MK Battery Manual

How critical is recharge voltage?
Why are all VRLA batteries so charge
sensitive?
All lead-acid batteries give off hydrogen from the negative plate
and oxygen from the positive plate during charging.
VRLA batteries have pressure-sensitive valves. Without the ability
to retain pressure within the cells, hydrogen and oxygen would be
lost to the atmosphere, eventually drying out the electrolyte and
separators.
Voltage is electrical pressure. Charge (ampere-hours) is a quantity
of electricity. Current (amperes) is electrical flow (charging speed).
A battery can only store a certain quantity of electricity. The closer
it gets to being fully charged, the slower it must be charged.
Temperature also affects charging.
If the right pressure (voltage) is used for the temperature, a battery
will accept charge at its ideal rate. If too much pressure is used,
charge will be forced through the battery faster than it can be
stored. Reactions other than the charging reaction occur to
transport this current through the battery—mainly gassing.
Hydrogen and oxygen are given off faster than the recombination
reaction. This raises the pressure until the pressure relief valve
opens. The gas lost cannot be replaced. Any VRLA batter
y will dry
out and fail prematurely if it experiences excessive overcharge.
Note: It is the pressure (voltage) that initiates this problem—
a battery can be “over-charged” (damaged by too much voltage)
even though it is not fully “charged.”
This is why charging voltage must be carefully regulated and
temperature compensated to the values on page 11.
How long does it take to recharge
a fully discharged VRLA battery?
A specific time is difficult to determine because recharging
depends on so many variables:
• Depth of discharge
• Temperature
• Size and efficiency of the charger
• Age and condition of the battery
See the following Charging Guides for an estimated time based
upon the initial charge current the battery accepts.
12
It will take about 60% of the charge time to bring a VRLA battery
from 0% charged to 90% charged. It will take the remaining 40%
of the total charging time to put the last 10% of the charge back
into the battery.
Charge is a quantity of electricity equal to rate of flow (Amperes)
times time (hours), and usually expressed in Ampere-hours (Ah).
0% state of charge is defined as the depth of discharge giving a
terminal voltage of 10.50 Volts – measured under a steady load at
the 20-hour rate at 80ºF. (The 20-hour rate is the 20-hour capacity
divided by 20 hours.)
Typically, the charge that must be returned to a VRLA battery to
achieve a 100% state of charge is from 105% to 115% of the
charge removed.
60% of time
State of
Charge
40% of time
0% 90% 100%
EXAMPLE: 3
1
⁄2 hours 6 hours
Typical Charging Time vs. 90% and 100% State of Charge
Charging Guides
Typical Charge Time vs.
Initial Charge Current to 90% Full Charge
(Using an automatic temperature-sensing, voltage-regulating charger
set at 13.8V. Totally discharged battery at 11.80–12.0 volts.)
Initial Amperes
Part No. 13 hrs* 6 hrs* 3
1
⁄2 hrs*
8GU1, 8GU1H, 8AU1, 8AU1H 3815
8G22NF, 8A22NF 51223
8G24, 8A24 71733
8G27, 8A27 82141
8G30H, 8G31, 8G31DT, 8A30H, 8A31, 8A31DT 92445
8G4D, 8GGC2, 8A4D, 8AGC2 17 42 83
8G8D, 8A8D 20 50 100
*approximate
HOW TO USE THIS CHART: When charger is first turned on,
read amps after about one minute. Initial ampere reading will
indicate approximate charging time.
EXAMPLE
If an 8G24 reads about 17 ampere charge current
when first turned on, the battery will be at 90%
in about 6 hours.
IMPORTANT: Always use an automatic temperature-
sensing, voltage-regulated charger! Set charger at 13.8
to 14.1 volts at 68°F for gel, or 14.4 to 14.6 volts at 68°F
for AGM. Do not exceed 14.1 volts for gel or 14.6 volts
for AGM.
FULLY CHARGED FULLY DISCHARGED
Specific
Gravity
1.265
Specific
Gravity
below 1.225
Specific
Gravity
1.190
Specific
Gravity
1.120
s
POS = PbO
2
NEG = Pb
ACID = H
2
SO
2
s
POS = PbSO
4
NEG = PbSO
4
Electrolyte = H
2
O
As battery discharges,
the sulfate from the electrolyte
forms on the plates.
As battery recharges,
the sulfate is driven back
into the electrolyte.