ARTICLE FOCUS:

• Height-minimizing designs
• Choosing plastics, base metals
• Designing contacts

Today's cutting-edge medical devices require battery holders that are smaller, stronger, and smarter.

Thinner device designs and reduced component profiles are being accomplished, for example, with new height-minimizing designs calling for a small cutout to be made in the surfaces of printed circuit boards (PCBs). These designs make it so that the battery doesn't sit on the PCB; instead it is positioned so that the battery is partially through the PCB.

Another option for board designers is to lay a coin cell battery directly on the PCB and use the boards’ traces as the negative contact, while an attached stamped retainer is put on top of the coin cell to act as the positive contact.

These design changes often allow an extra few millimeters of clearance that is needed to save a design from unnecessary compromises.

Plastics, base metals considerations
To make holders stronger, new materials, manufacturing processes, and assembly techniques are being used along with emerging surface mounting techniques. A relevant example here is the use of liquid crystal polymer plastics for surface mounting battery holders, which now survive processing in a 260°C reflow soldering environment despite a wall thickness of only 1 mm or less. LCPs are also known to have a good resistance to chemicals, making them suitable for repeated sterilizations without significant loss of mechanical properties unlike standard plastics, which fail to withstand even one hot steam sterilization.

For AAA to D size batteries, the most common plastic used to make holders is polypropylene with a food grade rating that meets US and European regulatory agency requirements (21 CFR and EC 1935) to ensure long-term patient safety. There are thousands of polypropylenes on the market today, so care in selecting one with the right temperature range, impact strength, and FDA approval is critical.

One such material is polypropylene. In addition to meeting the above requirements, it has an advantageous price point and is often certified free of latex, food allergens, and genetically modified organisms. Polypropylene also meets environmental directives in the US, Europe, and Asia for RoHS, BPA, BHT, BHA, PFOA, and heavy metals. Plus it’s "green" because it’s made without using ozone-depleting substances.

Base metals include copper, steel, stainless steel, and phosphor bronze, with phosphor bronze being the best choice for medical applications due to its resistance to corrosion. A good finish is necessary since oxide films on base metals increase contact resistance.

Batteries are usually nickel-plated on the anode and cathode, so normally a holder's contacts are nickel-plated to prevent galvanic corrosion. When reliability and safety requirements are high, gold is well-known as the preferred choice for the finish of the holder's contacts from a variety of standpoints. Gold-flashed contacts have the best resistance to corrosion and the lowest electrical resistance.

Designing battery contacts
The battery contacts in any holder are always subject to many design decisions. Using simple, straightforward shapes decreases production time, increases the stamping die's life, and lowers costs. However, redundant contact points reduce the likelihood of an intermittent connection with the battery, so this is always worth the increased cost when reliability is a large concern. Contact shapes now include wicking features, so the solder paste on the pads move up onto the contact’s legs for greater joint strength.

Contact pressures normally range from 50 to 150 grams, depending on the battery size of the holder, the current drain, and whether the contacts are also holding the batteries or not. Some designs use one flexible and one fixed contact. The problem with a fixed contact is that when the direction of shock is away from it, there is an intermittent connection as the battery moves away while the contact remains in place. Designs where both contacts are flexible and easily move with the cell are preferable because even under shock and vibration the connection with the battery is secure.

More design innovations
Some designs have the contacts hold the battery while also making electrical contact; others use a plastic battery holder or molded-in section of the housing to receive the battery and use the contacts as electrical conduits. The latter is best because when you contacts are required to do two things, one is usually compromised. One example is a design that uses large pressure contacts to hold the battery. To withstand being crushed, the contacts must be very strong and thus require a large force to remove the batteries. When this device is used by the elderly or disabled, this can make it impossible for them to replace the batteries.

Another thing to consider is how heavy batteries are compared to the holders carrying them. For example, four D batteries weigh about 1.25 lbs, while a typical polypropylene holder weighs under 0.1 lbs. For such a holder to function, it is necessary to specify that no regrinds be used by the molder when making the housing. The addition of an elastomer to polypropylene also can improve toughness and impact resistance, while striking a balance between stiffness and strength.

Further, designs often can be greatly improved by small features in the plastic that allow for better capturing of the batteries and reduction or elimination of weight shifting, which causes intermittent contact, plastic fractures, crushed contacts, and a host of common problems.

These are some of the ways now being used to make smaller, stronger, and more reliable. Future innovations, based on feedback from engineers and consumers, will enable battery holders to continue achieving higher levels of durability, usability, and connectivity.