Product Design Short: Specifying and Sourcing Batteries
An underrated, undervalued struggle of creating a new consumer electronics product is powering the device.
Batteries and power are critical in electronic device design. Batteries are a part of the device that users count on, and only think about when they cause trouble (they don’t hold their charge after a few years, they catch fire, etc.). Selecting a battery is an art many engineers don’t appreciate or excel in. I challenge you to consider learning it, as it gives you more power (pun intended) in making sure you meet all of the competing demands placed upon the device design.
To outline those competing demands, let’s go over the two major directions many requirements come from, and how they relate to product design: top-down and bottom-up.
Design approaches and component selection
Top-down directives originate externally to the device — from the outside of the device, how it looks to the user, and from intended user experience. Bottom up directives are driven by the selected internal components — PCBAs, batteries, and design considerations for them. In many cases, your top-down directives will come from industrial designers or users, who want your device to be slim or of a certain shape. Your bottom-up directives come from your electrical engineer, who needs you to keep that PCB edge a millimeter this way because otherwise his board design goes awry.
The best mechanical designs balance requirements from both the top-down and bottom-up, although some applications apply weight to the requirements differently.
Designing your device only around a battery and existing PCBs would be an example of bottom-up design. That approach can result in boxy, cumbersome, and unattractive enclosures. Selecting electrical components without considering top-down factors can have a negative impact on size, form, weight, impact resistance, user input locations, and other “top-down” factors. These can make or break a product’s success; no one wants to buy smart phones that look like TI-83s. However, if your device doesn’t fit electrical components that help the device function as intended (batteries included), you could end up with death and lawsuits (remember the Samsung Galaxy Note issues?)
The best product development results from collaborative design.
A note on custom batteries
If you’re space constrained and thought you could design your board and just fill in the rest of the space with whatever battery, I regret to inform you that, unless you’re Apple, it’s an unlikely proposition. We’ve heard of lithium ion pouch cell manufacturers being accommodating of in-between sizes if that’s needed, though — but no custom shapes.
Although I’ve floated this idea to a few electrical engineers, combining several batteries in a product doubles the required amount of charging circuitry. A single battery implementation is easier and safer for most applications.
Before you dive deep in your battery search
You may be in a position where you’ve got either a limited amount of space, or target amount of space, in which to house a battery. A few pieces of information are helpful before you start your battery search based on volume and shape alone. Work with an electrical engineer or PCB designer to understand your requirements if you don’t have this information.
- What are your current requirements?
- What are your voltage requirements?
- What is the desired run time of the device?
- Does the battery need to be rechargeable? Replaceable? Both?
- Is there room for overcurrent and overvoltage protection on the board for discharge? What about for charging if you are planning on it being rechargeable?
Many other factors are important to your electrical designer, but these are critical. These criteria will help you sort batteries into “workable” and “not workable” buckets.
Battery spec basics
The most important properties to filter for are C-rate and capacity.
Capacity
Capacity is the raw amount of energy available in the battery. Unfortunately, it is not as simple to find the run time as dividing capacity by the current draw. You must consider how fast the car is trying to empty its gas tank to get an accurate understanding of its run time.
C-rate
The C-rate is a proportion of the battery’s ability to supply current to its capacity. For example:
- 300 mA of supplied current from a battery with a capacity of 300 mAh indicates that the battery is running at a C-rate of 1C.
- That same 300 mAh battery supplying 600 mAh is running at a C-rate of 2C.
- That 300 mAh battery supplying 50 mA of current is running at 1/6 C.
And so on. The C-rate is often specified on a data sheet for both charging and discharging.
Charging or discharging a battery above the stated C-rating (called “fast charging” for charging case) stresses battery cells. It is especially harmful over sustained periods. In almost all cases, this treatment reduces cells’ effective capacity. Stress may also increase its risk of failure, making it more dangerous to the user.
Most batteries will supply higher C-rates above certain temperatures. Note that charging a battery below certain temperatures can be very bad for it as well.
A note on safety factors
Many battery device manufacturers test their batteries to supplied specifications. However, it is always wise to buffer their specifications to ensure adequate power. This can avoid costly redesign of boards or externals.
Battery shapes
Cylindrical
Cylindrical batteries are common and include the readily available and powerful 18650 format, as well as smaller sizes like AA, AAA, and LR44.
Pouch
Lithium pouch batteries are growing in popularity within consumer electronics. They are an efficient packaging that is often thin and fits snugly alongside a PCB enclosure. Pouch batteries need room to expand as a result of gas generation. They are somewhat flexible and may be slightly compressed in a single direction, but need room to expand in other directions. They also need protection from impact. High temperatures and humidity can cause degradation in these batteries.
Prismatic
Prismatic batteries are the hard-cased version of pouch cells. The hard case adds protection but also weight.
Button cells
Button cells are reminiscent of watch batteries and key fob batteries. Many are rechargeable. You can find them in many places, from powering hearing aids to keeping the time on your computer. They work great as a clock back up for any kind of powered device, and can fit into many form factors. They come in many chemistries and can be very small.
Use with caution in any child product, and make them difficult to remove in any consumer product. If swallowed, the voltage across a lithium button cell produces hydroxide in the esophagus. This can result in severe chemical burns or death.
Other shapes
Circular lithium polymer cells, flexible cells, and ultralight batteries are a few neat battery types new to the market. New or unique form factors limit your supply chain, but could work for your application.
Where to start
I start my search on PowerStream because their categorization technique is intuitive for building a brand new device. Many battery sources sort batteries by application. This method doesn’t work well for new product development, as the new device doesn’t exist yet. PowerStream sorts their products by battery types and shapes. For example, I’ve found use for old GPS batteries in smart water bottles. Searching by shape and chemistry is faster than looking through application areas to a new product designer.
PowerStream also provides a wealth of data sheets, as well as cost information at different volumes. They sell some batteries from stock to ship quickly for prototypes. For some SKUs and for large quantities, they seem to have manufacturing partners in the US and in China.
Once you’ve found a battery that meets your needs, forward its specifications to your PCB designer or electrical engineer.
Case Study: a rechargeable prismatic lithium ion cell
I’m going to run through GMB042030S’s data sheet to demonstrate the process.
Say you’ve identified that that this battery might be able to work for your design.
- The volume and weight, 4.2 mm x 20 mm x 30 mm size and 6.9 g , seems about right for your volumetric needs
- The capacity of 170 mAh seems about right. For references, phones at this time of writing often have 2000–3000 mAh batteries, so this one is relatively light on capacity compared to a cell phone.
C-rates
For our example battery, we see many references to the C-rate in its specifications.
“C5A” means the manufacturer tested the battery over a 5 hour period, and I’ll substitute “C” going forward.
Standard discharge is shown as 0.2 C. We can calculate what that means in terms of current in units of mA by multiplying the rated capacity of the battery, 170 mAh, by 0.2. This gives us 34 mA as a standard discharge rate.
We see also that the standard charge rate is 0.5C, meaning standard recharge rate of the battery is 0.5 x 170 = 85 mA.
The indicated fast charge rate of 1C translates to an allowable fast charge rate is 170 mA. The data sheet claims to take about 2.5 hours for a full charge.
The maximum continuous discharge current is rated at 1.5C, or 1.5 x 170 mAh = 255 mA. This number is critical.
Make sure your application does not require more than the maximum continuous discharge current for any significant period of time, or you will be designing your device such that it repeatedly damages your battery.
Capacity
As we calculated earlier, for 0.2 C / 34 mA constant current, specification 1 indicates that the device will last about 5 hours. Running at double this, you would not expect to see it last 2.5 hours due to running it at a higher C-rate, but you may see two hours of run time due to the added stress on the battery. These numbers aren’t exact, and require some testing, but are a good starting point for prototyping the device.
Operating temperature range
Note that the discharge temperature has a wider range than the charge temperature. The charge temperature minimum of 0 degrees Celsius is important to note. Charging lithium ion batteries below freezing can cause permanent, irreversible plating of the anode.
Once you’ve got a few in mind
Don’t forget to check if the battery you’ve chosen is readily available off-the-shelf, or if you’ll need to get a specific battery supplier involved. If you’re making prototypes and the size you need isn’t available off-the-shelf, I’ve had luck buying batteries off of eBay.
Summary
Figure out what C-rate and capacity you need for your application before diving into battery selection. Keep in mind the variety chemistries, shapes, and manufacturers when looking for a battery . You may find that your ideal batter looks far different than you originally envisioned.
Happy hunting!
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We hope you enjoyed this product design short. Let us know if you have any specific questions or suggested topics for future posts by contacting steph{@}swopedesignsolutions.com.
