MK Battery Manual

13
How can continual undercharging
harm a battery?
In many respects, undercharging is as harmful as overcharging.
Keeping a battery in an undercharged condition allows the positive
grids to corrode and the plates to shed, dramatically shortening life.
Also, an undercharged battery must work harder than a fully
charged battery, which contributes to short life as well.
An undercharged battery has a greatly reduced capacity. It may
easily be inadvertently over-discharged and eventually damaged.
How can you tell if an
VRLA battery is fully charged?
By using a voltmeter.
How can you tell if a VRLA battery has
been damaged by under- or overcharging?
The only way is with a load test. Use the same
procedure you would use with a wet cell battery:
a. Recharge if the open circuit voltage is below 75%.
b. If adjustable, set the load at
1
⁄2 the CCA rating or
three times the 20 hour rate.
c. Apply the load for 15 seconds. The voltage should
stabilize above 9.6 volts while on load.
d. If below 9.6 volts, recharge and repeat test.
e. If below 9.6 volts a second time, replace the battery.
What is a float charger?
What float voltage is recommended?
This type of charger continually delivers a pre-set voltage to the
battery, regardless of charge conditions.
These chargers are used in stationary, emergency back-up power,
emergency lighting, and other applications.
The frequency of discharge and temperature will dictate a more
exact setting. For example, the more frequent the discharge, the
higher the suggested recharge voltage, to a maximum of 2.35 volts
per cell (at 20°C/68°F).
Our recommended float voltage is 2.25 to 2.3 volts per cell for
gel and absorbed models.
Open Circuit Voltage vs. State of Charge Comparison*
% Open Circuit Voltage
Charge Flooded Gel AGM
100 12.60 or higher 12.85 or higher 12.80 or higher
75 12.40 12.65 12.60
50 12.20 12.35 12.30
25 12.00 12.00 12.00
0 11.80 11.80 11.80
NOTE: Divide values in half for 6-volt batteries.
* The “true” O.C.V. of a battery can only be determined after the battery
has been removed from the load (charge or discharge) for 24 hours.
What is a thermal runaway?
The appropriate charge voltage depends on the battery temperature
(see page 11). A warmer battery requires a reduced voltage. If the
voltage is not reduced, current accepted by the battery increases.
When the current increases, the heating increases. This can contin-
ue in a loop feeding on itself with the battery temperature and
charging current rising to destructive levels.
Gel batteries are much less susceptible to thermal runaway than
AGM batteries. Batteries may become more susceptible with
increasing age. Without a recombination reaction, flooded batteries
convert most excess charging energy to gas, not heat. This makes
them almost immune from the thermal runaway.
Thermal runaway can be prevented with:
• Temperature compensation monitoring at the battery—not
at the charger.
• Limiting charging currents to appropriate levels (see page 11).
• Allowing for adequate air circulation around the batteries.
• Using timers, or Ampere-hour counters.
• Using smart chargers that recognize the signature of a
thermal runaway event which will shut the charger down.
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.