Development of a stretchable battery pack for textile integration

Kristof

Pinnoo

INTRODUCTION

As electronics become smaller and more integrated, the next step is to integrate them into textile. Research to create textile-integrated electronics continually develops, but there is also a growing need for textile-integrated batteries. This will allow standalone applications, without any rigid parts attached. Many uses can be imagined, such as health monitoring, interactive clothing or fashion.
The goal of this thesis is to investigate how a battery-pack can be integrated into textile. After investigating the key components
and formulating a design methodology, a demonstrator is made to illustrate the functionality of such a battery-pack, along with a driver circuit to power a stretchable LED-matrix and a charging circuit.
ENERGY STORAGE

Investigation shows that, for textile applications, two options are available to store energy. The first is lithium rechargeable batteries, which offer the largest energy density, but require protection against fault conditions. Because of this, they are inherently unsafe in abnormal operation. The second option is using supercapacitors, which offer a higher power density and are able to deliver larger current peaks to the application.
If the energy consumption of the circuit is really low, one could also opt for a non-rechargeable solution. Solid-state primary batteries, offer a safe solution with a high energy density. However, as they cannot be recharged and since replacement in textile is expensive and difficult, this is only useful in applications which they can power a long time.
1. Protection
  As lithium batteries need protecting, a small circuit is added to each battery-cell (= primary protection). An IC, along with a few passive components, can sense over- and undervoltage conditions, overcurrent conditions and short-circuits. By controlling two FET-switches in the ground-connection path, thecurrent flow can be blocked easily. A fuse is often added to protect against overcurrent conditions.
2. Balancing
  When batteries are placed in parallel, they will balance themselves out automatically. If the voltage differences are large, they current that flows can be large. This should be avoided.
  Series-connecting batteries is more critical: if cells are imbalanced, the protection circuitry will stop charging and discharging too soon, which will limit the effective capacity of the battery-pack. The cells can be balanced using passive balancing, where current is drained and dissipated from the cells with a higher charge. An alternative method in which no current is lost, is active balancing. In this case, current is redistributed over the cells, which can be done using capacitors, inductors or small DCDC convertors.
  The IC that will facilitate this balancing often offers fault detections as well, but on the level of the entire battery-pack. This is called secondary protection.
  It was demonstrated that selecting a good configuration of the cells is critical to maximize the reliability. Parallel connections are preferred over series connections.
DESIGN METHODOLOGY

Using a few simple steps, designers can quickly construct a battery-pack, suited for their application.
1. Calculate the required energy for the application, starting from the average current consumption, the supply voltage and the required operating time of the application.
2. Now the energy category can be determined. Applications requiring an energy storage of more than 5 Wh, will not be able to use a textile-integrated battery-pack without using a very large surface area.
3. Depending on the energy category and the required flexibility of the battery-pack, a battery-size category and the number of cells can be selected.
4. Next, the configuration of the cells can be determined, keeping in mind the implications with regard to balancing.
5. Depending on the configuration the maximum output current might not be sufficient to accomodate the maximum current consumption of the application. In that case, a new design or a change in configuration is required.

STRETCHABLE CIRCUIT

To integrate the battery onto textile, the circuit is produced on a flexible and stretchable substrate. Off-the-shelf components can be used on this substrate. They are placed on rigid islands, on which most of the routing is located as well. By interconnecting these rigid islands with meander interconnects, stretchability is assured.
The circuit is designed on a FCB (flexible circuit board), consisting of 50μm of polyimide and 18μm of copper. This is then attached to a rigid carrier using tape. The copper patterns are etched, after which the components are mounted. This allows the circuit to be tested. After confirming that all the components of the circuit function as desired, the islands and interconnects are patterned using laser-ablation. The polyimide, which supports the interconnect and islands, increases the reliability of the circuit.
The next production step is to laminate the top of the circuit with a thermoplastic polymer. After removing the carrier, this is also performed at the bottom of the circuit. The module can then be punched out.
Reheating this lamination once again allows the attachment onto textile.
DEMONSTRATOR

To demonstrate and test some of the findings of this thesis, a demonstrator was made. It was designed to power a LEDmatrix consisting of 12 strings of 3 RGB-LEDs in series, where each color of each LED consumed 5mA. Using a single color at any given time, this demonstrator, consisting of 8 200mAh flexible rechargeable batteries, could power this matrix for more than 5h.
This demonstrator is accompanied by a LED-driver and a charge circuit, allowing the demonstrator to be charged from a USB port. It allows charging of the entire demonstrator (dual cell charging), or just one of the 2 rows (single cell charging).
CONCLUSION

After investigation, the options to store energy on a substrate, suitable for textile integration, protection circuits were reviewed, as well as balancing techniques. Flexible Li-ion polymer batteries proved to be the best option with regards to energy-density and supercapacitors when a high powerdensity is required. This information was combined into a demonstrator, made on a FCB.