower charge current and
provides saturation, and the float
charge compensates for the loss caused by self-discharge.
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The battery is fully charged when the current drops to a pre-determined level or levels out in stage 2. The float voltage must be reduced at full charge.
During
the constant-current charge, the battery charges to 70 percent in 5–8 hours;
the remaining 30 percent is filled with the slower topping charge that lasts
another 7–10 hours. The topping charge is essential for the well-being of the
battery and can be compared to a little rest after a good meal. If deprived,
the battery will eventually lose the ability to accept a full charge and the
performance will decrease due to sulfation. Sulfation is simply absorbtion of sulphates from
electrolyte during discharging. The float charge in the third stage maintains
the battery at full charge.
The
switch from Stage 1 to 2 occurs seamlessly and happens when the battery reaches
the set voltage limit. The current begins to drop as the battery starts to
saturate, and full charge is reached when the current decreases to the three
percent level of the rated current. A battery with high leakage may never
attain this low saturation current, and a plateau timer takes over to
initialize the charge termination.
The
correct setting of the charge voltage is critical and ranges from 2.30 to 2.45V
per cell. Setting the voltage threshold is a compromise, and battery experts
refer to this as “dancing on the head of a needle.” On one hand, the battery
wants to be fully charged to get maximum capacity and avoid sulfation on the
negative plate; on the other hand, an over-saturated condition causes grid
corrosion on the positive plate and induces gassing.
To
make “dancing on the head of a needle” more difficult, the battery voltage
shifts with temperature. Warmer surroundings require slightly lower voltage
thresholds and a cold ambient prefers a higher level. Chargers exposed to
temperature fluctuations should include temperature sensors to adjust the
charge voltage for optimum charge efficiency. If this is not possible, it is
better to choose a lower voltage for safety reasons.