Charge balancing of rechargeable batteries

Details Charge balancing of rechargeable batteries Charge balancing of rechargeable batteries

1990-05-01

1991-07-02

Skapars, Anthony

Chemistry: electrical current producing apparatus, product, and

With control means responsive to battery condition sensing...

429 91, H01M 1048

Patent

active

050284991

DESCRIPTION:

BRIEF SUMMARY
The invention relates to charge balancing of cells of rechargeable batteries principally but not exclusively having a negative electrode composed at least partly of an alkali or alkaline earth metal, for example lithium.
The process by which cells in a rechargeable battery are brought to a state where each contains the same amount of electrical charge is known as charge balancing. If a number of cells in a battery are connected electrically in series it is of crucial importance that each cell can be fully charged otherwise the performance of the whole string of cells is limited by the cell that contains the least electrical charge, i.e. the weakest link. If one cell becomes out of balance with respect to the others then the efficiency of the battery is reduced. A battery deteriorates far more rapidly if the charges on the cells are out of balance than if they are balanced, because it is likely that gross overcharging or discharging will occur during the discharge/recharge cycle and this can catastrophically damage cells in many types of cell systems. One approach to recharging a battery is to monitor the charging of the cells. However, when the first cell reaches full charge then any cell which started at a lower state of charge will never be fully charged and the battery can never reach its maximum voltage level. Alternatively, recharging can be continued to overcharge the cells to bring all of them to full charge. In this way charge balancing is achieved with lead-acid batteries, the overcharging ensuring that every cell is fully charged. However, this is not possible with some types of battery as the cells would be damaged and the battery ruined. For example, in the lithium-aluminium/iron sulphide cell, overcharging leads to the formation of iron halide complexes at the positive electrode which eventually leads to irremedial damage. Furthermore, over-discharge of a battery brings about the anodic dissolution of the aluminium in the negative electrodes, irretrievably damaging the electrodes.
If battery cells are connected electrically in parallel the problem does not arise as the cells will settle to approximately equal charge levels giving approximately equal voltages and the battery can operate effectively.
Charge balancing is necessary to smooth the operation of the battery by equalising the charge of all of the cells. It is also necessary in order to avoid problems during the start up of the battery from different discharges of the separate cells.
Present solutions require each cell to be charged separately to achieve balanced charging and avoid over-charging or discharging of the cells. The individual cells are monitored and the charging carried out under computer control to ensure that all the cells have equal charges. The battery must be specially constructed and sophisticated switching means are required to enable charging to be discontinued on those cells that become fully charged and to continue on those that are still not fully charged.
A particular type of rechargeable battery is that based on the electrochemical reaction between a negative electrode of a lithium-aluminium alloy and a positive electrode of iron sulphide. The invention is particularly concerned with this type of battery but is also applicable wherever similar reversible reactions occur. In one variation Lithium-Aluminium could be replaced by Lithium-Calcium. Alternatively lithium could be replaced by sodium, potassium, calcium or other alkali or alkaline earth metals which are sufficiently reactive. Additionally iron sulphide could be replaced by iron disulphide or nickel sulphide or certain transition metal sulphides or oxides.
The inventor has discovered that it is the presence of hydrogen in a lithium battery cell that causes it to self-discharge and leads to problems of out-of-balance charges. Although lithium batteries are made under the driest conditions possible it is practically impossible to ensure that all water is removed from the battery chemicals and thus there will always be some residual water. When the battery

REFERENCES:
Patent Abstracts of Japan, vol. 5, No. 99 (E-63)(771), Jun. 26, 1981, JP, A 5642971 (Tokyo Shibaura Denki K.K.) Apr. 21, 1981.
Patent Abstracts of Japan, vol. 10; No. 129 (E-403)(2186), May 14, 1986, JP, A 60258870 (Matsushita Denki Sangyo K.K.) Dec. 20, 1985, International Search Report.

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