How do I know if a charger is
“gel friendly” or “AGM friendly”?
Unfortunately, many chargers on the market claim to be gel
“friendly” or “OK for sealed batteries”, but are not. Some
overcharge the batteries, while others may not fully charge
the batteries. Some chargers claim to be “smart”. Some “smart”
chargers do a good job, others do not. The best choice of charger
often depends on the application.
Use only “voltage-regulated” or “voltage-limited” chargers.
Standard constant current or taper current chargers must not be
used. The voltage must fall in the range of the chart on page 11.
Almost all applications require temperature sensing and voltage
compensation. Beware, many chargers measure the ambient
temperature which could be significantly different from the battery’s
internal temperature.
Low frequency current ripple (to about 333 Hz) can be detrimental
to sealed batteries depending on the application. On applications
where the charger is connected continuously to a float voltage,
especially where simultaneous charge and discharge may occur,
the level of current ripple must be a consideration.
If you are not sure if a charger is performing properly, follow this
procedure:
a. Using a fully discharged VRLA battery (OCV about 11.8V)
and a digital voltmeter, record the initial open circuit
voltage at the battery terminals.
b. Using an automatic charger as described above,
set voltage if adjustable (14.1V for gel, 14.4V for
AGM models).
c. Connect and start charging. Record initial on-charge
voltage and current .
d. Each hour or so, check and record the on-charge voltage
across the battery terminals. Except for occasional, brief
“blips” or pulses, the voltage should not exceed the voltage
limits noted in “b” above.
e. At the end of charge (when the current is very low or goes
to zero) check and record the voltage. Note that the charger
may have turned off by then.
f. The disconnected battery should be at 100% or above after a
24 hour rest.
During the charging time, the charger should not have exceeded
the limit (except for occasional, brief pulses). This indicates that
the charger is working properly.
Keep in mind that the voltage limit is at 68°F/20°C. Charging at
higher or lower temperatures will change this limit.
A temperature-sensing charger should always be used, as
manual adjustments are never accurate and will damage any
VRLA battery.
