The Fusion Design team takes a holistic approach to enclosure design that begins with an in-depth understanding of the client’s business/product objectives and the expected experience of the intended user. This understanding allows us to make the right decisions about balancing often opposing goals within the system (engineering trades).
We need to understand the constraints and user expectations thoroughly so we pursue an understanding of the following:
Will it be a low volume product or a high volume product?
Will it be a lightweight product or a heavy product?
Does it need a plastic, metal or composite enclosure?
Will it be in an aggressive environment or in a non-aggressive environment?
Does it need to have a replaceable battery or a rechargeable battery?
If it will be a reusable battery, will a charger be necessary? Will charging be done wirelessly or with a plug in power cord?
Will it need water ingress protection?
Will the battery be installed in the factory, or will the user need to install it?
Is it a rugged application or a passive one?
How will it be mounted or supported? Will it be pole mounted, or can it go in a pocket?
Will it be a moving application or is it in a fixed location (ie. Data center)
Will it have a backup battery source?
Will it need a charging circuit?
Will it be a large battery requiring special structural mounting?
Will it be mounted on a circuit board or will it live in a battery chamber that is part of an enclosure?
How long will the battery need to last?
How many recharge cycles are needed (battery life)?
Considering the price point, does it need to have margins for a more expensive battery that is more reliable and/or with higher capacity (future planning)?
What will be the mechanical configuration (rectangular, cylindrical, clipped, battery contact interfaces, mounting method)?
Can standard battery formats be used or are custom formats required (more costly)?
Is weight a concern? For example, for a wearable military product, the user might carry 25 pounds of batteries.
Choosing the Right Battery
There are many types of batteries available and the choice is not a simple decision because there are many different factors to take into account.
How much power will be needed and how long will it need it to last?
How long between charge cycles?
What types of connectors are available/needed?
How will status monitoring be done?
Will it drive motors (potentially high current demand)?
What will be the current usage capacity over time?
What are the power cycle needs?
What type of battery management system is needed?
Is the provider a reliable and cost effective source?
After selecting a battery and performing required load testing ,the enclosure and mounting schemes are developed. Here are some of our tips and best practices for developing battery mounting schemes:
Check for Adequate Clearance
Heat causes batteries to swell and therefore clearance is needed to absorb the swelling. Some clearance is necessary within limits. Too much clearance may cause your battery to rattle around in your enclosure. Loose parts give the impression of cheap design and can actually lead to damaged batteries. The best practice is to allow for battery expansion but also to add some compliancy (i.e. foam) to prevent relative motion (rattling).
Here is an example of a very compact design where the battery is nested in a compliant cavity for this hand held application:
Manage the Heat
It is often necessary to perform thermal analysis to manage heat dissipation. Battery placement is critical. Typically, a small battery should be near an enclosure wall. Since it can get extremely hot, a conductive pad can be placed between it and the housing wall. Adding a structural wall and a foam element to the opposite side of the battery is often desirable. The foam will push the battery into the conductor pad causing the heat to be carried into the enclosure where it would be dissipated via convection. This arrangement is found in most wearable devices.
Often both simulations and real-world testing are required. Thermal profiles will be necessary over an entire PCB to minimize obstruction and to ensure proper airflow. Every battery circuit must have a fuse of some sort, either electronic or physical.
Handle Battery Mounting Challenges With Flexible Products
Most wearables wrap around a wrist, ankle, neck or ear and these curved surfaces create extra challenges because most batteries are rectilinear, some type of rectangle. In a curved situation, space is limited and that impacts the allowable size and shape of the battery. The form factor needs to be addressed early on because a design can be wrecked if the battery is too big for the desired aesthetic.
Here is an example of a curved earpiece that accommodated a rectangular battery:
Define Ruggedization Strategies
Careful design and placement are required in order to reduce susceptibility to shock and vibration. Mechanical structures can be attached to strategic locations on a board to ensure meeting ruggedization requirements. These include ribs, stiffeners, hold-down clamps or brackets, spring retaining clips, adhesives, rubber pads, and encapsulation materials.
Battery performance is subject to environmental factors such as air density and temperature. Special design considerations may be needed for altitudes higher than 19,685 feet (6000 meters) above sea level. This may impact batteries for aircraft and drones. In general, colder temps make chemical reactions slow down, so less electricity will be provided.
Another temperature concern is the exposure it may have to solar radiation. Will the product be left in a car on a hot day? For a small device, there is a very high-temperature challenge, even if it might not be directly in the sun, it will still be inside the car. Proper cooling is required to carry the heat away from the battery. It is also a good idea to have a automatic shutdown function if the device gets too hot.
Expected shock loading is another consideration. Here is an example of a Homeland Security Lock that needed to be extremely rugged. It has been tested under extreme shock loading (2500g) and does not open.
Consider Battery Cabling
Cabling is an oversight that is often not considered in the choice of a battery. How will it be connected? How will it charge? Are the cables and connectors properly strain relieved?
It is necessary to consider how the battery and cabling will be affected by vibration. Wire routing to and from the battery needs to be planned so edges and relative motion are not able to damage connectors or wire insulation. The rule of thumb is not to have a lot of loose wire in a product. The existing wire should be properly routed and tied down to make it properly constrained.
Ensure Strain Relief With Connectors
Connectors are often a failure point. A connector must be mounted in a way that relieves strain and vibration conditions need to be factored in. A best practice for vibration applications is to use a latching connector which prevents the connector from working itself off the mating piece.
For this example, the cables and connectors in it that are properly strain relieved. The cable is routed up through the end of the earpiece in a way that constrains it for ruggedness.
Address Safety Concerns
Safety concerns may affect the location of the battery / connectors / cables as well as the choice of enclosure material and fasteners. Make sure connectors are not reversible. If a connector gets plugged in the wrong way (reversed), this may cause damage to the circuit or even a fire.
In this stud finder example, we had to do minimum part count for 9-volt batteries, so we tried to reduce and eliminate cables. Our design allowed for the insertion of a 9-volt battery in only one orientation.
For another example we helped design a telecom robot that moves around the office and plugs itself in between usages. The charging station had to be designed so the robot could not plug itself in the wrong way and cause damage.
