Odyssey PC1200 Owner's Manual - page 19
11
Publication No: US-ODY-TM-001 - April 2011
www.odysseybattery.com
®
PEukERT’S ExPONENT
The capacity of a battery depends on the rate at which it
is discharged. For example, the ODYSSEY
®
31-PC2150
battery is rated at 92Ah when discharged at the 10-hour
rate (9.2A for 10 hours) and 100Ah when discharged at the
20-hour rate (5.0A for 20 hours). This variability in battery
capacity makes it problematic to estimate the support time
one can expect for a given current draw. This is where the
Peukert equation, shown below comes in handy.
Capacity = I
n
x t
log T
2
– log T
1
n = ------------------––––––
log I
1
– log I
2
In the first equation, n is called the Peukert’s coefficient;
I
1
and I
2
are two discharge current rates and T
1
and T
2
are
the corresponding discharge durations. Knowing the rated
capacity of the battery and the coefficient (given in the table
below) one can use this equation to estimate the battery
support time for a given amp draw.
Suppose you have a 15A load you want to run off the
ODYSSEY 31-PC2150 battery, which is rated at 92Ah and
you want to know approximately how long it will support
the load. From the table below, n is 1.112 for the Group 31
battery; its capacity is 92Ah and I is 15A. Substituting these
values into the Peukert equation yields a support time t of
about 4-1/2 hours before the battery is fully depleted.
Table 1
Some battery monitoring systems (BMS) offer the option
of entering the Peukert coefficient so that the BMS can
estimate the time left for the battery to be fully discharged
at a given discharge rate. If you have such a system, you
can use the table above to find the value of your battery
model. An important point to keep in mind here is that this
estimation is valid only for an ambient temperature
of 25°C (77°F).
CyCLE LIfE AND DEPTH Of
DISCHARGE (DOD)
Applications in which the battery is frequently discharged
and recharged are called cyclic. A complete cycle starts with
a charged battery that is discharged and then brought back
to a full charge. Battery life in these applications is stated as
the number of cycles the battery will deliver before its capacity
drops to 80% of its rated value. For example, suppose a
battery is rated at 100 amp-hours (Ah) and has a published
cycle life of 400. This means that the battery can be cycled
400 times before its delivered capacity drops to 80Ah.
Proper charging and DOD are the two key factors that
determine how many cycles a battery will deliver before it
reaches end of life. The DOD is simply the ratio of capacity
extracted from the battery to its rated capacity expressed
as a percentage. If a 100Ah battery delivers 65Ah and then
recharged it is said to have delivered a 65% DOD cycle.
The relationship between DOD and cycle life for ODYSSEY
batteries is shown in Figure 1. The lower the DOD the higher
the number of cycles the battery will deliver before reaching
end of life.
figure 1
The true dual purpose design of ODYSSEY batteries
is reflected in the cycle life results shown in the graph
below. The two ODYSSEY 65-PC1750 battery samples
were discharged at 25A (the reserve capacity rate) for 108
minutes (80% of 135 minutes, which is the 100% rate for this
battery), rested for an hour then charged for 5 hours at 14.7V
with a 50A current limit. Since five hours are not sufficient to
charge them completely, both batteries were given 20-hour
charges every ten cycles. Sample 1 cycled 581 times and
Sample 2 lasted 544 cycles before reaching end of life.
100
1000
10000
100000
1000000
Number of cycles
Depth of discharge, DOD %
Charge profile: Full recharge at 14.7V with a 1C current limit
odyssEy
®
Battery Model
peukert
Coefficient
odyssEy
®
Battery Model
peukert
Coefficient
PC310
1.072
PC1350
1.147
PC535
1.078
25-PC1400
(all variations)
1.034
PC545
1.107
34-PC1500
(all variations)
1.133
PC625
1.107
PC1700
1.091
PC680
1.129
65-PC1750
1.081
PC925
1.100
PC1800-FT
1.150
PC1200
1.106
31-PC2150
(all variations)
1.112
PC1220
1.132
PC2250
1.180
75-PC1230
(all variations)
1.074
0
20
40
60
80
100
120
140
0
50 100 150 200 250 300 350 400 450 500 550 600 650
Run Time in Minutes
Cycle
End of Life -
Sample 1 - Cycle 581
/
Sample 2 - Cycle 544