ENERGY STORAGE Understanding lithium-thionyl chloride batteries By Malcolm Southern, technical director at Just Batteries. A s many engineers will know, the battery industry has not yet achieved the perfect battery or rather cell technology. In almost every choice there is always a compromise to be made when selecting the chemistry which best suits a specific application. There are some fundamentals to keep in mind when considering a lithium- thionyl chloride (Li-SOCL2) cell or battery for a specific application. Generally speaking a higher capacity cell is most commonly sought after. Herein lays a common mistake made when briefly studying a potential products specification sheet. This mistake occurs when the current delivery ability of the cell is overlooked. There are some basic principles about lithium-thionyl chloride technology that should be considered. Two basic methods are employed by the manufactures, which produce this technology. Each method offers advantages and disadvantages. The two methods employed are referred to as the bobbin cell and the spiral wound cell. “Self–discharge is often overlooked when a particular brand of cell is considered.” The bobbin cell allows for a higher capacity. Typically this is a capacity ranging from 17 Ah to 19 Ah at a nominal voltage of 3.6 V. The spiral wound cell has a much lower capacity which is generally between 13 Ah to 14.5 Ah depending on the brand in question. So what is the difference? It all has to do with the current requirements of a particular circuit. The spiral wound cell has a much larger electrode surface area which enables a far greater current to be drawn. Typically a spiral wound D cell can deliver a continuous current of about 1,800 mA and a pulse current of about 4,000 mA. These currents vary depending on the brand of cell used. A factor that must be kept in mind is the effect on the cell of what is referred to as passivation. In brief, passivation can 48 be described a love hate relationship at best. On the one hand passivation allows for the excellent shelf life of Li-SOCl2 cells. It is an essential reaction that must occur in the production process. It effectively takes the cell out of short circuit when the electrolyte is added during the production process. Passivation can thus be described as a protective layer that forms on the lithium metal layer contained inside the cell. This layer inhibits the electrolyte from making contact with the lithium material contained within the cell. It is important to remember that passivation is reversible. The effects of passivation are evident when one attempts to draw current from the cell. By placing a load on the cell the passivation layer on the lithium metal is broken down but during this process there is a drop in the cell voltage. Depending on the level of passivation this drop can result in the cell voltage dropping to below 2.0 V in extreme cases but it gradually recovers. The end result is that the cell cannot deliver the current or the voltage that a circuit may require and hence this could result in a situation which could have been avoided. There are a number of remedies that can be applied to deal with this factor. Generally speaking excessive temperatures will affect the rate and depth of passivation as well as the rate of self-discharge. Self–discharge is often overlooked when a particular brand of cell is considered. A number of brands feature 19 Ah rated capacity cells when the better quality brands only offer a 17 Ah equivalent. The difference here is that the better quality cell generally has a much better self- discharge rate, so one could end up with much less capacity when the cell is put to work in the field due to the rate of self-discharge. In summary, always know your circuit’s current requirements and make sure that you select the appropriate cell that can deliver the required levels of current. Remember that the cell will passivate over time, especially if current is not drawn from it on a regular basis. Environmental factors play a significant role in a cell’s performance so make sure you understand the environment in which the cell will be expected to perform. Lastly, remember that a greater capacity at a lower price does not necessarily mean better lifetime at a bargain price. Always consider the ramifications of a battery failure or not meeting the stated lifetime expectancy. ESI ESI AFRICA ISSUE 4 2013