This application is based on French Patent Application No. 00 08 153 filed Jun. 26, 2000, the disclosure of which is hereby incorporated by reference thereto in its entirety, and the priority of which is hereby claimed under 35 U.S.C. xc2xa7119.
1. Field of the Invention
The present invention relates to a storage cell battery incorporating a safety device. The device operates in the event of an anomaly in the operation of one or more cells to increase the response time and minimize the risks, in particular of fire. This is why the invention is more particularly intended to be applied to lithium rechargeable storage cell batteries, some components of which are inflammable. Improving the safety of the user is one imperative for xe2x80x9cconsumerxe2x80x9d applications, for example if the battery is installed in an electric vehicle.
2. Description of the Prior Art
Storage cells have an enclosure containing an electrode assembly impregnated with electrolyte and made up of at least one positive electrode, at least one negative electrode, and at least one separator disposed between the electrodes. The cells can be combined to form a battery by series or parallel connections. They are placed together in a closed container. An anomaly in the operation of the battery can be caused by the malfunctioning of one of the cells (short circuit, etc) or an external disturbance (impact, temperature rise, etc). The anomaly causes the battery to become heated and increases the risk of fire. The proximity of a large number of cells multiplies the risk accordingly, which makes external intervention necessary for permanently safeguarding the battery. The risk is particularly high in the case of lithium cells, which in particular contain organic solvents that are readily volatile at relatively low temperatures in the range from 70xc2x0 C. to 120xc2x0 C., with flash points in the range of ambient temperature up to 100-200xc2x0 C.
Safety devices have been proposed whose object is to extinguish a fire using an extinguishing agent that can take the form of a gas, a liquid, a powder, granules or a mixture of the above.
The device can be installed inside the cell itself, as described in the documents JP-9 161 754 and U.S. Pat. No. 5,523,178. This has the drawback of increasing the total volume of the battery in proportion to the number of cells constituting it. The energy per unit volume of this kind of battery falls, and soon becomes incompatible with user expectations. In other cases (JP-03 112 570), an extinguishing agent stored externally is introduced directly into the interior of each cell, which requires an injection conduit specific to each cell. Once again, the resulting overall size is prohibitive for a consumer application, for example in an electric vehicle, where the space available for the battery is very constricted.
In the particular case of sodium-sulfur (Naxe2x80x94S) cell batteries, which operate at high temperatures (300xc2x0 C.-400xc2x0 C.), several documents (JP-4 288 169, JP-4 300 563, JP-4 303 466 and JP-5 031 207) describe fire-fighting equipment that injects a granulated extinguishing agent into the container of a battery via an injection nozzle (JP-4 288 169 or JP-5 031 207) or preferably via a distribution manifold so that the agent is distributed homogeneously (JP-4 300 563 or JP-4 303 466). Injection is commanded by a fire detector, such as a temperature probe, placed on the container. The fire-fighting equipment is disposed outside the battery. It includes in particular a storage tank containing the extinguishing agent granules and an inert gas and a device for pressurizing the gas contained in the storage tank. The storage tank and the device are connected to each other and to the container by pipes and valves.
It takes a long time for the fire-fighting equipment to respond, in particular because this entails a time-delay to pressurize the storage tank using a gas supply device. To reduce this time-delay, it has been proposed to keep the storage tank pressurized by connecting it to a pressurized cylinder (JP-5 084 318). However, pressurized gas leaks into the valves and the pipes and when fire breaks out there is insufficient gas to entrain the extinguishing agent. A compressor can be added to the equipment to remedy this drawback (JP-5 089 908). This improvement requires an additional component, and therefore increases the volume of the equipment, increases its cost and requires more complicated maintenance. Moreover, if the storage tank is pressurized, the powder and the granules of extinguishing agent can find their way into the seats of the valves and the connecting flanges, which increases leaks.
Also in the case of Naxe2x80x94S cells, the document JP-5 317 440 proposes fire-fighting equipment that does not include any auxiliary device for pressurizing the storage tank. The equipment includes a device for supplying pressurized gas to the extinguishing agent storage tank and a device for admitting pressurized gas to the battery container. If fire breaks out in the container, the gas is emitted simultaneously toward the container and toward the storage tank by an array of pipes and a set of automatic valves whose control circuit is connected to the fire detector. Once the pressure in the storage tank has reached the required value, the control device interrupts the emission of gas by the emission device. The extinguishing agent entrained by the gas from the supply device is then fed into the container.
The devices previously described have the drawback of reacting too slowly for the safety of the user. The outbreak of fire is detected by a temperature sensor which causes valves to be opened via a control circuit. The gas must then flow through the pipes before reaching the container. These successive operations necessitate time-delays that can be as much as several seconds.
In the case of a lithium rechargeable cell battery, in particular, because of the presence of very volatile organic solvents, this time period is sufficient to reach an unacceptable level of risk. In particular, this battery has a high risk of fire due to products generated by thermal runaway of the reaction.
An object of the present invention is to eliminate the drawbacks of the prior art and in particular to shorten the time-delay between detecting an anomaly and the removal from danger of a cell battery in order to enhance the safety of the user. The invention relates more precisely to a safety device, in particular a fire-fighting device, solving this problem.
The present invention provides a storage cell battery including:
at least one cell having an enclosure containing an electrode assembly impregnated with an electrolyte and comprising at least one positive electrode, at least one negative electrode and at least one separator disposed between the electrodes,
a common container provided with a first orifice, and
a safety device including an anomaly detector, a storage tank containing a non-inflammable gas under pressure and connected to the first orifice, and a control system in the form of a pyrotechnic mechanism for controlling the admission of the gas into the common container.
The function of the safety device according to the invention is to prevent the onset or to stop the evolution of a fire affecting the cells and which is likely to propagate. The combination of a pyrotechnic mechanism with a very short response time and a non-inflammable gas injected at high speed into the container because of the pressure fills the common container virtually immediately and improves the efficiency of flame inhibition. With the danger averted in this way, the user can act himself or call on external help.
The anomaly detector includes a system for measuring the value of the parameter to be detected and a system for comparing the measured value to a reference value. The measuring system can be a temperature sensor, for example, a pressure sensor or a shock detector. The comparator system can be a probe for measuring the outdoor temperature or a sensor for measuring the outside pressure.
On detecting a value, for example a temperature value, greater than the fixed reference value, the pyrotechnic mechanism is fired. The mechanism includes a pyrotechnic composition and an initiator which can be electrical, mechanical or magnetic. An electrical initiator is preferably used, with a locally heated filament that causes very energetic combustion of a powder composition. Its operating time-delay is of the order of a few milliseconds.
A first variant of the mechanism includes an explosive composition whose detonation opens a vent mechanically. The vent is inserted into the wall of the inert gas storage tank at an element for communication with the container, for example.
In a second variant, the mechanism contains a composition whose combustion generates gases producing a pressure sufficient to actuate a piston. The pressure increase is sufficient to push the piston and the movement of the piston frees the passage leading to the container for the inert gas coming from the storage tank.
The inert gas storage tank contains carbon dioxide CO2 or argonite (equi-volume mixture of nitrogen N2 and argon Ar). It is a pressurized cylinder, for example. Argonite is preferred in the case of use in an electric vehicle. CO2 must be stored in the liquid state and the storage pressure varies with temperature. Storage requires a pressure of almost 200 bars, given the temperature under the hood of a vehicle (50-60xc2x0 C.), compared to only 52 bars at room temperature (20xc2x0 C.).
In one embodiment, the container is further provided with a second orifice. This is the case in particular if the battery is cooled by sweeping air through the container, the additional orifice being an air intake.
The battery according to the invention can equally be provided with some other type of cooling device, for example one circulating a fluid in a double wall of the cell enclosure.
In a preferred embodiment, at least one of the orifices contains a check valve, preferably a low-pressure check valve. Air is expelled from the container at a high flowrate by the sudden introduction of the inert gas. The valve then closes again, possibly allowing a slight leak to prevent backflow of air.
The invention will be better understood and other advantages and features will become apparent on reading the following description of embodiments of the invention, which refers to the accompanying drawings.