Apparatus and method for conducting nail penetration test for secondary battery
An apparatus and method for conducting a nail penetration test for a secondary battery are provided. The apparatus for conducting the nail penetration test includes: a stage on which the secondary battery, which is an object of the nail penetration test, is fixed; a nail penetration unit including a nail elevating/lowering means; a voltage measuring unit configured to repeatedly measure a short-circuit voltage of the secondary battery with a time interval during the nail penetration test; and a controller operably coupled to the voltage measuring unit. The controller periodically receives a short-circuit voltage from the voltage measuring unit, determines a short-circuit current allowing the received short-circuit voltage to be applied between outermost nodes of an equivalent circuit which models the secondary battery whenever the short-circuit voltage is received, and visually outputs a change in the determined value of the short-circuit current according to time through a display unit.
1. An apparatus for conducting a nail penetration test for a secondary battery, the apparatus comprising:
a stage on which the secondary battery, which is an object of the nail penetration test, is fixed;
a nail penetration unit comprising a nail which penetrates into the secondary battery and a nail elevating/lowering means configured to elevate or lower the nail;
a voltage measuring unit coupled to electrodes of the secondary battery and configured to repeatedly measure a short-circuit voltage of the secondary battery with a time interval while the nail penetration test is in progress;
a display unit configured to visually display information; and
a controller operably coupled to the voltage measuring unit,
wherein the controller controls the nail penetration unit to lower the nail such that the nail penetrates into the secondary battery, periodically receives the short-circuit voltage from the voltage measuring unit, determines a short-circuit current which allows the received short-circuit voltage to be applied between outermost nodes of an equivalent circuit which models the secondary battery based on the equivalent circuit whenever the short-circuit voltage is received, and visually outputs changes in a value of the determined short-circuit current according to time through the display unit.
2. The apparatus of claim 1 , wherein the equivalent circuit comprises, as a plurality of circuit elements, a serial resistor, at least one RC circuit, and an open circuit voltage source which varies a voltage depending on a state of charge of the secondary battery, the plurality of circuit elements being connected to each other in series.
3. The apparatus of claim 2 , wherein the controller determines the short-circuit current by using an equation below:
i short =( V short −V RC −V OCV ) R 0
where i short is a short-circuit current, V short is a short-circuit voltage measured by the voltage measuring unit, V RC is a voltage applied by the RC circuit, V OCV is an open circuit voltage depending on a state of charge of the secondary battery, and R 0 is a resistance value of the serial resistor.
4. The apparatus of claim 3 , wherein the controller time-updates V RC by using an equation below:
V RC [ k+ 1]= V RC [ k ] e −Δt/R*C +R (1 −e −Δt/R*C ) i short [ k ]
where k is a time index, V RC [k] is a value of V RC right before a time update, V RC [k+1] is a value of time-updated V RC , Δt is a time update period of V RC , R and C are respectively a resistance value and a capacitance value of a resistor and a condenser included in the RC circuit, and i short is a predicted value of a short-circuit current determined in a previous calculation period;
the controller time-updates SOC, which is a state of charge of the secondary battery, by using an equation below:
SOC[ k+ 1]=SOC[ k ]+100 *i short [ k ]Δ t/Q cell
where k is a time index, SOC[k] is a state of charge right before a time update, SOC[k+1] is a time-updated state of charge, i short is a short-circuit current determined in a previous calculation period, Δt is a time update period of a state of charge SOC, and Q cell is a capacity of the secondary battery; and
the controller determines an open circuit voltage V OCV of the secondary battery corresponding to the time-updated state of charge by using the time-updated state of charge and a predefined “state of charge-open circuit voltage lookup table”.
5. The apparatus of claim 1 , wherein the controller determines R short , which is a short-circuit resistance of a nail penetration point, by using an equation below:
R short =V short /i short
where R short is a short-circuit resistance of a nail penetration point, V short is a short-circuit voltage of the secondary battery, periodically measured by the voltage measuring unit, and i short is a predicted value of a short-circuit current corresponding to a short-circuit voltage of the secondary battery, periodically measured, and
visually outputs changes of the short-circuit resistance according to time through the display unit.
6. The apparatus of claim 1 , wherein the controller measures Q short , which is short-circuit Joule's heat generated from a nail penetration point, by using an equation below:
Q short =i short *V short
where Q short is short-circuit Joule's heat generated from a nail penetration point, V short is a short-circuit voltage of the secondary battery, periodically measured by the voltage measuring unit, and i short is a predicted value of a short-circuit current corresponding to a short-circuit voltage of the secondary battery, periodically measured, and
visually outputs changes in the short-circuit Joule's heat according to time through the display unit.
7. The apparatus of claim 1 , wherein the controller determines Q cell , which is resistance Joule's heat generated from a resistance characteristic of the secondary battery, at a penetration point of the secondary battery by using an equation below:
Q cell =i short *|V short −V OCV |
where Q cell is resistance Joule's heat generated from a resistance characteristic of the secondary battery at a nail penetration point, V short is a short-circuit voltage of the secondary battery, periodically measured by the voltage measuring unit, i short is a predicted value of a short-circuit current corresponding to a short-circuit voltage of the secondary battery, periodically measured, and V OCV is a predicted value of an open circuit voltage depending on a state of charge of the secondary battery, and
visually outputs changes of the resistance Joule's heat according to time through the display unit.
8. A method for conducting a nail penetration test for a secondary battery, the method comprising:
(a) fixing the secondary battery on a stage;
(b) allowing a nail to penetrate into the secondary battery;
(c) repeatedly measuring a short-circuit voltage through electrodes of the secondary battery with a time interval;
(d) whenever the short-circuit voltage is measured, determining a short-circuit current which allows the measured short-circuit voltage to be applied between outermost nodes of an equivalent circuit which models the secondary battery based on the equivalent circuit; and
(e) visually outputting changes of the determined short-circuit current.
9. The method of claim 8 , wherein the equivalent circuit comprises, as a plurality of circuit elements, a serial resistor, at least one RC circuit, and an open circuit voltage source which varies a voltage depending on a state of charge of the secondary battery, the plurality of circuit elements being connected to each other in series.
10. The method of claim 9 , wherein the operation (d) comprises determining the short-circuit current by using an equation below:
i short =( V short −V RC −V OCV )/ R 0
where i short is a short-circuit current, V short is a short-circuit voltage measured by a voltage measuring unit, V RC is a voltage applied by the RC circuit, V OCV is an open circuit voltage depending on a state of charge of the secondary battery, and R 0 is a resistance value of the serial resistor.
11. The method of claim 10 , wherein the operation (d) comprises:
(d1) time-updating V RC by using an equation below:
V RC [ k+ 1]= V RC [ k ] e −Δt/R*C +R (1 −e −Δt/R*C ) i short [ k ]
where k is a time index, V RC [k] is a value of V RC right before a time update, V RC [k+1] is a value of time-updated V RC , Δt is a time update period of V RC , R and C are respectively a resistance value and a capacitance value of a resistor and a condenser included in the RC circuit, and i short is a predicted value of a short-circuit current determined in a previous calculation period;
(d2) time-updating SOC, which is a state of charge of the secondary battery, by using an equation below:
SOC[ k+ 1]=SOC[ k ]+100 *i short [ k ]Δ t/Q cell
where k is a time index, SOC[k] is a state of charge right before a time update, SOC[k+1] is a time-updated state of charge, i short is a short-circuit current determined in a previous calculation period, Δt is a time update period of a state of charge SOC, and Q cell is a capacity of the secondary battery; and
(d3) determining an open circuit voltage V OCV of the secondary battery corresponding to the time-updated state of charge by using the time-updated state of charge and a predefined “state of charge-open circuit voltage lookup table”.
12. The method of claim 8 , further comprising:
determining R short , which is a short-circuit resistance of a nail penetration point, by using an equation below:
R short =V short /i short
where R short is a short-circuit resistance of a nail penetration point, V short is a short-circuit voltage of the secondary battery, periodically measured by a voltage measuring unit, and i short is a predicted value of a short-circuit current corresponding to a short-circuit voltage of the secondary battery, periodically measured; and
visually outputting changes of the short-circuit resistance.
13. The method of claim 8 , further comprising:
determining which is short-circuit Joule's heat generated from a nail penetration point, by using an equation below:
Q short =i short *V short
where Q short is short-circuit Joule's heat generated from a nail penetration point, V short is a short-circuit voltage of the secondary battery, periodically measured by a voltage measuring unit, and i short is a predicted value of a short-circuit current corresponding to a short-circuit voltage of the secondary battery, periodically measured; and
visually outputting changes in the short-circuit Joule's heat.
14. The method of claim 8 , further comprising:
determining Q cell , which is resistance Joule's heat generated from a resistance characteristic of the secondary battery, at a penetration point of the secondary battery by using an equation below:
Q cell =i short *|V short −V OCV |
where Q cell is resistance Joule's heat generated from a resistance characteristic of the secondary battery at a nail penetration point, V short is a short-circuit voltage of the secondary battery, periodically measured by a voltage measuring unit, i short is a predicted value of a short-circuit current corresponding to a short-circuit voltage of the secondary battery, periodically measured, and V OCV is a predicted value of an open circuit voltage depending on a state of charge of the secondary battery; and
visually outputting changes of the resistance Joule's heat.