PhD Thesis - Cranfield University

Chapter 3
provided by the battery manufacturer) to arrive at a more practical equation that can be
expressed as,
n−1
⎡C
⎤
C
⎢ h ⎥
T =
⎣ ⎦
( 3-23)
n
I
where C is the nominal capacity of the battery and h is the hour rating at that capacity. (For
example, a 100Ah battery rated for 20 hours translates to C = 100 and h =20 ). With this, the
estimated amphour of the battery can be evaluated by a multiplication of time T obtained
from (3-23) with the discharge current I.
To demonstrate the effectiveness of the battery capacity estimation, a comparison using
discharge profile data of a 27Ah – 10hour VRLA battery against the calculated capacity was
performed. Figure 3.4 shows the comparison between the two. In the calculation, a Peukert
coefficient of n=1.13 was used. As shown, the estimated Ah is in good agreement with most
of the measured Ah points but shows a large error at very high currents. However, in a
supervisory system that manages the power level exerted by the batteries and mitigates high
power stresses to another device, the use of this capacity estimation method proves
sufficient.
Capacity (Ah)
35
30
25
20
15
10
5
0
1
10
20
30
40
50
60
70
80
90
100
110
68
120
130
140
Discharge Current (A)
150
160
170
Calculated
Measured
Figure 3.4 Comparison of a battery measured capacity and an estimated capacity using an adpatation of
Peukert’s equation
180
190
200
210
220
230
240