How to Select the Optimal Battery Technology for Your Application

Choosing a battery for a given application involves more than just knowing the application requirements. An understanding of the underlying technology of each battery type, its characteristics, limitations, and advantages, is required in order to select the right type of battery.

The batteries described here are all based on electrochemical processes that produce electricity. We’ll be looking at both disposable and rechargeable battery technologies.

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Before going further, note that a battery pack is defined as being made up of one or more such cells bundled together into a single unit. Sometimes additional circuitry is included for protection, fuel gauging, or temperature sensing.

Disposable Batteries

These types of batteries are non-rechargeable, and are discarded after use. They typically come in well-defined standard sizes such as AA, AAA, C, D and various coin types.

The next sections briefly describe the technologies and uses of the major types of disposable batteries.

Alkaline, Zinc-Carbon, and Lithium Batteries

These can be easily found everywhere. They are available as cylindrical or button types, and 6V or 9V types. The typical open-circuit voltage of a fresh single cell is nominally 1.5V, but can be as high as 1.65V. The end of discharge voltage is around 0.9–1.0V.

Alkaline cells have between 3–5 times the gravimetric energy density of zinc-carbon batteries with around 100 to 190 Whr/Kg (watt hours per kilogram), depending on the discharge rate. Whereas non-rechargeable lithium batteries have around 200–270 Whr/Kg.

Since the available sizes are standard, these figures can be used to roughly estimate the weight of the battery needed for a given application.

They are capable of moderate discharge rates at the expense of reduced capacities. For example, AA batteries can be discharged at 800 mA to 1.0A at room temperature — less in fully enclosed spaces.

Coin-Type Batteries

These are mostly zinc-air or silver oxide chemistries which have nominal voltages of about 1.3V — 1.6V, and lithium manganese dioxide types at about 3.0V.

Except for lithium manganese, these batteries have very low discharge current capabilities.

On the other hand, a lithium manganese coin cell such as a CR2032 can be discharged at 100mA or more, and at low discharge rates it has a very flat discharge curve.

So, unless the extra cost is a consideration, lithium manganese is usually recommended for applications requiring coin cells.

Special Types

If the application requires a battery that has a shelf life of 15–20 years, has high energy density, and can operate from -55°C to greater than 85°C, then a lithium thionyl chloride battery should be considered.

The main drawback is that it is only capable of a very low discharge rate. This means a large decoupling capacitor is usually required to improve its transient response. Lithium thionyl chloride batteries are also expensive.

Rechargeable Batteries

There is a wide range of choices for rechargeable batteries. Due to that, there is an equally wide set of issues to consider when selecting the proper technology to match the application.

Lead Acid

Lead acid batteries have low gravimetric and low volumetric energy densities, respectively at around 34–40 Whr/Kg and 60–75 Whr/L. Thus, for a given amount of energy storage, they are very large and heavy.

Their cell voltage is around 2.1V, however, they are usually available as 6V or 12V batteries. Self-discharge is high at 5–20%/month. What distinguishes them is their very high discharge rate capability and their relatively low cost. Also, charging them can be very simple.

Lead acid batteries are most commonly used in automobiles. They are almost never used in consumer electronics.

NiCad Batteries

NiCad batteries have been mostly supplanted by other types with better cost-performance ratios. However, they are still widely in use at this time so it’s good to understand their pros and cons.

NiCad batteries are not recommended for use in new designs.

Nickel Metal Hydride (NiMH) Batteries

NiMH batteries have replaced NiCads in most applications where they would have been considered before.

Two things to watch out for are cycle life, which is quite low, and self-discharge, which is quite high.

There are some low, self-discharge types on the market, but they typically have lower energy densities. Also, charging them requires specific, though widely available, chargers.

Li-ion Batteries

Li-ion batteries actually fall into many sub-types even though, at a fundamental level, they store energy in similar ways. This is, by far, the technology with the highest energy density and the widest selection.

Li-ion batteries are also by far the most common battery technology used to power modern electronic products. They mainly come in three different form factors as shown in figure 4 below.

The cylindrical and prismatic types have rigid, pre-formed metal cases.

The pouch type, known somewhat incorrectly as lithium-ion polymer or LiPo, has a casing made up of laminated nylon, aluminum and polyester materials.

This type offers the most shape flexibility since they are easily customizable. Some suppliers will even do custom shapes for moderate production volumes.

Figure 5 shows some examples of curved or irregular-shaped pouch-type batteries.

Yet another type, known as solid-state batteries, can be made into extremely thin and flexible, but offer low capacity.

The flip side of li-ion technology, however, is they require stringent precautionary measures in order to provide safe operation and handling.

Pay particular attention to safety when using lithium-ion batteries, otherwise you may find your product catching on fire like happened with the Samsung Galaxy Note 7 back in 2016/2017.

Mechanical Considerations

Lithium-ion batteries have to be securely mounted inside the enclosure. If the battery can rattle around when the device is jerked or dropped, and it gets dented or bruised, internal shorts in the core can occur. These, in turn, can cause overheating, or even explosions.

In addition, lithium-ion cells will swell as they age and the battery compartment has to accommodate this increase in size. Again, this can also cause bruising of the core.

Electrical considerations

The li-ion cell must have a Protection Circuit Module (PCM), usually bundled together to make it into a battery pack. The PCM must provide Under-Voltage Protection (UVP), Over-Voltage Protection (OVP), and Over-Current Protection (OCP).

Li-ion cells do not take well to being over discharged. They will suffer permanent damage. They also cannot be charged much beyond their maximum charge voltage. Even 0.5V of overcharge is too much, and they will run a risk of thermal runaway.

Finally, they have to be protected against over-current discharge, or else they will overheat, and swell up.

This all seems very complicated, but, fortunately, there are chips that integrate all these components. Most battery vendors provide packs that already include the PCM. I always recommend that you only use lithium-ion batteries that have the PCM built-in so as to reduce your chance of an accident during early testing, and to decrease your own liability.

On top of the PCM, li-ion batteries have strict multi-stage charging requirements that include pre-charging, full rate charging and taper charging.

They also have both low and high temperature limits on when charging is allowed. Specifically, li-ion batteries should not be charged at low temperatures — below 0°C to 10°C, depending on the manufacturer.

Doing so repeatedly will cause lithium plating — a very dangerous condition that can cause the battery to catastrophically fail, sometimes hours after charging, when the battery is just sitting idle.

PCM’s have a temperature sensor, usually a Negative Temperature Coefficient (NTC) thermistor. This provides temperature readings to the charger controller that, in turn, disables charging beyond the safe limits.

Again, there are various chips that can handle this task with various cost/performance tradeoffs. The system itself should also shut down if the battery temperature gets too high during use.

Regulatory Approvals

Because of all of the above, li-ion batteries have to be certified before they can be used in most commercial products. For the North American market, the cells have to be UL1642 certified. The battery packs should be IEC62133 certified.

In addition, to be able to ship the cells by air, they should be UN38.3 certified. Also, note that they fall under the Dangerous Goods Regulations of IATA.

This requires, among other things, that batteries shipped in bulk meet certain state of charge requirements, as well as packaging and labeling requirements. Li-ion batteries cannot be shipped in bulk on passenger aircrafts.

Common Type of Li-ion Batteries

Almost all common types of li-ion batteries use graphite, or graphite with some percentage of silicon or silicon oxide as their anode, or negative electrode.

The positive electrode, on the other hand, can be one of several material types, from which the type names are derived. Common cathode materials include Lithium Manganese Oxide (LMO), Lithium Cobalt Oxide (LC), Lithium Iron Phosphate (LFP), and Lithium Nickel Manganese Cobalt Oxide (NMC).

They each offer various tradeoffs among nominal voltage, energy density, maximum charge and discharge rates, specific power, and cycle life.

The C-rate is defined as the actual stored energy of the battery, in Whr (watt-hours), divided by the nominal voltage.

To get the Whr of a battery from the energy density given in Wh/Kg, simply divide it by the weight of the battery. A 200 Whr/kg, 1 kg, battery has a stored energy of 200 Whr. If its nominal voltage is 20V, then the C-rate is 10A.

Conclusion

The choices of battery types can be a bit overwhelming at times. However, the large majority of products will use either disposable alkaline batteries, disposable lithium coin cell batteries, or rechargeable lithium-ion batteries.

Only in rare cases will any of the other battery technologies be used.

Although the large majority of new products use rechargeable lithium-ion batteries you need to be aware of the extra complications these batteries create.

From a technical standard most of the complications due to lithium-ion batteries are taken care of by the built-in PCM, and the wide availability of easy to use battery charger chips.

However, they do add extra product cost, and require several additional levels of certification.

In some cases disposable alkaline batteries make the most sense, at least initially. Perhaps their use could be one way to simplify your Minimum Viable Product (MVP). Later you have the option to release a version with a rechargeable lithium-ion battery.

For more articles like this one aimed at helping entrepreneurs and startups develop new electronic hardware products be sure to check out my Predictable Designs blog.

Originally published at https://predictabledesigns.com on June 28, 2019.