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Engine starting battery sizing
Standard conditions
To size an engine starting battery, you need to know:
- the total cylinder volume in cubic inches or in liters or the starter‘s power of the engine
- the system voltage
The standard sizing conditions are the following:
- Total cylinder volume in cubic inches or in liters or Starter‘s power
- Engine type: 4-cycle diesel
- Total cranking time: 3*10 seconds
- Battery temperarture: 20-25°C (68-77°F)
- Oil temperature: 20-25°C (68-77°F)
- Oil type: SAE30 or multiviscosity oil
- Starter motor efficiency: 60%
- Battery minimum voltage: 71% of the nominal system voltage (12, 24 or 32 V)
- Battery state of charge: 100%
The base cranking current is determined by Table 1. The engine‘s cylinder volume
must be equal to or smaller than the figure stated in the table.
If the starter‘power is known,
Base cranking current =
Starter‘s power
Battery voltage x Starter efficiency
Then, refer to the cell performance SPH range or SBH range at 30 seconds for a
minimum voltage of 0.85 volt per cell (71% of the nominal system voltage) to determine
the cell size required. The cell type corresponds to a current which is at least equal or
superior to the base cranking current.
Non standard conditions
To select the appropriate battery for conditions other than those listed under standard
conditions, a total correction factor may be applied to the base cranking current to
compensate these conditions.
The total correction factor is the product of all relevant correction factors for each
parameters:
- Engine type ( Table 2 )
- Total cranking time ( Table 3 )
- Battery temperarture ( Table 4 )
- Oil temperature ( Table 5 )
- Starter motor efficiency ( Table 6 )
- Battery state of charge ( Table 7 )
- Battery final voltage ( Table 8 )
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Required cranking current = Total correction factor x Base cranking current
Then, refer to the cell performance SPH range or SBH range at 30 seconds for a
minimum voltage of 0.85 volt per cell (71% of the nominal system voltage) to determine
the cell size required. The cell type corresponds to a current which is at least equal or
superior to the product of the base cranking current and the total correction factor C.
National / International sizing standards
To size a battery in accordance with national or international standards, certain sizing
parameters become fixed. The following table Table9 explains this situation for each of
the standards/methods.
Table9: Sizing parameters for national / international standards
NFPA
20-1996 (3)
NFPA
99-1996
NFPA
110-1196
US VA
Hospital
AT & T
Alternator
charging
Float
Float
Float
Float
Float
Alternator
Total cranking time
90 sec
45 sec
45 sec
50 sec
60 sec
Battery temperature
4.5°C/40°F
10°C/50°F
10°C/50°F
10°C/50°F
0°C/32°F
Sizing parameters
Charging method
(4)
Oil temperature
% of the nominal
system voltage (1)
Battery state of charge
25°C/77°F
25°C/77°F
25°C/77°F
70
100%
100%
Number of cell (2)
100%
85%
83%
100%
100%
75%
10/20/26
10/20/26
9/18/24
(1) The nominal system voltage are 12, 24, 32V
(2) The 3 numbers correspond to nominal system voltages of
(3) The battery capacity required for the listed conditions must
the installed capacity
(4) It‘s allowed to modify the set value
12, 24,32V, respectively
be doubled to determine
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Table1: Base cranking current in Amperes for 12, 24, 32 volt systems
Based on 3 x 10 second cranks at 20°C to 25°C (68°F to 77°F)
Cylinder
Volume
Cubic Inch
Cylinder
Volume
Liter
50
75
100
125
150
175
200
250
300
350
400
450
500
600
700
800
900
1000
1200
1400
1600
1800
2000
2500
3000
3500
4000
4500
5000
6000
7000
8000
9000
10000
0.82
1.23
1.64
2.05
2.46
2.87
3.28
4.10
4.92
5.74
6.55
7.37
8.19
9.83
11.47
13.11
14.75
16.39
19.66
22.94
26.22
29.50
32.77
40.94
49.16
57.35
65.55
73.74
81.94
98.32
114.71
131.10
147.48
163.87
Table2: Engine type
Engine type
4-cycle diesel
2-cycle diesel
4-cycle gas
Correction factor
1.00
1.25
0.90
12 Volts
24 Volts
32 Volts
128
160
191
224
254
283
310
362
411
458
506
544
586
665
740
812
882
949
1077
1199
1315
1427
1536
1793
2036
2266
2486
59
78
96
112
127
141
155
181
206
229
251
272
293
333
370
406
441
474
538
599
658
714
768
896
1018
1133
1243
1349
1452
1648
1834
2012
2184
2350
331
356
404
449
493
535
576
672
763
850
932
1012
1089
1236
1376
1509
1638
1763
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Table3: Battery temperature
Temperature
Table 4: Oil temperature
Battery temperature factor
Oil temperature factor
°F
°C
SPH
SBH
SAE 30
77
68
65
60
55
50
45
40
35
32
30
25
20
15
10
5
0
25
20
18
16
13
10
7
4
2
0
-1
-4
-7
-9
-12
-15
-18
1.00
1.00
1.01
1.01
1.02
1.03
1.04
1.06
1.09
1.11
1.12
1.16
1.21
1.27
1.34
1.43
1.54
1.00
1.00
1.01
1.05
1.08
1.12
1.17
1.22
1.27
1.31
1.33
1.40
1.47
1.60
1.65
1.77
1.90
1.00
1.00
1.02
1.06
1.12
1.20
1.26
1.34
1.42
1.47
1.50
1.60
1.71
1.84
2.00
2.20
2.45
SAE 10W
SAE 20W
SAE 40
0.90
0.94
0.97
1.01
1.06
1.09
1.10
1.15
1.20
1.26
1.32
1.40
1.49
0.89
0.89
0.90
0.94
0.98
1.03
1.09
1.15
1.21
1.24
1.27
1.33
1.41
1.49
1.60
1.72
1.88
1.05
1.05
1.08
1.15
1.23
1.31
1.39
1.47
1.55
1.61
1.65
1.78
1.95
2.20
Table5:
Table6:
Table7:
Total cranking time
Starter motor efficiency
Battery state of charge
Total cranking
time (second)
Correction
factor
Starter motor
efficiency
Correction
factor
Battery
state of
charge
Correction
factor
30
40
50
60
70
80
90
100
110
120
180
300
1.00
1.04
1.08
1.11
1.15
1.18
1.21
1.24
1.27
1.30
1.43
1.54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
1.09
1.07
1.05
1.03
1.02
1.00
0.98
0.97
0.95
0.94
0.92
0.91
0.90
0.88
0.87
0.86
0.85
0.83
45
50
55
60
65
70
75
80
85
90
95
100
1.29
1.25
1.22
1.19
1.16
1.13
1.11
1.08
1.06
1..04
1.02
1.00
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Table 8: Battery final voltage
Batteryfinal voltage in %
Correction factor
of nominal system voltage
SPH
SBH
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
0.98
1.00
1.02
1.07
1.10
1.13
1.16
1.19
1.25
1.29
1.32
1.36
1.40
1.48
1.52
1.56
0.99
1.00
1.02
1.05
1.07
1.09
1.11
1.13
1.18
1.20
1.23
1.26
1.29
1.36
1.39
1.43