2013 EUSPEN Challenge: Oticon


Design a means to replace 6mm batteries in hearing assist devices. Keeping in mind that the process needs to be simple enough that it can be done with limited dexterity and vision.

Photo taken of the Oticon hearpiece with the battery out from the side view.  This image was used to create a 3D CAD file that was used to prototype battery exchange tool. Photo of the top of the Oticon ear piece.

Left: Side view of Otticon hearing assist device with the gate open showing the 6mm diameter battery. Right: Top view of the hearing aid.


In May/June 2013, my advisor asks me if I wanted to go to Europe for a design challenge. At which point, I asked about the topic of the challenge and he firmly replied "refresh your optics." Having no idea what was to come, I read over my engineering optics book and prepared for the challenge.


Functional Requirements

  • Replace battery in hearing aid
    Process: Open hearing aid, remove battery, add new battery, close hearing aid
  • Limited dexterity
  • Limited visual guides

Design Parameters

  • Limited size of hearing aid (Diameter 6 mm x Height 2mm)
  • Replaced by someone with limited dexterity
  • User with limited vision
  • Only mechanical designs, no motors or electronics


As soon as the announcement was made the first thing I did was to do my own assessment. This is part of the PREP process () where each team member does their own assessment of the project to come up with ideas and concepts without others involved. Doing work on your own at first helps in ensuring that that best ideas go forward during the peer review process. The second stage of the PREP process is peer review, but in order for that to happen other members of the team need to do the self assessment in order to make sure that no one idea dominates and that everyone has had a fair opportunity to design. The three startegies that I identified where: 1) fix hearing aid and process around it, 2) move hearing aid during the process, 3) combination. The one that I dentified as the most promissing is a combination of moving both hearing aid and tooling to make the change. Due to the small size of the hearing device and the limited dexterity, the approach that I found most favorable is to put the ear piece in a thermaformed part holder similar to the apple ear buds casing (like that shown in the reference section). In order proceed, I separated each process that needed to occur and put strategies for each section.

Remove Battery
Add New Battery
Pin pusher

Independent Design

Details of the hearing aid device where limited. We were given one hearing aid per group, and two non-dimensioned drawings hardcopy only (one top view and one side view). From experience, I knew that no matter the design we needed to have the hearing aid in CAD form to be able to 3D print custom components. Overnight, I used a caliper in conjunction with the two drawings to create a CAD of the device. How was it done? I took a photo with my phone of the two drawings. I took the images and used Omax Layout to draw lines and splines on top of the images.The dxf file was then imported into Solidworks and started to create the CAD drawing. The drawing was scaled knowing the dimensions of the battery. What I produced in a period of four hours is a functional CAD that was used to find the pivot point, lever arm, and part envelope. To see an exceprt of my desing notebook click here.

Scan of Dr. Rojas Design notebook which shows a preliminary design for an earpiece that where the battery is replaced.
Excerpt image from my notebook that shows the ear piece inside of a little box that where the hearing aid fits into and rotates as part of the disk.

CAD design showing the three components of the ear piece: 1) main body, 2) opening slit, 3) battery Side view of the CAD model showing the location of the force applied and the pivot point in order to open the battery reservoir.

CAD that I produced overnight in order to find the pivot point and get shape of the hearing aid. Left: Solidworks CAD of hearing device. Right: motion of the battery holder showing the location of the pivot point.

Open Hearing Aid

The lowest resistance to opening the hearing aid is by inserting a little wedge between the rotating arm (holding the battery) and the housing. The arm rotates about a pivot point (identified with the CAD model).

Scan of Dr. Rojas design notebook showing the critical distance of the lip of the earpiece that is required to open the battery reservoir.
Schematic of opening of the hearing aid from my design notebook.

Remove Battery

The battery can only removed in one direction. Since we are not allowed to modify the hearing aid the direction of battery removal and insertion must be the same. Due to the geometry of the battery a wedge is not ideal. Using gravity to remove the battery is a viable option but does not necessarily help with the insertion of a new battery. Therefore, a pin seems to be the most viable option.

Add Fresh Battery

Since a pin is already being used to remove the old battery. Occam's Razor dictates (KISS) to keep it simple and use the same method of removal to do the insertion of the battery.

Close Hearing Aid

The process of closing the hearing aid can be done in several ways; however, one thing to consider is that if the user is to pick up the device then the battery may fall out if the device is not closed prior to removal. Therefore, the rotating arm needs to be closed in as part of the process.

Team Design for Prototype Only

Why it's important to work in a team: peer review, the best ideas get incorporated together to create the best design possible, and brainstorming. In the team discussion several important things were discussed. We didn't want the user to interact with small latches or systems, so the whole operation should be consolidated to placing the hearing assist in the device and either by pushing motion or by a twisting motion the entire desing should be replace the battery. Due to the limited resources time and manufacturing facilities (3D printer). We decided to create a prototype Battery Replacement Device (BRD) that conveys the idea keeping in mind manufacturing feasibility and time.The BRD consists of a disk like system where the hearing aid clips inside, as shown below.

Battery design opening and closing sketch from Dr. Rojas Design notebook.
Excerpt from my design notebook the new design that instead of a box has the system in a cylinder disk like structure.

CAD image of prototype design for replacement of battery for earpiece.

CAD image of the prototype design

Step 1: Open Hearing Aid

As a team we converged on opening the system with a wedge like system that clips on to the rotating arm. For symmetry I centered the pivot of the arm and made it concentric with the BRD. The first step that the user has to do is to load the device.

Step 1.1: Load Device ................................................................Step 1.2: Open by twisting opening ring

Top view of CAD design for earpiece prototype that shows the process of opening the device to be loaded. CAD design illustration showing how the earpiece opens.

Step 2: Remove Battery

Although we wanted the BRD to operate in one motion, the manufacturing restrictions led us to give the user a guide rail that would allow us to replace the battery. Once in the open configuration the user has to move a pin up in order to remove the battery. David came up with the wonderful idea to use a revolver system to have multiple batteries. With the battery size and the possible configurations there are only four.

Step 2.1: Push Battery......................Step 2.2: Push battery down

Battery removal mechanism pushing down the old battery. Cross section of device shoing the battery being removed from the earpiece.

Step 2.3: Battery at bottom section................................................................Step 2.4 Rotate Cartrige to remove

Cross section of CAD showing battery in cartridgeBottom of prototype showing battery catridge that rotates about center to place a new battery.

Step 3: Add Fresh Battery

Using the carosell design, as one battery is removed the fresh battery is added. With the new battery in place the battery displacement mechanism pushes a fresh battery into the hearing device.

Step 3.1: Push the new battery in place....................................................Step 3.2: Complete the loading

Process of adding a fresh battery to the earpiece using the catridgeCross section of CAD inserting the new battery.

Step 4: Close Hearing Aid

In the same manner of openning the BRD, the closing of the lid is done by rotating the most exterior shell clockwise.

Step 4.1: Rotate clockwise to close the device

Process of closing the hearing aid with the new battery.

Other Features

Multiple batteries replacements and keeps old batteries.


Identifying the appropriate analysis to perform. While there was much discussion in performing the analysis for a simple cantilver beam flexure within my team. The truth at the end of the day is: An engineer uses analysis to determine the limitations, dimension, etc of a feature; while a hacker can build one item and hope that it works based on experience. I was the one who lead the team to victory by incorporating the fabrication, cantilever analysis, scaling, and cost projections.

Power Analysis

While my team mates believed that no analysis was needed given the time restriction, I begged to differ. Therefore, I did a preliminary analysis to determine the efficiency of the unit based on the torque analysis.

Excel image shoing the efficiency of the unit at 87%

Ratchet Analysis

Ratchet design and cantilever used to lock in place at a new battery.


The inspiration for the primary holding mechanism is the apple ear buds case that holds the earbuds in place.

Apple earphone container static.  The earpiece housing should have a similar configuration that hugs the earpiece.

How to Succeed

Understand how the price of a product dominates the cost in manufacturing. Have a means to CAD something very quickly and make sure to bring a laptop and a lab notebook. Familiarize yourself with tools software like: ProE, Solidworks, MasterCAM, Excel, etc. Also understand manufacturing process and cost. How to design parts for injection molding, casting, millitng, turning, thermoforming, etc. Any good design will take into account limitation of the manufacturing process and cost. Also communication is key between team members. Phuc & David where wonderful to work with to get this done as a team. Thank you.

Recommendations to future students


I have a working design to integrate all the elements to make a cheap ~$5.00 unit. I just have to finalize and fabricate.