Lithium-ion thermal management

Online Editor
RVmagnetics has now presented a sensing system called MicroWire

High energy density, little or no memory effect, and low self-discharge, make the lithium-ion batteries a good enough fit for hybrid or fully electric applications. These are quickly taking over the NiMH and nickel-cadmium secondary batteries market for portable and hand-held devices, additionally, they are the main choice when it comes to EVs and HEVs. However, being developed in the 1980s these batteries still aren’t fully deployed commercially in quantities relevant to the EV/HEV market demand. The reasons are safety, cost, low-temperature performance, which can all be summed up as pure thermal management.

After a certain amount of performance (which can depend on numerous variables like overcharging) the positive feedback produced by the hotter cells in Lithium-ion batteries generates more current (and therefore more heat). Additionally, uneven cooling can inflame inherent temperature gradients, even more as the cells are designed to be connected in parallel (all current must pass through each cell).

The uneven heat distribution, overheating are not only a cause of higher costs, energy waste, and shorter shelf life for the battery, but, in a much serious sense, it has the possibility of thermal runaway. Thermal runway created by overcharge, over-discharge, short circuit, high temperature - has chances to cause separator melting, fire, explosion, steaming, and other dangerous effects.

The temperature that is too high inside the battery can affect the electrodes and electrolyte to decompose exothermically, the separator to shrink and the battery to short-circuit internally. This carries the same explosive risks mentioned before. Other reasons for thermal runaway may be physical damage e.g. due to an accident causing battery deformation excessive ambient temperatures may occur.

There have been different strategies developed to deal with this issue. Starting from battery cooling technologies, all the way to pre-calculated guidelines developed to use the batteries. However, neither an industry integrator nor an individual customer has found these widely useful. There hasn’t yet been a solution that would add no changes to the production process, wouldn’t affect the size or the weight of the battery, and would actually be able to perform the job.

To try and identify the main issue here is easy. The customer does not have enough information about what happens inside the battery. When is it the time to stop using it? What is the exact amount of charge to be put in it? Does it get too hot? Is it time to integrate any battery management system? Is the physical environment suitable for where the battery is placed (the temperature, the physical abuse on the battery, vibrations, etc.)?

Answering these questions is indeed not an easy task! As there isn’t a possibility to conduct invasive sensing on the battery as a preventive maintenance strategy, neither would it be economical. The customer may not (and rightfully so) want to stop the production to conduct measurements on the battery system. Thus, there is a need in the market of Battery Management Systems, for a solution that would provide real-time(live), precise information about the local physical environment from within the battery. At the same time, this solution should not have high production costs, should be a plug-and-play solution that provide extreamly limited or no change at all to the industrial design.

RVmagnetics has now presented a sensing system called MicroWire. The new sensor measures physical variables such as pressure, temperature, the magnetic field directly, and variables such as electric current, torsion, bending, load, and vibration indirectly. This sensor is as thin as human hair (3 µm to 70 µm) and resistant to chemically aggressive environments. Naturally, the MicroWire sensor can be embedded directly into the battery cell, even in hazardous, acidic environments. Ultimately, it is a system for non-destructive testing and measurement from inside the battery - without any wiring.

A pair of coils consisting of excitation and pick-up coil is able to excite magnetic field and gather the signals of the MicroWire from up to 10cm. As the sensitivity of the sensing coil is 10000x/second – it is now possible to conduct predictive maintenance. Finally having a smart and safe rechargeable battery that will inform you about its health, and the needed maintenance activities.

Measuring Principle

MicroWire consists of a metallic alloy core (diameter approx. 1 µm to 50 µm), which is with and a layer of glass coating (thickness 2 µm to 20 µm) which makes it possible to have it embedded into chemically aggressive environments. The sensors are usually 1 cm to 4 cm long.

The sensing is conducted in a contactless nature. It is important to mention that the MicroWire is a passive element with neither any wiring attached to it nor any data being stored in it. The data itself is gathered through the sensing head and the electronics. The sensing head consists of two coils (the excitation coil which generates the magnetic field, and the sensing coil that reads the MicroWires response in the local physical environment). The MicroWire’s operating temperature range extends from –273 ° C to +600 ° C, its sampling frequency is up to 10,000 measurements per second.

The measurement of the temperature and the mechanical load remains unaffected by magnetic noise, even if the sensor is magnetic. The measurement is very different from the method used in conventional magnetic sensors. Conventional sensors require high permeability in order to be highly sensitive and a high saturation field in order to maximize the measuring range. MicroWires, on the other hand, have a relative permeability µr of about 1 (like a vacuum) and a saturation field strength of about 300 µT (3 Oe).

The output signal is digitized in a control unit (e.g. ARM). The data can then be transferred via USB or Bluetooth, Wi-Fi, SigFox and processed in the software preferred by the customer (whether analytical, AI, data management, predictive maintenance, or other).

This all makes up for an R&D solution to solve the thermal management issues without compromising safety, it can add to the efficiency of the battery, increase the shelf life and overall make the batteries smart.