Lead-Acid Discharge Curves as Examples

To illustrate the influence of kinetic parameters, discharge curves of a lead-acid

battery are compared to the equilibrium voltage in Fig. 1.9. The figure shows

Figure 1.9 Discharge curves relative to the dell 312-0883 drawn amount of Ah. The dashed curve shows

the equilibrium voltage according to the Nernst equation. It reflects the dell K903K dilution of the acid


with progressing discharge (cf. Fig. 1.2).

Flooded traction cell with tubular plates (350 Ah at 5-hour rate).

Table 1.2 Transference numbers in sulfuric acid and

potassium hydroxide at room temperature. For diluted

solutions of sulfuric acid given in Ref. 10, but also true for

concentrations used in batteries. For potassium hydroxide


true for a wide concentration range given in Ref. 11.

Sulfuric acid tt . tHt . 0:9 t . tHSO 4

. 0:1

Potassium

hydroxide tt . tKt . 0:22 t . tOH . 0:78

Copyright . 2003 by Expert Verlag. All Rights Reserved.


discharge curves at various loads relative to the amount of Ah drawn from the

battery. The dashed curve at the top represents the changing equilibrium voltage due

to the gradually decreasing acid concentration, according to the Nernst equation

(Eq. (11), Fig. 1.2). If all the DELL JKVC5 Battery partial-reaction steps were fast enough, i.e. if no kinetic

hindrance occurred, the increased discharge rate would cause only a voltage drop


that would shift the dashed line in parallel to lower voltages.

Figure 1.9 shows that not only a considerable voltage drop can be observed

with increasing discharge current, but also a growing decline of the curves. So, with

increasing load, the Dell Inspiron 1750 Battery dischargeable share of the capacity is more and more reduced by

the impact of kinetic parameters, and the current amount that can be drawn from the


battery is markedly reduced, although the end-of-discharge voltage is lowered with

increased load. Mainly acid depletion at the electrode surface reduces the rate of the

FUJITSU FPCBP225 reaction. Furthermore, some of the undischarged material may be buried underneath

the growing PbSO4 layer. This FPCBP226 layer grows very fast at high loads, resulting in a thin


but compact covering layer that prevents further discharge very early.

1.4 HEAT EFFECTS

Electrochemical reactions, like chemical reactions, are always connected with heat

effects, determined by the (positive of negative) reversible heat effect, already

mentioned in Eq. (4). When current flows through the cell, additional heat is Lenovo 51J0499 Battery


generated by ohmic resistances in the electrodes and the electrolyte, but also by

polarization effects, which together cause ‘Joule heating’.

1.4.1 The Reversible Heat Effect

The reversible heat effect

Qrev . T ? DS e31T

represents the unavoidable heat absorption or heat emission connected with


electrochemical reactions. It is related to the thermodynamic (equilibrium)

parameters of the DELL KM742 concerned reaction, and is strictly connected with the amount of

material (in electrochemical equivalents) that reacts. Thus, the reversible heat effect

does not depend on discharge or recharge rates. When the cell reaction is reversed,

the reversible heat effect is reversed too, which means it gets the opposite sign. Thus,


energy loss in one direction means energy gain when the reaction is reversed, i.e. the

dell KM769 effect is ‘reversible’.

The reversible heat effect per time unit can be related to current flow, because

each multiple of the cell reaction requires the current amount n ? F:

dQrev


dt .

Qrev

n ? F

?i W e32T

with n: number of exchanged electrons; F: Faraday constant (.96485 As/

equivalent); i: current in A.


Qrev/nF has the dimension V. So it is equivalent to a voltage, although it is not a

voltage that can be measured, but for Dell P434 caloric evaluations it is convenient to use the

difference

Ucal . Uo

Qrev


n ? F

V e33T

as ‘calorific voltage’ (or thermoneutral potential Etp (12)). Ucal is a hypothetical

voltage that includes the reversible heat effect, and is used instead of the 9 Cell Dell Inspiron 1440 battery equilibrium

voltage for caloric calculations.


Combination with Eqs. (31) and (32) shows that Ucal also can be written

Ucal .

DH

n ? F e34T

Ucal is a fictive equilibrium voltage that includes the reversible heat effect and is

convenient with heat calculations (cf., e.g. Eq. (41)).

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