WO2011086562A1

WO2011086562A1 - Method of pulse charging

Info

Publication number: WO2011086562A1
Application number: PCT/IL2011/000053
Authority: WO
Inventors: Eran Ofek
Original assignee: Individual
Current assignee: Individual
Priority date: 2010-01-18
Filing date: 2011-01-18
Publication date: 2011-07-21
Anticipated expiration: 2012-07-18

Abstract

A method of pulse charging of a pack of battery cells provided with battery terminals is disclosed. A plurality of battery cells are organized as a pack. The aforesaid method comprises the steps of: (a) providing a charging device connectable to a source of electric energy; (b) electrically connecting said charging device to the terminals of battery cells; and (c) pulse charging the pack of battery cells by means of applying a train of voltage pulses to the terminals. The step of electrically connecting of the charging device to said terminals of the battery cells is performed individually to each cell. The step of pulse charging is performed by means of applying the train of voltage pulses over groups of battery cells in the pack, in a cyclic consecutive manner.

Description

METHOD OF PULSE CHARGING

FIELD OF THE INVENTION

The present invention relates to a method of battery recharging, and, more specifically, it pertains to a method of pulse charging.

BACKGROUND OF THE INVENTION

All batteries depend for their action on an electrochemical process whether charging or discharging and we know that these chemical reactions are in some way dependent on temperature. Nominal battery performance is usually specified for working temperatures somewhere in the + 20°C to +30°C range, however the actual performance can deviate substantially from this if the battery is operated at higher or lower temperatures.

Arrhenius' law tells us that the rate at which a chemical reaction proceeds, increases exponentially as temperature rises. This allows more instantaneous power to be extracted from the battery at higher temperatures. At the same time higher temperatures improve electron or ion mobility reducing the cell's internal impedance and increasing its capacity.

At the upper end of the scale the high temperatures may also initiate unwanted or irreversible chemical reactions and/or loss of electrolyte which can cause permanent damage or complete failure of the battery. This in turn sets an upper temperature operating limit for the battery.

At the lower end of the scale the electrolyte may freeze, setting a limit to low temperature performance. But well below the freezing point of the electrolyte, . battery performance starts to deteriorate as the rate of chemical reaction is reduced. Even though a battery may be specified to work down to -20°C or -30°C the performance at 0°C and below may be seriously impaired.

Note also that the lower temperature working limit of a battery may be dependent on its State of Charge. In a Lead Acid battery for instance, as the battery is discharged the Sulphuric Acid electrolyte becomes increasingly diluted with water and its freezing point increases accordingly.

Thus the battery must be kept within a limited operating temperature range so that both charge capacity and cycle life can be optimized. A practical system may therefore need both heating and cooling to keep it not just within the battery manufacturer's specified working limits, but within a more limited range to achieve optimal performance.

Thermal management however is not just about keeping within these limits. The battery is subject to several simultaneous internal and external thermal effects which must be kept within control.

The operation of any battery generates heat due to the I²R losses as current flows through the internal resistance of the battery whether it is being charged or discharged. This is also known as Joule heating. In the case of discharging, the total energy within the system is fixed and the temperature rise will be limited by the available energy. However, this can still cause very high localized temperatures even in low power batteries. No such automatic limit applies while charging as there is nothing to stop the user continuing to pump electrical energy into the battery after it has become fully charged. This can be a very risky situation.

Battery designers strive to keep the internal resistance of the cells as low as possible to minimize the heat losses or heat generation within the battery but even with cell resistances as low as 1 milliohm the heating can be substantial.

In addition to Joule heating the chemical reactions which take place in the cells may be exothermic, adding to the heat generated or they may be endothermic, absorbing heat during the process of the chemical action. Overheating is therefore more likely to be a problem with exothermic reactions in which the chemical reaction reinforces the heat generated by the current flow rather than with endothermic reactions where the chemical action counteracts it. In secondary batteries, because the chemical reactions are reversible, chemistries which are exothermic during charging will be endothermic during discharging and vice versa. In most situations the Joule heating will exceed the endothermic cooling effect so precautions still need to be taken. Lead acid batteries are exothermic during charging and Valve Regulated Lead Acid (VRLA) batteries are prone to thermal runaway. Nickel Metal Hydride (NiMH) cells are also exothermic during charging and as they approach full charge, and the cell temperature can rise dramatically. Consequently, chargers for NiMH cells must be designed to sense this temperature rise and cut off the charger to prevent damage to the cells. By contrast Nickel based batteries with alkaline electrolytes (NiCads) and Lithium batteries are endothermic during charging. Nevertheless thermal runaway is still possible during charging with these batteries if they are subject to overcharging.

The thermochemistry of Lithium cells is slightly more complex, depending on the state of intercalation of the Lithium ions into the crystal lattice. During charging the reaction is initially endothermic then moving to slightly exothermic during most of the charging cycle. During discharge the reaction is the reverse, initially exothermic then moving to slightly endothermic for most of the discharge cycle. In common with the other chemistries, the Joule heating effect is greater than the thermochemical effect so long as the cells remain within their design limits.

The thermal condition of the battery is also dependent on its environment. If its temperature is above the ambient temperature it will lose heat through conduction, convection and radiation. If the ambient temperature is higher, the battery will gain heat from its surroundings. When the ambient temperature is very high the thermal management system has to work very hard to keep the temperature under control. A single cell may work very well at room temperature on its own, but if it is part of a battery pack surrounded by similar cells all generating heat, even if it is carrying the same load, it could well exceed its temperature limits.

The net result of the thermo-electrical and thermo-chemical effects, possibly augmented by the environmental conditions, is usually a rise in temperature and as noted above, this will cause an exponential increase in the rate at which a chemical reaction proceeds. It is also known that if the temperature rise is excessive the active chemicals expand causing the cell to swell. Subsequent problems include:

• Mechanical distortion of the cell components which may result in short circuits or open circuits Irreversible chemical reactions can occur which cause a permanent reduction in the active chemicals and hence the capacity of the cell is reduced

Prolonged operation at high temperature can cause cracking in plastic parts of the cell

The temperature rise causes the chemical reaction to speed up increasing the temperature even more and could lead to thermal runaway

Gases may be given off

Pressure builds up inside the cell

The cell may eventually rupture or explode

Toxic or inflammable chemicals may be released

As battery engineers strive to cram more and more energy into ever smaller volumes, the applications engineer has increasing difficulty in retrieving the energy. The great strength of new technology batteries is unfortunately also the source of their greatest weakness.

The thermal capacity of an object defines its ability to absorb heat. In simple terms for a given amount of heat, the bigger and heavier the object is, the smaller will be the temperature rise caused by the heat.

For many years lead acid batteries have been one of the few power sources available for high power applications. Because of their bulk and weight, temperature rise during operation has not been a major problem. But in the quest for smaller, lighter batteries with higher power and energy densities, the unavoidable consequence is that the thermal capacity of the battery will be decreased. This in turn means that for a given power output, the temperature rise will be higher.

The result is that heat dissipation is a major engineering challenge for high energy density batteries used in high power applications. Cell designers have developed innovative cell construction techniques to get the heat out of the cell. Battery pack designers must find equally innovative solutions to get the heat out of the pack.

The operating temperature which is reached in a battery is the result of the ambient temperature augmented by the heat generated by the battery. If a battery is subject to excessive currents the possibility of thermal runaway arises resulting in catastrophic destruction of the battery. This occurs when the rate of heat generation within the battery exceeds its heat dissipation capacity. There are several conditions which can bring this about:

• Initially the thermal I²R losses of the charging current flowing through the cell heat up the electrolyte, but the resistance of the electrolyte decreases with temperature, so this will in turn result in a higher current driving the temperature still higher, reinforcing the reaction till a runaway condition is reached.

• During charging the charging current induces an exothermic chemical reaction of the chemicals in the cell which reinforces the heat generated by the charging current.

• Or during discharging the heat produced by the exothermic chemical action generating the current reinforces the resistive heating due to the current flow within the cell.

• The ambient temperature is excessive.

• Inadequate cooling.

Unless some protective measures are in place the consequences of the thermal runaway could be meltdown of the cell or a build up of pressure resulting an explosion or fire depending on the cell chemistry and construction.

The thermal management system must keep all of these factors under control.

The relevance of the formulated problems can be proved by simulations performed by means of mathematical model of a lithium ion battery.

This thermal model is developed based on the pseudo two-dimensional (P2D) model and a thermal electrochemistry coupled model. The diffusion coefficient of Li ions in the solid phase and electrolyte, the reaction rate constants of the electrochemical reactions, the open circuit potentials and the thermal conductivity of the binary electrolyte depend on the temperature in the model presented here.

The energy balance is given by

with the boundary conditions determined by Newton's cooling law:

dT

= h - T_∞- T)

where h is the heat transfer coefficient, T_∞ is the environmental temperature, is the total reaction heat generation rate, Q_rev is the total reversible heat generation rate, Q_ohm is the total ohmic heat generation rate. The heat fluxes are defined by:

Q_m = FaJ - ( - t₂ - U)

The temperature dependent open circuit potential of electrode is approximated by Taylor's first order expansion around a reference temperature:

^~dU^~

U, = U,_ref + (T -T_ref)

dT _

where is the open circuit potential under the reference temperature T_Tef.

Several geometries, specifically, a ID geometry which consists of three sequentially connected lines to represent the positive electrode, the separator and the negative electrode, respectively, a 2D geometry which consists of two rectangles to denote the solid phase in the electrodes are considered. The considered geometries are shown in Fig. 1. The vertical coordinate in the 2D geometry indicates the radial direction of the solid particles. Since we ignore the diffusion of Li ions in the x-direction in the particle, the corresponding diffusion coefficient is set to zero in this direction. The concentration of the binary electrolyte, the potential in the electrolyte, the potential in the solid phase and the pore wall flux are solved in the ID geometry. The concentration of Li ions in the solid phase is solved in the 2D geometry. The pore wall flux is extruded from the ID domain and projected to the top boundary of the 2D geometry by using "subdomain extrusion coupling variables". The concentration of Li ions on the top boundary in the 2D geometry is projected to the ID domain by using "boundary extrusion coupling variables". The battery is discharged for 3000s at C/2 rate first and then discharged at 3C rate until the cell voltage drops to 2.5V. The change of the applied current density is implemented by using the smoothed Heaviside function "flsmhs" and is shown in Fig. 2.

Fig. 3 shows the temperature on the cell surface at 1C discharge process under three different cooling conditions where the heat transfer coefficient is 10.0, 1.0 and 0.1 W/m2/K, respectively, and two limiting conditions: the isothermal condition and the adiabatic condition. The thermal effect on the cell voltage is shown in Fig. 4. The cell provides more discharge capacity when it is placed in a better heat isolation environment (i.e. adiabatic condition). In a better isolated environment, the cell temperature increases faster during the 1C discharge process which results in the higher diffusion coefficient for the binary electrolyte and reduces the diffusion limitations.

The reduction of the diffusion limit in the binary electrolyte can be verified by comparing the concentration profile of the electrolyte under different cooling conditions. Fig. 5 shows the concentration profiles of the electrolyte at the end of 1C discharge process under the two limiting conditions. The concentration profile under the adiabatic condition is flatter than that in the isothermal case, which indicates a better diffusion property in the electrolyte under the adiabatic condition than under the isothermal condition.

Fig. 6 shows the cell temperature during the 1C discharge process at different current rates as the heat transfer coefficient is 1.0 W/m2/K. As expected, the cell gets hotter as the discharge current rate increases. It is also noticed that the wave part which appears in beginning of the temperature curve at low current rate (less than 2C) does not exist in the high current rate cases. The wave part on the temperature curve is characterized by the reversible heat generation during discharging. Under low current rate discharging, the reversible heat is roughly equivalent to the ohmic heat, but becomes unimportant as the discharge current rate increases. The P2D model mentioned in section 2 is also useful for simulating the discharge process with pulse. Fig. 7 shows the cell voltage during the C/2 discharge for 3000s followed by a 3C pulse discharge until the cell voltage drops to 2.5V. The corresponding temperature on the surface of the cell is also plotted in Fig. 8. The surface temperature at the end of the 3C pulse is slightly less than that in the pure 3C discharge process. Fig. 9 shows the concentration of the binary electrolyte at the two ends of the cell during the pulse discharge process. At the beginning of the pulse, the concentration of the electrolyte changes extremely, after that it relaxes and tend to a stable value.

US Patent 5945811 ('511) discloses a pulse charging method and charging system for use with non-aqueous secondary batteries, employing a pulse charge controlling method all the way from the start to the end of charging. The pulse charging method has an on-duty ratio of pulses in a next specified charge period reduced when an average battery voltage has exceeded a charge control voltage during a specified charge period, has an on-duty ratio of pulses in a next specified charge period increased when the average battery voltage has not exceeded the charge control voltage and has the pulse charging ended when an on-duty ratio of pulses has reached a specified value. The pulse charging system comprises an on-duty ratio reducing means for having an on-duty ratio of pulses reduced, an on-duty ratio increasing means for having an on-duty ratio increased and a means for determining pulse charge ending for having the pulse charging ended when an on-duty ratio of pulses has reached a specified value.

It should be emphasized that any charging process is fraught with heat generation resulting in life shortening of the rechargeable batteries. It is a long-felt and unmet need to provide a method and means for recharging batteries which reduces the charging temperature and extends the service life of the rechargeable batteries. Prevention of or protection from the thermal runaway should be provided. SUMMARY OF THE INVENTION

It is hence one object of the invention to disclose s method of pulse charging of a pack of battery cells provided with battery terminals. The aforesaid method comprises the steps of: (a) providing a charging device connectable to a source of electric energy; the charging device adapted for providing a voltage pulse train to the terminals; (b) electrically connecting the charging device to the terminals of battery cells; and (c) pulse charging the pack of battery cells by means of applying a train of the voltage pulse train to the terminals.

It is a core purpose of the invention to provide the step of electrically connecting the charging device to the terminals of the battery cells is performed individually to each cell. The step of pulse charging is performed by means of applying the train of voltage pulses over the battery cells in a cyclic consecutive manner.

Another object of the invention is to disclose a time interval between said charging pulses at said step of pulse charging which are applied to each cell is sufficient for dissipating heat generated by a charging current conducted across said cell.

A further object of the invention is to disclose the interval between the charging pulses comprising at least one voltage pulse of an opposite polarity.

A further object of the invention is to disclose the train of charging pulses comprising a plurality of sutbrains. The charging pulses belonging to one subtrain are identically to each other. The charging pulses belonging to one subtrain are consecutively distributed over said battery cells.

A further object of the invention is to disclose a duration of the charging pulse increasing over time of charging.

A further object of the invention is to disclose a duration of the pulse interval increases over time of charging.

A further object of the invention is to disclose a step of monitoring battery pack parameters and optimizing charging process.

A further object of the invention is to disclose a device for pulse charging of a pack of battery cells provided with battery terminals. The aforesaid device is connectable to a source of electric energy. The pulse charging device comprises a generator of a voltage pulse train provided to the terminals.

It is a core purpose of the invention to provide the device further comprising a commutating circuitry. The commutating circuitry is adapted to commutate the voltage pulses over the battery cells in a cyclic consecutive manner.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be implemented in practice, a plurality of embodiments is adapted to now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which

Fig. 1 is a scheme presenting geometries and variables coupling between the geometries;

Fig. 2 is a graph of current density profile in the discharge process including a 3C pulse;

Fig. 3 is a graph of the temperature on the cell surface during 1C discharge process under different cooling conditions;

Fig .4 is a graph of the cell Voltage for 1C discharge process under different cooling conditions;

Fig. 5 is a graph of the concentration profiles of the binary electrolyte at the end of the 1C discharge process under the isothermal condition and the adiabatic condition;

Fig. 6 is a graph of the temperature on the cell surface during discharge process under different current rates;

Fig. 7 is a graph of the cell Voltage at C/2 discharge for 3000s followed by a 3C pulse discharge;

Fig. 8 is a graph of the temperature on the cell surface in the discharge process with 3C pulse;

Fig. 9 is a graph of the concentration of the binary electrolyte at the two ends of the cell;

.1.0 Fig. 10 is a block diagram of the pulse charging device connected to the battery pack;

Fig. 1 1 is a graph of the train of the pulses applied to one battery cell;

Fig. 12 is a graph of voltage on the battery cell which is charged by the pulse train;

Fig. 13 is a graph of the train of the pulses applied to three battery cells;

Fig. 14 a-c are a schematic presentation of cylindrical battery cell and a graph of the dependence of the form factor on cell quantity;

Fig. 15 a-c are a schematic presentation of brick battery cell and a graph of the dependence of the form factor on cell quantity;

Fig. 16 is a pseudo-color pattern of the temperature distribution over battery cell; and

Fig. 17 is a pseudo-color pattern of the temperature distribution over multi-cell battery created in the course of the pulse charging process.

DETAILED DESCRIPTION OF THE INVENTION

The following description is provided, alongside all chapters of the present invention, so as to enable any person skilled in the art to make use of said invention and sets forth the best modes contemplated by the inventor of carrying out this invention. Various modifications, however, are adapted to remain apparent to those skilled in the art, since the generic principles of the present invention have been defined specifically to provide a method of pulse charging an electric battery and a device for performing thereof.

While a battery is being charged or discharged, the heat generation cause by flowing current raises the temperature until balance is achieved between heat generation in the cell and heat dissipation to the environment. Thus the two parameters of heat generation within the battery and heat dissipation from the battery determine the temperature T changes of the battery according to the formula

dt C dt dt where dQ_sen/dt is generated energy per unit of time, dQ_&ssldt is dissipated energy per unit of time, flatt is specific-heat capacity of a battery.

Due to manufacturing tolerances or uneven temperature conditions across the pack or non uniform ageing patterns, some cells accept less charge than others. The result is that in a series chain, a weak cell with reduced capacity will reach its full charge before the rest of the cells in the chain and become overcharged as the charger attempts to charge the overall cell chain to its nominal voltage. As already noted, overcharging causes the cell to overheat resulting in expansion of the active chemicals as well as the possible gassing of the electrolyte. These factors in turn cause the internal pressure to rise, resulting in overstress and possible damage to the cell. This will be repeated with every charge- discharge cycle causing the cell to become more stressed and hence even weaker until it eventually fails. On the other hand, if for some reason the weak cell cannot reach full charge, perhaps due to a very high self discharge, or in an extreme case, a short circuited cell, then the good cells, rather than the weaker cell, could possibly become overcharged.

Damage to weaker cells can also continue during the discharge cycle. When discharged in a series configuration, the capacity of the weakest cell in the chain will be depleted before the others. If the discharge is continued (to discharge the remaining good cells), the voltage on the low capacity cell will reach zero then reverse due to the IR voltage drop across the cell. Subsequent heat and pressure build up within the cell due to "cell reversal" can then cause catastrophic failure.

The initial tolerance spread which caused these interactions may be very low but it can build up over time as the damage increases with every charge-discharge cycle until the weak cells eventually fail. The term "cyclically consecutively charging" hereinafter refers to consecutively charging battery cells of a battery pack. The sequence of charging is a closed-loop cycle.

Reference is now made to Fig. 10, which shows a pulse charging device 100 electrically connected to a battery pack 150 comprising a plurality of battery cells 160. The pulse charging device 100 is energized by a power source 110. The pulse charging device comprises a pulse generator adapted to generate a train of voltage pulses characterized by variable pulse durations and intervals between pulses and a commutation circuitry 130 which distributes the pulses belonging to the generated train over the battery cells 160 is a consecutive cyclic manner.

Reference is now made to Fig. 11, presenting a train of voltage pulses applied to one battery cell. The pulses in the train are separated by time intervals which are sufficient for dissipation of heat generated by charging current. Thus, cell charging is performed by the train of pulses of relatively short duration in comparison with the interval therebetween. Optionally, the aforesaid pulse train comprises a voltage pulse of opposite polarity.

Reference is now made to Fig. 12, showing a time curve of voltage at the terminals of the battery cells in the process of charging. In a unlimited manner, the battery cell is provided with charging current. At the initial period of charging, the cell is charged in a continuous manner. Further, the cell is charged by the train of pulses of increasing relative pulse duration.

Reference is now made to Fig. 13, which illustrates the core of the present invention. In this particular case, a train of voltage pulses is distributed over a group of three battery cells in a consecutive cyclic manner. The charging process is organized in such a way that each battery cell is charged in a time period when the other two cells dissipate the heat generated by the charging current therein. Thus, the proposed technical solution reduces likelihood of a battery fault because of thermal runaway. It is herein acknowledged that in some embodiments of the invention, any number of battery cells can constitute a chargeable pack. Reference is now made to Figs 14 a-c, depicting dependence of heat dissipation on a form factor of a cylindrical battery.

Comparing volumes of two cylindrical batteries of equal length and different radii R_\ and i?₂, we have

V_] = nR h; V₂ = nR₂ ²h .

Introducing a factor n =

we have the ratio of dissipation surface areas

The results of numerical simulation of the form-factor n for the cylindrical battery are presented in Fig. 14b.

Reference is now made to Fig. 15a-c, depicting dependence of heat dissipation on a form factor of a parallelepiped-like battery. Analogous calculations for parallelepiped-like battery give the following expression V₂ = h₂w₂d₂ ;

h₂— +— d₂ + h₂d₂

n n

The results of numerical simulation of the form-factor n for the parallelepiped-like battery are presented in Fig. 6b.

Reference is now made to Fig. 16, presenting pseudo-color patterns characterizing temperature distribution over the battery body induced by charging/discharging current. Reference is now made to Fig. 17, showing pseudo-color patterns characterizing temperature distribution induced by charging/discharging current in the multi-cell battery. As said above, the charging voltage pulses are applied to battery cells in a consecutive cyclic manner such that the interval between charging impulses applied to each battery cell is sufficient for dissipating the heat generated by the induced charging current. Specifically, a cell 5 is under action of charging pulse, while other cells 1-4 dissipated received heat and cool down. The temperature distribution pattern of the battery cells corresponds to a charging protocol (cell sequence of charging). In this case cell of sequence 1-2-3-4-5, the cell 5 is the hottest one and cell 1 is coolest.

The proposed technical solution provides the following advantages:

1. High charging speed;

2. High charging/discharging efficiency;

3. Long battery life time;

4. Efficient heat dissipation.

5. Prevention from heat runaway.

Claims

Claims:

1. A method of pulse charging of a pack of battery cells provided with battery terminals, said method comprising the steps of:

(a) providing a charging device connectable to a source of electric energy; said charging device adapted for providing a voltage pulse train to said terminals;

(b) electrically connecting said charging device to said terminals of battery cells; and

(c) pulse charging said pack of battery cells by means of applying a train of said voltage pulse train to said terminals;

wherein said step of electrically connecting said charging device to said terminals of said battery cells is performed individually to each cell; said step of pulse charging is performed by means of applying said train of voltage pulses over said battery cells in a cyclic consecutive manner.

2. The method according to claim 1, wherein at said step of pulse charging a time interval between said charging pulses applied to each cell is sufficient for dissipating heat generated by a charging current conducted across said cell.

3. The method according to claim 1, wherein at said step of pulse charging said interval between said charging pulses comprises at least one voltage pulse of an opposite polarity.

4. The method according to claim 1, wherein at said step of pulse charging said train of charging pulses comprises a plurality of sutbrains; said charging pulses belonging to one subtrain are identically to each other; said charging pulses belonging to one subtrain are consecutively distributed over said battery cells.

5. The method according to claim 4, wherein at said step of pulse charging a duration of charging pulse increases over time of charging.

6. The method according to claim 4, wherein at said step of pulse charging a duration of pulse interval increases over time of charging.

7. The method according to claim 1 further comprising the step of monitoring battery pack parameters and optimizing charging process.

8. A device for pulse charging of a pack of battery cells provided with battery terminals, said device connectable to a source of electric energy; said device comprising a generator of a voltage pulse train provided to said terminals; wherein said device further comprises a commutating circuitry; said circuitry is adapted to commutate said voltage pulses over said battery cells in a cyclic consecutive manner.

9. The device according to claim 8, adapted to apply said charging pulses to each cell so that time intervals between said charging pulses is sufficient for dissipating heat generated by a charging current conducted across said cell.

10. The device according to claim 8, wherein said interval between said charging pulses comprises at least one voltage pulse of an opposite polarity.

11. The device according to claim 8, wherein at said train of charging pulses comprises a plurality of sutbrains; said charging pulses belonging to one subtrain are identically to each other; said charging pulses belonging to one subtrain are consecutively distributed over said battery cells.

12. The device according to claim 1 1, wherein a duration of charging pulse increases over time of charging.

13. The device according to claim 11, wherein a duration of pulse interval increases over time of charging.

14. The device according to claim 8 further comprising means for monitoring battery pack parameters and optimizing charging process.