Product Design Short: Primary vs. Secondary Batteries
If you’re looking to design a device, getting to know more about batteries can empower you to make better decisions about powering your product or system.
Batteries comprise the core of many consumer electronics products, as well as vehicles, emergency service aids, healthcare products, and millions of other devices. If you’re in the design business, there are a few battery basics which can empower you to make better, more informed decisions about powering your product.
I’ve developed several truly unique devices — where there is no existing technologies to reference — and I’ve worked with existing product manufacturing titans to evaluate their designs. In my research, I’ve thought a lot about how to select a power solution that provides the end user with the best experience, while not sabotaging electrical requirements or supply chain. I have also found that most battery specifications focus on providing relevant information to electrical engineers, but not product designers or mechanical engineers.
While this information is not meant to substitute working alongside an experienced electrical engineer or PCB designer, it will aid anyone involved in the product development process to understand the complexities and the major impact battery choices can have all over the map.
Let’s get to it!
When delving deep into battery world, you’ll find there are two broad categories of battery chemistries: primary and secondary. The core difference is as follows:
A secondary battery is rechargeable, while a primary battery is not.
Primary batteries include most common alkaline batteries, traditional watch batteries (non-rechargeable coin cells), lithium metal batteries (with lithium anodes — NOT lithium ion), dry cells, and many other types. These batteries cannot be recharged, and can pack a lot of energy into them as a result. However, they must be replaced after they are used.
Secondary chemistries include the increasingly common lithium ion, lead acid (common as car starter batteries), nickel-cadmium (NiCad), and nickel-metal hydride batteries (NiMH).
Whether you choose a primary or secondary battery should be driven by many factors. To name a few:
- User experience
- Manufacturing supply chain
- User supply chain
- Servicing requirements
- Bill of materials cost
- Cost to user/cost of power
- Current requirements
- Voltage requirements
- Power requirements
- Availability of an external power source
- Sustainability
- Device recycling (regulations and recycling programs vary globally)
A battery which excels in one application may perform poorly in others. The optimal choice depends very much on your particular set of requirements.
I’ll do a quick comparison of strengths and weaknesses based on using a rechargeable battery.
Advantages of a secondary (rechargeable) batteries over primary batteries
- Low internal resistance, thus better peak power output
Secondary batteries handle current spikes much better than their primary counterparts due to their low impedance. The need to handle current spikes isn’t limited to power tools — batteries for digital applications must handle the extremes of processors’ variable current demands. - Reduced waste
Rechargeable batteries often produce less waste per ounce over a product’s lifetime.
- Lower cost of power for the user
- Simplified supply chain for end user
In the case of primaries, batteries need to be replaced every time the charge is depleted, so they must have extra batteries in case the product dies.
Disadvantages of rechargeable batteries
- High self-discharge rates
Rechargeable batteries are not suitable for long-term energy storage compared to their primary cousins. - Recharge interruptions
It may not be possible to charge a device (like an implantable pacemaker). In some cases, it may provide significant interruption to the user if the product must be recharged. If users do not have access to a second, hot-swappable battery or battery pack, then they may not be able to continue using the device as intended.
- Lower energy density
Secondary batteries have much less energy capacity than a primary battery of the same size. - Charging circuitry complications
Device designers need to protect sensitive electronic components and the battery from overvoltage and overcurrent during charging, which is most safely implemented by including it within the product being charged. You’ll usually need to provide a charger for your device as well, which can take the form of a standard power supply or cable, or a custom charger. Especially if you are so tight on space that you cannot fit protections on your device PCBA, a custom charger should be included to prevent death/lawsuits/frying electronics. - Increased BOM cost
Requiring users to supply standard primary (non-rechargeable) batteries (AAs, AAAs) reduces bill of materials costs, and reduce weight of the shipped product. “Batteries not included” can help manufacturers maintain their margins. Lower sticker costs may result in more sales because consumers are less turned off by the sticker price, even if they must buy them separately. Not having to provide chargers saves on cost as well.
Some applications lend themselves exclusively to one type of battery. For example, power-demanding applications often require secondary chemistries due to their lower internal resistance. Many cases can accommodate both — it is then up to the product design team to determine the best fit for their use case.
Case Study: Siri remote
A product category in which you may find both primary and secondary batteries is television system remotes.
Most household remotes use alkaline batteries, like AAAs. Apple prioritizes slimness and sleekness in designing their products, so alkaline batteries are not used in their modern remote controls.
The first Apple TV remote used a primary coin cell battery similar to those in many car key fobs. The size and shape of a coin cell battery allowed for a sleeker design which fit the Apple aesthetic better than designing around a cylindrical AAA alkaline battery.
Apple overhauled their remote with the release of the Siri remote for the Gen 4 Apple TV. Along with a redesign of the electronics to accommodate voice commands and touch control, Apple’s design teams changed the battery to a rechargeable (secondary) lithium ion battery.
The original Apple remote’s primary coin cell battery lasted years, after which the user physically removed it and replaced it with a new one. The Siri remote’s rechargeable lithium ion battery only lasts a few months, and must be recharged using Apple’s proprietary Lightning cable.
Since users of the Apple ecosystem often possess personal Lightning cables in their homes to charge their iPhones, the use of a lightning cable to charge the remote doesn’t add much friction.
The Siri remote does have more recycling implications than its predecessor. “The lithium-ion battery in the Siri Remote should be removed by Apple or an authorized service provider,” the Apple website says. “Dispose of this product and/or its battery separately from household waste, and in accordance with local environmental laws and guidelines.”
In summary: product design teams at Apple considered aesthetic targets, user experience, power demands of the new electronics, and ultimately space constraints in deciding what battery to include in the Siri remote. This decision affected many people who produce and ship the product as well as the user, not to mention how the product is recycled after use.
Summary
The battery type and how it’s either replaced or recharged fundamentally alters the user’s experience and affects the sustainability, cost, supply chain, and form of your device or system.
Beyond primary versus secondary, specific chemistries vary with regards to electrical properties, ease of recycling, safety, availability, cost, and more.
Don’t forget to think outside the primary/secondary dichotomy as well. Rechargeable secondary battery packs (think a drill battery) and corded devices could be the right choice for your particular application.
Deliberate wisely!
And remember not to measure batteries with metal calipers!
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Much thanks to Battery University for providing tomes of invaluable information throughout my years of battery research.
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If you have battery or product development questions you’d like answered in future blog posts, or you’re interested in talking to an engineer or product development specialist about your specific application, please comment below or reach out to steph{@}swopedesignsolutions.com. Learn more about our team and what we do at swopedesignsolutions.com.
