JLG sky trak mmv Service Manual page 580

Electrical System
When the engine is started and the alternator (1) furnish-
es current to the load (2) and charge current (3) to the "A"
battery (4). All the current now flows through the "B" bat-
tery (5), since it is in series with the load and the "A" bat-
tery.
Alternator Output
"B" Battery
12 Volts
Alternator
24 Volts
1
"A" Battery
12 Volts
When this happens, a condition exists where the "B" bat-
tery overcharges because it does not need recharging,
and the "A" battery does not get the charge it requires.
Charging current for the "A" battery is impeded by the al-
ready charged "B" battery and reduced by the amount of
current that flows to the load (6). Even if there were no
load at this point, the same problem would exist due to
the imbalance of the battery charges.
When operating the system in this manner, the life of the
batteries is dramatically shortened. The "A" battery fails
prematurely because lead sulfate builds up on the battery
plates, reducing the capacity. The "B" battery fails when
its electrolyte "gasses" evaporate and damage occurs to
its plates and separators in the dry battery cells as a re-
sult of overcharging. Failure time can be as short as a few
weeks for heavy 12V loads.
10.100
5
2
Load
Current
"A" Battery
Charge
Current
3
4
If there is no 12V load, a 24V system with a two-battery
"stack" can operate well without an equalizer. However,
the batteries must be of the same type and age. Without
a battery equalizer, current drawn from one battery and
not the other will cause an imbalance. This imbalance ac-
cumulates over time. Because the effects of the imbal-
ance are cumulative, battery degradation occurs even
with very small loads, such as a radio, clock or small
lamp.
The equalizer was developed as a solution to the problem
described above. Technically, equalizers are specialized
DC-DC converters. These devices convert electrical pow-
er from one voltage level to another at relatively high effi-
ciencies. Used in battery balancing applications, the
converters are called "equalizers" .
The term "equalizer" is derived from the fact that these
devices balance or equalize the charge states in the sys-
tem batteries. In the case of 12/24V, dual voltage system,
12 Volt
the equalizer output voltage is equal to half of the input
Load
voltage. This is true for any input voltage within the oper-
ating range of the device. The operation is as follows:
6
Equalizer Operation: Engine OFF
The following figure shows an equalizer (7) installation,
with engine OFF, and a 12V load (8) applied. Assume
MM4470
that the batteries are equally charged to start with. As the
load draws current (9) from the "A" battery (10), the volt-
age on that battery begins to drop. Then, in order to main-
tain the output (11) condition of half of the input (12)
voltage, the equalizer output current increases. This rais-
es the voltage across the "A" battery until both battery
voltages become equal.
Notice that the current (13) required to maintain the out-
put voltage is taken from the "B" battery (14). In other
words, the load power is actually furnished by both batter-
ies, not just the "A" battery. The "B" battery will not furnish
all of the power, because, if its voltage begins to drop, the
voltage at the equalizer output would then be greater than
half the input voltage and the output current would reduce
to zero. The "A" battery would then furnish the load cur-
rent.
Model MMV Rev. 12-04