Page 63 - Battery Reference Book

1/48 Introduction to battery technology
Table 1.22 Effect of discharge current on operating parameters 120
of a 2V lead-acid battery
110
Discharge End-point Electrical Capacity Energy
current em.$ resistance (Ah) density 100
(A) (V) (a) (W mg)
- 90 Discharge
-
100 1.58 0.0155 29.7 46.5 - net total
m
600 1.26 0.002 10.0 12.5 Y Joule + AH
al 80
$
c
.g 70
2. Chemical heating effect due to dissociation of water -
'0
to hydrogen and oxygen towards the end of charge. 4- m
I! 60
This only applies to certain types of battery, e.g. .-
m
lead-acid ;
L
50
Hz+ 402 =HzO+57410cal 0
2 -
e 40
This type of heating occurs to an appreciable extent -
al
only when batteries are fully charged, i.e. on over- !
charge, and will not be discussed further. 3c
3. Ohmic heating effects due to Joule heating effects
related to operating current and the electrical res- 2c
istance of the battery. From Equation 1.86,
1c
Joule heating = RToT caUt s I I I I I I I I I I I I I I I
4.18 C 2 4 6 8 1012141618202224262830
4. Localized heating effects due to tracking of current Discharge time (rnin)
along low-conductivity paths on the top of the
battery. Figure 1.32 Discharge of a 2V lead-acid battery: chemical (AH),
ohmic (Joule) and net total (Joule + 4H) heating effect
1.22.1 Consideration of chemical (AH) and
ohmic (Joule) heating effects in a lead-acid
battery loor
90 -
Information on the chemical and ohmic Joule heating
effects during the discharge of a lead-acid battery is - EO - "*,
given in Table 1.20, which tabulates the extent of those -
E
effects during the discharge at various rates of a 2V ;
Y
lead-acid battery. P 70-
This information is summarized in Figures 1.32 and c
1.33 which, respectively plot discharge time and cur- ._ 60-
-0
rent versus the chemical (AH) and ohmic (Joule) -
m
contributions to the heat produced during a total dis- I
T, 50-
charge. It can be seen that, in this particular example, .-
C
regardless of the discharge time, the chemical heat- L
3
P
ing effect is very similar to the ohmic heating effect. TJ 40-
-
It should be noted that the above comments apply 0
only to the total discharge of a lead-acid battery E 30-
m
when both the chemical and Joule effects contribute I
al
to the production of heat. Obviously, during charge, 20 -
the chemical effect is one of cooling (Le. AH is pos-
itive), whilst the Joule effect still leads to heating -
(Equation 1.86 - Joule heating is positive whether cur- 10
rent is being withdrawn during discharge (positive) 1
I
I
I
I
I
I
I
or supplied during charge (negative), as the square 0 100 200 300 400 500 600 700
of the current is used in the calculation of heat pro- Discharge current (A)
duction). During the total battery charge in this par-
ticular example, there is a small net heating effect of Figure 1.33 Discharge of a 2 V lead-acid battery: chemical (4H),
the battery as illustrated in Figure 1.34, which shows ohmic (Joule) and net total (Joule + AW heating effect