Report

1 Considerations About UN38.3 T4 Test, and its Applicability to Large Lithium Batteries. How to improve this test? T4 Constant Acceleration Test – Force vs Mass Currently, the UN38.3 T4 test is based on constant acceleration, which means higher force is applied to larger battery mass. Batteries over 6200Wh (approximately 120kg) are not required to be tested at all. Force UN38.3 T4 Shock Test Force vs Mass F ma 50G Cell Test Only 150G 12kg 50kg Mass ~120kg Does a bigger battery actually experience higher force during transport? >120kg (>6200Wh) But do larger batteries actually experience higher forces, or constant accelerations under actual shipping conditions? Let’s consider a case of large shock in shipping Battery Drop from Height, and compare it to the constant acceleration of UN38.3 T4. Let’s study one battery of mass m and one battery 10x that mass. 2 3 T4 Constant Acceleration Test – Force vs Mass Constant acceleration would require the ratio of velocity change over time to be constant for all masses of batteries tested to the constant acceleration. UN38.3 T4 test is for 50G acceleration for 12kg<X<6200Wh batteries. But is this representative of the actual physics of battery transportation? Acceleration is a measure of velocity change over time. dv dt 50G acceleration is similar to stopping a mass travelling at 50m/s and stopping on 0.1s, or travelling at 10m/s and stopping in 0.02s. What is the actual situation? How much does a battery pack accelerate? What is the force? dV - Velocity Change - m/s a Velocity Change vs Impact Time for 50G Impact 60 50 40 30 20 10 0 0 0.02 0.04 0.06 0.08 dt - Impulse Time - Seconds 0.1 4 Consider a Drop Test – Constant Velocity Compare it to a drop test. Large Mass =10xSmall Mass Small Mass Mass=m Mass=10m mg mg d An object in freefall will have a velocity related to the distance of freefall. v 2 gd For a given height of freefall, d, both a large mass and a small mass will have the same velocity at impact. But is the acceleration on impact the same? Semi Rigid Surface 5 Consider a Drop Test – Constant Velocity Small Mass v Large Mass =10xSmall Mass 2 gd Momentum p=mv Kinetic Energy=1/2mv2 Momentum p=10mv Kinetic Energy=1/210mv2 Semi Rigid Surface The momentum of an object in motion is its mass x its velocity. The energy is the mass x velocity squared. The velocity for the small mass and large mass dropped from the same height is the same! When the object in motion strikes another object, the momentum is transferred to that other object (conservation of momentum principle), and the energy is conserved (conservation of energy principle). Impact Deformation Longer Impact Time Small Mass v 2 gd Large Mass =10xSmall Mass Momentum p=mv Kinetic Energy=1/2mv2 Elastic deformation of impact surface and mass due to small momentum. Energy Elastic Deformation. 6 Plastic Deformation of mass shape. Momentum p=10mv Kinetic Energy=1/210mv2 Elastic + Plastic Deformation of impact surface due to large momentum. Energy Elastic + Plastic Deformation. Semi Rigid Surface For a small mass and a large mass dropped from a height, the change in velocity, dv, is the same. Both are brought to rest and the momentum goes to zero. The kinetic energy is distributed through elastic deformation (like a spring) and plastic deformation (physical permanent damage to both surfaces). The higher the momentum, the more deformation. But the force acting on a small mass and a large mass is not proportional to the difference in mass, because the impact time is different due to the magnitude of the deformation! Impact Deformation Longer Impact Time Small Mass v 2 gd Large Mass =10xSmall Mass I=Fsdts=mdv Elastic deformation of impact surface and mass due to small momentum. Energy Elastic Deformation. Short Impact Time. Plastic Deformation of mass shape. I=FLdtL=10mdv Elastic + Plastic Deformation of impact surface due to large momentum. Energy Elastic + Plastic Deformation. Long Impact Time! Semi Rigid Surface For a change in momentum, an object must undergo an Impulse. Impulse is the Acting Force x Impact Time = Change in Momentum. 7 Impact Deformation Longer Impact Time Small Mass v 2 gd I=Fsdts=mdv Large Mass =10xSmall Mass I=FLdtL=10mdv Semi Rigid Surface Since the large mass is deformed by the semi-rigid surface more than the small mass, and the semi-rigid surface is deformed by the large mass more than the small mass, the resulting Impulse time is longer. (it takes more time for the large mass to travel through the deformation) Small Mass Impact Time < Large Mass Impact Time dts<dtl If the difference between the Small Mass impact time and the large mass impact time is Dt, then dtl=dts+Dt 8 Longer Impact Time Lower Force and Acceleration Small Mass v 2 gd Large Mass =10xSmall Mass I=Fsdts=mdv Small Mass Force x Small Mass Impulse Time = Small Mass x Change in Velocity. Impulse Momentum Change F s dt s mdv Fs mdv dt s ma s Let’s consider the difference between the Small Force, FS, and the Large Force, FL. I=FLdtL=10mdv Large Mass Force x Large Mass Impulse Time = Large Mass x Change in Velocity. Impulse Momentum Change dt L F L 10 mdv FL 10 mdv FL 10 mdv dt L dt S D t 10 ma L m 10 dv dt S D t 9 Longer Impact Time Lower Force and Acceleration Ratio of Small Force to Large Force mdv FS dt s FL m 10 dv dt S D t Fs FL dt S D t 10 dt s 10 dt s F s F L ( dt S D t ) mdv FS FL dt s m Even though the large mass is 10x the small mass, because Dt>0, the large force is not 10x the small force. 10 F s F L (1 10 dv Dt dt S dt S D t 10 (1 Dt dt S Fs FL ) ) The longer the Dt, the smaller the impact force. (This is why we design cars to crush in impact – to lower the impulse force!) Since Dt>0, FL<10FS 10 Longer Impact Time Lower Force and Acceleration 10 (1 Dt Fs FL ) T4 treats all size from 12kg to 6200Wh with the same acceleration. dt S 10 (1 Dt Now let’s consider the acceleration, aS for the small mass and aL for the large mass. ma S 10 ma L ) But… dt S 1 (1 Dt dt S aS aL ) Since Dt>0, aL is always less than aS. The larger battery experiences smaller acceleration. Therefore under T4 with a constant acceleration for different masses, larger batteries are subjected to a higher force then necessary. Another way to look at it is again to compare to a drop test… 11 T4 Constant Acceleration Test Higher Drop Acceleration G vs Impact time for Item Dropped from Height 100000 Acceleration G 10000 Graph showing lines of constant height drop test and resulting acceleration G for a given impulse time. UN38.3 T4 gives 0.011s impulse time for 50G shock. 1000 150g Drop Test Height 1m 2m 20m 200m Constant Acceleration Line 100 50g 10 But larger mass needs larger force to generate 0.011s pulse. It doesn’t reflect real world! 1 0.0001 0.001 0.01 0.1 dt - Impulse Time - Seconds 1 10 12 T4 Constant Acceleration Test Higher Drop Acceleration G vs Impact time for Item Dropped from Height 100000 Acceleration G 10000 Since the larger mass has longer impulse time… Increasing Impulse Time 1000 150g Drop Test Height 1m 2m 20m 200m Constant Acceleration Line 100 50g 10 1 0.0001 using a test of constant G is similar to increasing the height of a drop test for larger mass objects! 0.001 0.01 0.1 dt - Impulse Time - Seconds 1 10 13 T4 Constant Acceleration Test – Force vs Mass • So we have established two things: – Larger battery assemblies experience lower accelerations than smaller batteries during shock events in transportation due to the deformations in the system under load. • Therefore the constant acceleration test exposes larger batteries to disproportionally larger forces than they would experience in reality. – Battery Assemblies of higher mass experience forces in transportation that are not proportionally higher than batteries of smaller mass. • Therefore reduction in force from the constant acceleration line of T4 is warranted. • And the corollary of this is that the surface that an item is attached to for transport can only impart a maximum force on that item before it deforms under load. – An airplane floor, for instance, will deform significantly under the force that is necessary to accelerate a large battery assembly, thereby reducing the acceleration and the magnitude of the force. 14 T4 Constant Acceleration Test – Force vs Mass There are two descriptions of conditions to relieve testing below in current publication... <The last paragraph of 38.3> When batteries that have passed all applicable tests are electrically connected to form a battery assembly in which the aggregate lithium content of all anodes, when fully charged, is more than 500 g, or in the case of a lithium ion battery, with a Watt-hour rating of more than 6200 Watt-hours, that battery assembly does not need to be tested if it is equipped with a system capable of monitoring the battery assembly and preventing short circuits, or over discharge between the batteries in the assembly and any overheat or overcharge of the battery assembly. <P903 in Model Regulations> Proposal of the addition of the paragraph below into T.4 Shock Shock test for battery assembly with (test 1 or 2) can be applied if the battery assembly is contained in a strong, impact resistant outer casing and is equipped with a system capable of monitoring the battery assembly and preventing short circuits, or over discharge between the batteries in the assembly and any overheat or overcharge of the battery assembly. 15 16 T4 Constant Acceleration Test – Drop Test? So, the basic rationale for the T4 shock test needs to be clarified. Does it represent an object dropping onto a rigid surface from a given height? Or a crash from a given speed? UN38.3 T4 Shock Test By Drop Test Then Option 1 Option 1) Change the criteria from an Acceleration G based test to a Drop Test from a height, or a sled test from a corresponding velocity. Basic Problem: What height? What surface? PGII test requires 1.2m PGI test requires 1.8m SAEJ2464 tests at 2m (What height? What velocity? What surface?) ma d (Option 1 is more favorable as test equipment is more widely available) Proposal of the addition of the paragraph below into T.4 Shock Shock test for battery assembly with drop from height X can be applied if the battery assembly is contained in a strong, impact resistant outer casing and is equipped with a system capable of monitoring the battery assembly and preventing short circuits, or over discharge between the batteries in the assembly and any overheat or overcharge of the battery assembly. 17 T4 Constant Acceleration Test – Force Test? Does it represent a certain force that can act on the object? For instance, a surface can only impart a maximum force before it deforms. Then Option 2 Option 2) Change the criteria from an Acceleration G based test to a Force Based test. UN38.3 T4 Shock Test Force vs Mass Basic Problem: What force? Should it be [email protected] or something else? Force F ma Maximum Force (How to define?) 50G Cell Test Only 150G 12kg 50kg Mass ~120kg >120kg (>6200Wh) Proposal of the addition of the paragraph below into T.4 Shock Shock test for battery assembly with constant F can be applied if the battery assembly is contained in a strong, impact resistant outer casing and is equipped with a system capable of monitoring the battery assembly and preventing short circuits, or over discharge between the batteries in the assembly and any overheat or overcharge of the battery assembly.