GB2133923A - Replenishment system for multi- cell electric batteries - Google Patents

Abstract

A system for replenishing liquid in a battery having a plurality of cells 10 comprises a reservoir 13 having compartments 12 equal in number to the cells 10, each compartment 12 being connected to a cell 10 by a pipe 11 having a bore which is sufficiently small to inhibit free flow from the compartments 12 to the cells 10. The pipes 11 open into the cells 10 at the desired level of electrolyte therein. A gas pressure Pa applied to the tops of the compartments 12 to urge liquid therein into the cells 10 and ultimately to compress trapped gas volumes in the cells 10. Subsequent removal of pressure Pa allows excess liquid in the cells 10 to flow back to the compartments 12 until the bottoms of the pipes 11 are uncovered. <IMAGE>

Description

SPECIFICATION Replenishment system for multi-cell electric batteries This invention relates to a replenishment system for a multi-cell electric battery.

Large electric batteries, such as are used for powering electric vehicles, have a large number of cells and manual topping-up, or replenishment, of these cells is very time consuming. It has been proposed to top up the cells of such batteries by connecting groups of the cells in series by means of a supply pipe and drawing topping-up liquid through the pipe from a reservoir, each cell being provided with a valve or similar device which prevents further supply of topping-up liquid when the electrolyte in the cell reaches a predetermined level. In addition to a separate level control device on each cell, such a system requires the provision of a pump and multiway valves for selectively coupling the supply pipes either to the pump and reservoir or to external vents.

It is an object of the present invention to provide a replenishment system in which a valve or similar device is not required to establish an electrolyte level in each cell, in which gas pressures in the cells do not result in transfer of electrolyte between the cells and in which a separate installation which includes a pump and multiway valves is not required.

According to the invention a replenishment system for a multi-cell electric battery comprises a plurality of containers for replenishment liquid, the number of said containers being equal to that of the cells in said battery, means for introducing replenishment liquid to a predetermined level in each said container and a plurality of pipes connecting said containers with respective ones of said battery cells, said pipes opening into said cells at a desired level of electrolyte therein.

In a preferred embodiment the dimensions of said containers are such that when the liquid therein is at said predetermined level the volume of liquid in each container is approximately half the volume of the space within each cell above the tops of the cell plate separators.

In a particular embodiment there is provided means for supplying a gas under pressure to the tops of said containers.

An embodiment of the invention will now be described by way of example only and with reference to the accompanying drawings in which:~ Figure 1 is a diagram showing a plurality of battery cells and a replenishment system, and Figure 2 is a diagram of a single cell and a container for replenishment liquid, indicating the relationship between the liquid levels therein.

As shown in Figure 1 a plurality of sealed cells 10 form part of an electric battery and each cell 10 is connected by a flexible pipe 1 1 to one compartment 12 of a reservoir 13. In a particular example where the battery contains 108 cells two reservoirs 13, each containing 54 of the compartments 12 would be provided. The pipes 11 communicate with the bottoms of the compartments 12 and the tops of these compartments are provided with inwardly convergent openings 14 which serve to prevent liquid within the compartments 12 from splashing out and entering an adjacent compartment. The reservoir 13 includes a chamber 15 from which topping-up liquid can flow to the compartments 12. A sealable filler cap 16 is provided, through which topping-up liquid can be introduced into the chamber 15 and compartment 12.A drain connection 17 communicates with the chamber 15 through an electro-magnetic valve 18, a U-trap 19 and a weir 20. The valve 18 is open in its de-energised state so that during filling of the reservoir 13 the liquid level therein will not rise above the weir 20. An air pressure supply line 21 also communicates with the chamber 1 5 through a non-return valve 22 and the U-trap 19. Pressure is applied to the line 21 from a source 24 which may be a pump or a high pressure reservoir. A control device 25 is provided for regulating delivery of pressure by the source 24.

Preferably communication between the pipes 1 1 and compartment 12 is through U-traps 23 which are provided at the bottom of each compartment 12. As will be explained hereafter sufficient liquid will normally remain within each of the compartments 12 to enable the traps 23 to act as flame barriers in the event of an explosion in any of the cells 10. Alternatively a suitable flame trap and pressure relief valve may be provided on each of the cells 10.

Figure 2 shows diagrammatically a section through a single cell 10 and its connection to a compartment 12 through one of the pipes 1 The plates of the cell 10 are spaced apart by separators, one of which is indicated at 30. A volume VH exists above the separators 30 within the cell 10, and the capacity of the compartment 12 is approximately half the volume VH. In the present example the bottom of the compartment 12 is 1200 mm above the top of the cell 10 and the bore of the tube 1 1 is typically 2.0 mm, but does not exceed 5 mm. The tube 11 opens into the cell 10 at a level therein corresponding to the desired level of electrolyte. This level is typically half way between the top of the separators 30 and the top of the cell. Assume initially that the electrolyte level is at the top of the separators 30.

When the reservoir 13 (Figure 1) is charged with topping-up liquid this liquid will flow slowly from the compartment 10 through the restriction provided by the tube 1 1 into the cell 10 to reduce the clearance volume therein. A first equilibrium condition will obtain when the elevation head Pel corresponds to the pressure Pv1 in a reduced volume V1. The small bore of the tube 1 8 has the effect that gas from the cell 10 cannot readily pass upwardly through the tube 1 1 and flow of topping-up fluid thereby ceases, even though the bottom of the tube 11 is above the level of the electrolyte.

However, if before the aforesaid first equilibrium condition is reached, a gas pressure Pa, which is may be of the order of 100 mm Hg is applied through the line 21 and the valve 22 (Figure 1) to the top of the compartment 1 1, this pressure Pa acts to force liquid through the pipe 1 1. A further equilibrium condition will obtain when the clearance in the cell is reduced to a volume V2, and the pressure Pv2 in the volume V2 is equal to the sum of the pressure Pa and that arising from the new elevation head Pe2. If the air pressure Pa is now removed the pressure Pv2 urges some of the electrolyte back up the tube into the container 12 to provide an increased clearance volume V3 in the cell, and a new elevation head Pe3. This flow of electrolyte will cease when its level within the cell 10 reaches the bottom of the tube 11.If an equilibrium condition is reached in which the electrolyte level is above the bottom of the tube 1 1 when the pressure Pv3 in volume V3 equals pressure Pe3, subsequent gassing of the cell during charging will increase the pressure in the cell to transfer electrolyte therefrom to the container 12 until the desired level is reached. Gas subsequently generated in the cell 10 will pass upwardly through the trap 23 and thence to atmosphere through the chamber 15 and trap 19 (Figure 1). The cell 10 is provided with a pressure relief valve, indicated at 31 , which operates to relieve gas pressure in the cell 10 in the event that the pipe 1 1 becomes blocked, for example as a result of freezing of the liquid therein.

Preferably in use of the apparatus a battery charge must take place between a topping-up operation as described and the next successive topping-up. This ensures that the electrolyte level in the cell is lowered to the bottom of the connecting tube 11. Even if the electrolyte level in the cell 10 does not thereafter drop, filling of the compartment 12 and its subsequent pressurisation and de-pressurisation will have the effect only of transferring a quantity of liquid from the compartment 12 to the cell 10 and back again. Electrolyte from the cell 10 will not therefore flood over the top of the compartment 12 to enter adjacent compartments.

Alternatively the air pressure Pa may be made sufficiently high to ensure that the pressure Pv2 will always lower the electrolyte in the cell to the bottom of the tube 11, in which case the desired level is obtained without an intervening battery charge.

Since the aforementioned pressure Pa of 100 mm Hg may in some cases be sufficient to cause distortion of the containers of the battery cells, the pressure source 24 may be controlled by the device 25 to apply a sequence of lower pressures, each being in the order of 50 mm Hg, but in any case not greater than 75 mm Hg.

Typically the sequence will comprise a plurality of pressure applications of 30 seconds duration each, spaced at 30 second intervals. A first such pressure application will urge liquid through the pipe 1 1 until the pressure in the cell 10 is equal to sum of the applied pressure Pa and the new elevation head Pe. During the succeeding interval the excess air pressure will be vented back up the pipe 11, leaving the cell pressure substantially equal to the new elevation head Pe and a raised liquid level in the cell. Once the bottom of the pipe 11 has been covered, successive applications of pressure Pa will merely transfer liquid to the cell 10 and allow this liquid to return to the compartment 12 in the intervals.

Claims (8)

1. A replenishment system for a multi-cell electric battery, comprising a plurality of containers for replenishment liquid, the number of said containers being equal to that of the cells in said battery, means for introducing replenishment liquid to a predetermined level in each said container and a plurality of p.pes connecting said containers with respective ones of said battery cells, said pipes opening into said cells at a desired level of electrolyte therein.

2. A system as claimed in claim 1 in which the dimensions of said containers are such that when the liquid therein is at said predetermined level the volume of liquid in each container is approximately half the volume of the space within each cell above the tops of the cell plate separators.

3. A system as claimed in claim 1 or claim 2 which includes means for supplying a gas under pressure to the tops of said containers.

4. A system as claimed in claim 3 in which said gas supplying means comprises a pressure source and a control device for causing the pressure source to supply gas to said containers as a sequence of spaced substantially equal pressures.

5. A system as claimed in claim 4 in which said pressures do not exceed 75 mm Hg.

6. A system as claimed in any preceding claim in which the bottoms of said containers are positioned above the levels at which said pipes open into said cells.

7. A system as claimed in any preceding claim in which the bores of said pipes do not exceed 5 mm.

8. A replenishment system for a multi-cell electric battery, substantially as hereinbefore described with reference to the accompanying drawings.