Lead-acid cell specific energy

Lead-acid cell (automobile battery) — has low specific energy but generates high current over small time period to start vehicles... 

The low theoretical specific energy of the lead—acid cell is a result of high atomic weight of lead, which is one of the heaviest natural products. 

The theoretical specific energy is never delivered in practical cells. To transform the lead—acid cell into a practical power source, several design requirements must he met. Figure 2.51 shows the construction of a conventional SLI battery [123]. 

In an attempt to improve die power output of the lead—acid cell, its geometry was altered from prismatic to spirally woimd. The specific energy and power characteristics of these batteries of the SLI type are summarised in Table 2.10 [124]. 

Table 15.3 also shows that certain electrochemical systems, given, the same size are interchangeable, e.g. a lithium cell can replace two carbon-zinc (dry) cells or two silver oxide cells the same goes for three lead-acid cells compared to four alkaline manganese cells. In real life this is only possible to some extent, as certain specific properties of different electrochemical systems regarding their on-load characteristics, their energy content, and special constructive details resist this interchange. Two or three alternatives can always be found and should be evaluated. 

The reactions in the lead-acid cell proceed on two electrodes detached by a separator and immersed in solution with specific gravity 1.28. The whole system is placed in a container closed by a cover that is fitted with an outlet valve. This construction weighs about 30 kg and yields 1 kWh of energy. 

The theoretical specific energy for this battery is 2600 Wh/kg and the cell voltage is 2.2 V at 375°C [360-364]. A comparison between the performance of this battery and that of the lead acid battery (Pb/H2S04/Pb02) is given in Figure 36 [365],... 

Figure 3.3 shows the specific resistance (resistivity) of H2SO4 solutions as a function of H2SO4 concentration at different temperatures [2]. The acid concentration window of lead—acid battery operation is marked in the figure. It can be seen that within this window the specific resistance has the lowest values. Both decrease of CH2SO4 below 1.10 and increase above 1.30 relative density cause the specific electrical resistance to increase. When the cell temperature falls below 0 °C, the specific resistance increases rapidly and the battery loses both power and energy. The temperature window with sufficiently low electrical resistance is between 0 and 50 °C. However, even at lower temperatures the battery is capable to deliver sufficient electric current to start the engine of an automobile. 

Figure 2.6 shows the specific drawable energy of lead-acid traction batteries of different designs. The lower graph represents the capacity of the common PzS cells. Further development of this cell type for application in electric road vehicles of the PzF type yields accordingly higher values. 

The mass related (gravimetric) energy content, the specific energy (SE) of lithium batteries, is 100 to 500 Wh per kg depending on system and cell type. Preferably portable devices profit from a lithium power supply. For comparison classic lead-acid batteries show a specific energy between 35 and 55 Wh/kg and NiCd batteries, a bit more powerful, from 50 to 70 Wh/kg. The said higher (lithium) values have, however, been only realized by primary systems until now. 

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