Andrew Lamb Magefekt. Batteries

Transcription

1 Andrew Lamb Magefekt Batteries

2 Batteries Notes

3 Where the fuel goes

4

5

6 Energy density batteries

7 What is a battery A Battery is a reversible chemical process that requires the movement of Electrons to reduce its chemical potential energy.

8 Battery Pack design When is a battery pack full? When the first Parallel Module element at is maximum voltage When is a battery pack empty? When the first Parallel Module is at its lowest voltage When is a Battery Pack Balanced? When the parallel modules all reach the minimum voltage as the same time When is battery pack efficient? When the heat generated is minimized When is the life of a battery pack maximized? When the electrochemical stress is minimized ( High Frequency Switching, temperature, vibration) What is a good battery pack? One that does not fail in service

9 Battery Characteristic Batteries are considered a closed system where charge carriers move reversibly between two states. Batteries are a semi closed system designed to constrain a thermodynamically unstable system between two states. Battery systems are open to temperature that will retard performance. Battery systems are affected by the electrical load both on the millisecond scale and minutes. Batteries are the product of the materials and workmanship used.

10 Batteries respond to Temperature Thermal random walk model (Brownian Motion ) Ions will jump randomly due to thermal energy however the direction will be given by the reduction in chemical energy when discharging and the push from charging.

11 Threshold temperatures The energy available for motion is a function of e kt This can give the appearance of nothing happening up to a threshold However where there diffusion of species through a solid such as a membrane of carbon matrix, there is a minimum thermal energy required for the Ions to move a position in a solid

12 Mechanical Batteries Batteries laminated structures that are assembles and subject to mechanical damage. Korea Delkor batteries refused to deliver car batteries by rail as the shocks from joints in the track would dislodge the paste. The Pulsar Battery had the packing of paste assisted by the use of vibration during manufacture.

13 Electrical stress Electrical stress can cause electrochemical profiles that will reduce the life of batteries. Specifications will often have a recommended storage state of charge to reduce the electrochemical gradient across the membrane. As the current increases on a battery the ability of the charge carriers to keep up is tested and the voltage drops. High Frequency Loads will have the same affect close to anode and cathode.

14 Materials and manufacture Materials vary in quality. Membranes are rated as defects / m2. Therefore larger batteries have higher probability of membrane defects. Plastic laminate quality. Paste is graded by grain size and finish Anode and cathode material vary in alloy, purity and roll integrity. Trimming, welding, application, packing and sealing all play a part in the performance of the batteries.

15 Metal deposition Where the crystal structure and the surface energy to volumetric energy ratio is favorable then one metal will deposit on another by epitaxial growth. Metals can be drawn out of solution through electroplating however growth of metals rely on the atoms fitting on to lattice parameters of the parent metal to stay adhered. Lithium may diffuse or form intermetalics.

16 Battery life and performance depends on use and temperature Higher temperatures allow better diffusion and current. Higher potential will increase the increase stress and reduce life. Elevated storage temperatures will allow the redistribution charge carriers and reduction of capacity

17 Current work on battery failures Poor manufacture of cells Burrs in plates Swarf pushed into membranes Membranes caught in packaging welding Shorts to aluminium layer in packaging Membrane overheat Battery Manager Failures Capacitor short across potentials Wire link failures Packaging permeability

18

19

20 Unintended Current Flow Electrical currents do not always flow where we intend. Current will find the path of least resistance leading to local potential variation and stray currents.

21 Current Concentration The current concentration will vary through as conductor as a function of a of the resistance and may lead to significant potential gradients.

22 The harder you push Ions and electrons do not travel evenly through the battery. The Ions and electrons charging the cell will concentrate in the shortest path, giving rise to the local heating and chemical stress

23 Cell Gassing Electrical stress. Electrolyte breakdown

24 Root Cause under conformation TBA

25 Joining batteries

26 Solder joints Making a solder joint is casting alloy between two surfaces. The solidifying the alloy will grow from the contact surfaces drawing material into the gap. If liquid metal is not available to fill the gap created by the solidifying allow voids and cracks will form. Loose holes are more likely to crack, when soldered.

27 Excessive heat will allow the residual stress in the plastic membranes to shrink the film and can lead to failures close to solder points Plastic membrane

28 Bolt Loads Nut and bolt assemblies rely on the compression on materials captured. The load bearing and retention is limited by the compression and creep of the least robust compressed material Bolts in materials Bolts in a solid materials can only bear the load of the least robust materials. Compressed or tensioned materials will relax, casing the bolt to relax. Bolt resistance The material resistance of the Bolt will cause heating, as the bolt loosens, the contact reduces and resistance increases.

29 Bolts

30 Welding battery tabs Electric spot welding is the product of directing electric current across a high resistance joint to melt and fuse metal parts to be joined. The current will flow along the path of least resistance. A gap or low retention force may be used to produce high resistance.

31 Current path Managing the Current to produce the weld in the desired area requires the resistance of the preferred path to be much lower than the alternative. This compromise leads to complex connector geometry to direct current through joint.

32 Problems If the current, on time, pressure, contact area, material thickness, oxides, contaminates or temperature of metals vary there may be insufficient melting of material to achieve a reliable weld. If the weld is achieved it is still the product of the current and resistance of joint and will generate heat in use.

33

34

35 Thank you