2.25V
* 6 = 13.50V float charge
2.31V
* 6 = 13.86V typical "equalize" charge
2.37V
X 6 = 14.22V off-gassing
After
full charge, remove the battery from the charger. If float charge is needed
for operational readiness, lower the charge voltage to about 13.50V
(2.25V/cell). Most chargers perform this function automatically. The float
charge can be applied for an unlimited time.
Car
batteries and valve-regulated-lead-acid batteries (VRLA) are typically charged
to between 2.26 and 2.36V/cell. At 2.37V, most lead-acid batteries start to
gas, causing loss of electrolyte and possible temperature increases. The
exceptions are small sealed lead acid batteries (SLA), which can be charged to
2.50V/cell without adverse side effect.
Charging
lead-acid batteries with a power supply
Lead-acid batteries can be charged manually with a commercial power supply
featuring voltage regulation and current limiting. Calculate the charge
voltage according to the number of cells and desired voltage limit. Charging a
12-volt battery (6 cells) at a cell voltage limit of 2.40V, for example, would
require a voltage setting of 14.40V.
The charge current for small lead-acid batteries should be set between 10% and
30% of the rated capacity (30% of a 2Ah battery would be 600mA). Larger
batteries, such as those used in the automotive industry, are generally
charged at lower current ratings. Cells constructed of a non-antimonial lead
grid material allow higher charge currents but have a lower capacity. The
cylindrical Cyclone is sealed and can sustain a pressure of up to 3.5 Bar (50
psi). A pressurized cell assists in the recombination of gases.
Observe the battery temperature, voltage and current during charge. Charge
only at ambient temperatures and in a ventilated room. Once the battery is
fully charged and the current has dropped to 3% of the rated current, the
charge is completed. A good car battery will drop to about 40mA when fully
charged; a bad battery may not fall below 100mA.
After full charge, remove the battery from the charger. If float charge is
needed for operational readiness, lower the charge voltage to about 13.50V
(2.25V/cell). Most chargers perform this function automatically. The float
charge can be applied for an unlimited time.
State-of-charge reading based on terminal voltage
The state-of-charge of a lead-acid battery can, to a certain extent, be
estimated by measuring the open terminal voltage. Prior to measuring, the
battery must have rested for 4-8 hours after charge or discharge and reside at
room temperature. A cold battery would show slightly higher voltages and a hot
battery would be lower.
|
Open
circuit voltage
|
Charge
in %
|
|
12.6V
and higher
|
100%
|
|
12.4
- 12.6V
|
75
- 100%
|
|
12.2
- 12.4V
|
50
- 75%
|
|
12.0
- 12.2V
|
25
- 50%
|
|
11.7
- 12.0V
|
0
- 25%
|
|
11.7V
and less
|
0%
|
Estimated
state-of-charge of a 12V car battery.
Test the battery at room temperature. Allow 4-8 hour of rest after charge or
discharge.