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There are some worries concerning the safety of certain lithium batteries, particularly the SO2 and thionyl chloride batteries containing less than about 0.5 g of lithium that are used in watches, cameras, etc. Tight restrictions are placed on how these batteries may be carried on cargo flights. The safe disposal of partially discharged batteries is also a major concern: bulk users are urged to negotiate with a suitable waste-disposal contractor to take back used batteries for safe disposal. The US Environmental Protection Agency has ruled that lithium-sulfur di batteries are non-hazardous if fully discharged to deplete the reactive components to low levels.
All US military multi-cell lithium batteries now incorporate a discharge resistor which can be switched on when the battery is about to be discarded (called the complete discharge device, CDD). This will complete the discharging of the battery in about five days and batteries should be kept for this time before being dumped. Unfortunately, there have been a few incidents in the US Army in which batteries have exploded during, or at the end of complete discharging. These incidents are presently being investigated in order to see why they occurred. It is thought that some of the cells may have been faulty, rather than that there is a general problem. The usage of such batteries in the USA is up to 1000 times that in CF, so the probability of an incident in the CF is low and none has occurred.
High-power lithium cells need to be carefully ed to ensure safe operation. For example, if a cell is short-circuited, the large current will cause internal overheating, a rise in pressure and the cell could explosively rupture. In general, battery manufacturers do not like to use the word "explode" and have invented other terms. One interesting euphemism is "spontaneous disassembly" or "decrimpling". All except low-rate cells should incorporate safety vents to avoid a dangerous build up of internal pressure. These vents are especially ed weak points in the steel can, which rupture at a particular internal pressure. Such vents are not resealable, so the battery is then unserviceable (a safety report may require to be filled out and the battery sent away for analysis).
Electrical fuses and, frequently, thermal switches are fitted to avoid high cell internal temperatures. Blocking diodes are usually included to prevent charging. In a series-connected string of cells, one weak cell may have current forced through it by the other cells until its voltage reverses. Shunt diodes may be connected across cells (i.e. in parallel) to prevent the reverse voltage from reaching dangerous levels. Shunt diodes are recommended for 5 or more lithium-sulfur dioxide cells in series.
The load voltage of a lithium cell usually varies very little during discharging. Although this appears to be ideal, it does make it difficult to tell how much capacity remains in a partially discharged cell. It may be necessary to record the service history of each battery to give some indication of the capacity used. There is nevertheless a tendency to throw away perfectly good, partially used batteries simply because their state of charge cannot easily be ascertained. Researchers are investigating ways of cheaply estimating the residual charge.
