ELECTRICAL 5 - 24
November 2007 GEM Service Manual
Percentage of
Charge
Specific Gravity
Corrected to 80° F
Open-Circuit
Voltage
100 1.277 12.73
90 1.258 12.62
80 1.238 12.50
70 1.217 12.37
60 1.195 12.24
50 1.172 12.10
40 1.148 11.96
30 1.124 11.81
20 1.098 11.66
10 1.073 11.51
Table 1 State of Charge as related to Specific
Gravity and Open-Circuit Voltage
DISCHARGE (LOAD) TEST
There are two reasons for using a discharge unit on
a battery set:
1. To determine battery capacity.
2. To find defective cells or batteries in a battery
set.
Battery energy is measured in minutes obtained
upon discharging a fully charged battery set with a
known resistive load at a controlled temperature
until a shut-off voltage of 10.5 volts per battery for
12-volt batteries is reached, or 7 volts per battery
for 8-volt batteries.
NOTE: The following procedure is written
specifically for a 36 volt, 75-amp discharge unit,
which automatically shuts off at a specified
voltage. Follow all manufacturers instructions
when operating discharge units. Two units will
be needed to discharge the GEM 72-volt battery
set simultaneously, unless a 72-volt discharge
machine, part number 0615-00 can be acquired.
The discharge units are intended for use on
vehicles equipped with the standard 6-12 volt
battery configuration. (See Fig 14), though is
capable of operation on vehicles equipped with
9-8 volt battery configuration.
DISCHARGE PROCEDURE
1. Fully charge the batteries.
2. Turn the master disconnect switch to OFF.
3. Connect the discharge unit(s) to the battery set
as shown in (see figure 14)
4. After all the safety procedures specified by the
manufacturer have been followed, start the unit.
5. When unit shuts off, record the discharge time.
WARNING: IF THE BATTERY LEADS ARE
REVERSED, THE DISCHARGE MACHINE WILL
NOT RUN. HOWEVER, AN ELECTRICAL ARC
COULD OCCUR AND RESULT IN AN
EXPLOSION.
Fig. 14 Discharge Connection Locations
CALCULATING BATTERY CAPACITY
Battery capacity is expressed by the percentage of
time until shut-off voltage is reached. In order to
determine the accurate capacity of a battery set,
the manufacturer's rating at a 75-amp discharge
rate must be known. Also, this value is only
accurate for mature batteries, ones that have 50-70
cycles on them.
EXAMPLE:
• Manufacturer's rating - 56 minutes @ 75 amp
• Discharger shuts down after 42 minutes
• Battery capacity is 42/56 * 100% = 75.0%
If capacity is below 50%, batteries may be nearing
the end of their life and need to be replaced.
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