Series strings

In extreme cases of undercharge, batteries can experience cell reversal . This condition results in positive material being converted to negative material, and negative material to positive material, and can arise when cells in a series string develop appreciably different capacities. Cell reversal damages the active... 

FIGURE 17.7 Overcharge test of 14-cell series string of 18650 Li-ion ceiis showing the ceii voitage details. (For color... 

FIGURE 17.8 Temperature readings recorded during an overcharge test of a 14-ceii iithium-ion series string. 

A common test to determine the ability of cells to withstand voltage reversal is the forced-discharge test. The cells are deliberately discharged, at specified currents, to below 0 V by other cells in a series string or by an external power supply to determine whether a venting, rupture, or other undesired safety problem arises. 

Series/parallel with one battery reversed (Fig. 5.9). In this circuit, battery 3 has been reversed. As a result, batteries 1-3 are now in series and are charging battery 4. This condition can be avoided, if possible, by using a single series string of larger batteries. Further, as discussed in See. 5.2.1, the use of diodes in each series seetion wiU at least prevent one parallel stack from charging the other. 

If continued to the point of complete sodium depletion, electrolyte fracture will occur. But because of the presence of the metallic aluminum, the cell will remain electrically conductive. This characteristic permits batteries to be configured with long series strings (for more information refer to section 40.4.1 on Electrical Networking ). The quick decrease in cell voltage functions as a reliable indicator for the end of discharge and is used to provide overdischarge protection. 

FIGURE 40.13 Basic options for networking sodium-beta cells ([Pg.1301]

Sealed nickel-cadmium cells should not be charged in parallel unless each cell or series string of the parallel circuit has its own current-limiting resistor. Minor differences in internal resistance of the cells may result, after cycling, in extreme variation in their state of charge. This may lead to overcharge at excessive currents in some cells and undercharge in other cells. 

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