Powering Wireless and Bluetooth LE Products with Batteries

Since BLE was designed from the ground up for low power battery devices, it’s common to see a wide range of batteries used in designs. Besides the popular Coin Cell batteries you will also see Alkaline, Lithium and many other chemistries.

This guide is designed to give you a broad overview of the choices in powering BLE products and the practical issues surrounding their use. Every product and application is different, but with this information you can make better choices. As always, feel free to get in touch if you have questions.

BLE Current Consumption

Before we start talking about the batteries, we have to understand the BLE devices themselves and their power requirements. Most BLE devices today are System on Chip (abbreviated SoCs). Their current consumption profile has a large effect on the product lifetime.

Most modern BLE SoCs are very sophisticated devices that integrate almost everything needed to build a complete smart device:

• Processor – ARM Cortex-M0, M3 and M4
• BLE Radio – Complete radio and stack needed to connect to other BLE devices
• Memory – Flash and SRAM
• Peripherals – I2C, SPI, UART, PWM, GPIO

Power consumption in BLE SoCs is typically due to the processor and the BLE radio. The processor can draw several mA when running. The Bluetooth radio is another part that can consume significant amounts of power. Some BLE radios can reach a peak of 20mA when transmitting (or more if the output power increased). Which part consumes more depends mainly on the application and duty cycle.

Most BLE products are designed to stay asleep as much as possible to conserve power, waking up to process and send data. How much current they draw during sleep depends on the device, but a figure of around 1.5uA is typical of many SoCs. This figure accounts for a running a Real Time Clock (RTC) which wakes the system periodically to advertise or send data. When in sleep, the CPU is off, the BLE Radio isn’t transmitting, and most peripherals are stopped as well. If the device wakes up periodically, then the current peaks for a few milliseconds during transmission and reception. The average current of the system is then higher and depends on how often the system wakes up to send data.

It’s also possible to go to a deeper sleep mode where no RTC is running and the system in a low power mode, but this prevents a smartphone from connecting to a device. In these cases it’s possible to have a current draw of less than 200nA.

Note that we’re focusing here on BLE products that are running BLE in Peripheral mode. BLE was designed to be asymmetrical, with Peripheral devices designed to use as little power as possible. Central devices draw significantly more power, but since these are expected to be smartphones with large batteries, it’s not an issue.

Peak current consumption of BLE chipsets is affected by several factors:

• Power output of the BLE device – the higher the output power the more current is required. Many latest generation devices draw 3mA to 6mA peak, while older devices can draw 3x or more.
• Amount of data sent – larger advertising packets require the BLE radio to stay on the air longer, drawing more current during this time.
• Processing – the processor requires significant power while running the BLE stack as well as any other processing required

We’re not including any extra power required by sensors, other components or peripherals since they depend on the product, but they should be taken into account. Creating a low power product requires optimization on the hardware, software and other layers.

Another important factor to realize is that BLE devices need a certain voltage to operate. This is typically 1.8V to 3.6V, but some devices can accept up to 5V which can be beneficial in some cases.

Batteries, Lifetime and Current

Every battery, regardless of the chemistry, has several common specifications that we’ll describe:

Capacity – Probably the most important parameter of any battery. Capacity is the amount of energy in a battery and is typically specified in mAh or Ah. A battery with 100mAh can theoretically provide a steady current of 100mA for one hour, or 50mAh for 2 hours, or 1mA current for 100 hours. The actual capacity depends on the current being drawn and is better specified by Nominal and Effective parameters (unless the current draw is very low).

Nominal Capacity – This is the capacity under nominal conditions. Every manufacturer will specify the conditions under which this capacity is specified, and it typically includes temperature and current. For example, a capacity of 260mAh at 0.19mA.

Effective Capacity – This is the actual capacity of the battery in your product or application. This capacity can be lower or higher than the nominal capacity depending on how your product uses the battery.

Voltage – The voltage profile of a battery is the output voltage your system sees over time. Different battery chemistries have different voltage profiles, which can make a big difference in your product design.

The most important thing to understand about batteries is that their actual or effective capacity isn’t usually a linear relationship with current draw. The actual capacity of one battery where the current drawn is 10mA will be different than one where you’re drawing 10uA. The less current you draw, the more capacity the battery will seem to have, beyond just the linear relationship because the battery is subjected to less stress from peak currents. This is important in BLE applications where the average current is low, but the peak currents of BLE radio transmissions are higher. The same happens with every wireless radio.

BLE and Coin Cell Batteries

Caution: Coin cell batteries can be extremely dangerous to small children. It’s important to ensure proper product design to avoid giving children access to the batteries.

Coin Cell batteries are small, flat batteries that can be easily found in Key Fobs, BLE trackers and sensors. The size is very appealing for small devices that need to be placed on items and key chains. BLE was designed from the ground up to use these batteries.

There are many variants of these batteries, but the most popular are:

It’s easy to see that the larger the battery, the larger the capacity, and that is the case with every battery regardless of chemistry. What’s not so obvious is that the smaller the coin cell, the more it is affected by large current peaks. Whereas large batteries can cope better with large peaks during BLE transmission which can be 5mA to 19mA (depending on the device), smaller coin cells do much worse.

Interestingly, the CR2032 doesn’t follow the cost trend, being almost 1/3 of the cost of the others. The greater number of suppliers and economies of scale due to the popularity of this battery means it’s much cheaper.

Coin cells are popular because they’re small, but they also enjoy a very flat output voltage profile that makes product design simpler. Here is the discharge curve for a CR2032 from Energizer:

Out of 1300 hours, the battery has a voltage of 2.9V for 600 of them (almost 50% of the time), dropping to 2.8V after 900 hours. Only at the last 25% or so does the voltage begin to drop significantly but stays above 2.0V.

The importance of a flat voltage is that many BLE chipsets, sensors and devices operate in the 1.8V to 3.6V region. This means that the Coin cell battery can power these directly, without any power conversion. This is smaller, cheaper and simpler than adding regulators.

Coin cell batteries have one major disadvantage compared to other batteries and that is that they’re greatly affected by large current draws. Nominal current for a coin cell is around 300uA (may vary among batteries). But when the BLE device is transmitting, the current can easily spike to 5 to 20mA for a few miliseconds. Running the processor also draws a few mA.

So what happens when there are spikes? Here’s an example graph provided from Nordic:

Vload shows the voltage output profile when under load, compared to 0.5mA cont which is a continuous draw of 0.5mA. You can see that the capacity drops from around 225mAh (green line) to around 180mAh (blue line), a drop of 20% in capacity (your product will run 20% less).

Temperature Effects

Another issue you need to watch out for with coin cell batteries is that they can be affected by temperature, sometimes significantly. Here is the discharge curve for another energizer coin cell battery, showing this effect:

Temperatures can have a large effect on battery capacity and lifetime, and needs to be taken into account when selecting a battery.

Alkaline Batteries

Alkaline batteries, whether AA, AAA, C or D are also very popular in products using BLE, Wi-Fi and other wireless connectivity.

Compared to coin cells, alkaline batteries are physically larger, but provide much more capacity and can handle larger currents.

The actual capacities above change depending on the actual discharge rate. For alkaline batteries, it matters whether you are drawing 25mA, 100mA, 250mA, 500mA, or above.

Let’s look at the discharge curve for an Energizer EN91 AA battery:

Aside from capacity, the difference between a coin cell and alkaline is the discharge voltage. Whereas coin cells have flat voltages around 2.9V, alkaline batteries start at 1.5V and it drop down to around 0.8V. This puts it out of the 1.8V to 3.6V range of most devices.

In order to run most devices out of alkaline batteries, you will need to place batteries in series or use a DC/DC Boost. Two batteries in series can provide 3.0V down to 1.8V which is in the range used.

Two series alkaline batteries provide the exact same capacity as a single battery. When you take into account the inefficiency and the extra current consumption of a DC/DC converter (booster) two series alkaline batteries can provide longer life, but have variable voltage output which may not always be undesirable.

Topologies

The right topology to select depends on the circuit. A few items to take into account:

• Avoiding regulators (boost, buck) saves on both cost and increases system lifetime since you don’t need the costly boost and don’t need to worry about inefficiencies. This is best for system that can accept wide voltage ranges.
• Be careful of sensors that are calibrated by the manufacturer for a specific voltage. As voltage changes for sensors, their output may change and the readings may be affected.
• Whenever boosting the current, the input current from the batteries is the ratio of the output current to the input current, plus current due to efficiency. So, boosting from 1.5V to 3V will require at least twice the input current and more due to the inefficiency.

Lithium Batteries

Lithium batteries – the non-rechargeable primary kind, are similar to alkaline batteries in both the form factor and output voltage. In fact, they’re recommended for many devices where there is a very high current draw. Video Cameras and many other systems that need a lot of current can benefit from these versus alkaline.

You can see this clearly in the following graph for the ENERGIZER L91 Lithium battery:

The capacity of a lithium battery remains much more constant at currents up to 1000mA while alkaline batteries see a significant drop. For low currents, the difference is not significant. Their voltage is also slightly higher than Alkaline, reaching 1.6V or so.

Lithium batteries have another advantage and that is their operation in low temperature is much better than alkaline.

Bluetooth Low Energy products are usually low power by nature, but there can be cases where sensors or other devices require significant current. In those products it’s important to weigh the extra cost and the conditions of the system to see if Lithium batteries are needed.

Rechargeable Lithium Batteries

You’re more than likely familiar with rechargeable lithium-Ion and Polymer batteries from the countless of electronics that use them. These batteries have the one of the highest energy densities available and are available with capacities from 50mAh up to 10000mAh or more.

Although it is possible to have multiple cells in parallel or series, a single cell arrangement is the most typical. A single cell battery typically has an output voltage from 4.2V or 4.35V when fully charged down to 3V or 2.7V when empty. The nominal 3.7V often quoted is not the main voltage at which the battery operates.

Because the voltage gets to above 4V, it’s common to use an LDO or DC/DC Buck regulator to reduce the voltage to 3.3V to avoid destroying components that can’t withstand this voltage. A few BLE SoCs do have the ability to run up to 5V, so they may be considered to reduce the cost of regulating the voltage.

Lithium Ion and polymer batteries have very few issues with BLE peaks. The standard discharge current varies between batteries but is usually C/5, where C is the capacity of the battery. For a 100mAh battery, the standard discharge rate would be 20mAh which is higher than the peak BLE current of most devices.

However, even here it’s important to optimize the product power consumption.

One concern with these batteries is the self-discharge current. Although they’re better than NiMH and NiCad batteries, it’s not uncommon to lose about 2%-3% of their capacity a month. This can be larger than the current of the BLE system in sleep mode.

Battery Management

Optimizing BLE power consumption

Because of the small capacity of coin cells and the expected long lifetime of BLE products, optimizing the power consumption is a critical taks that has to done by experienced developers. Estimates can be useful in giving a ballpark figure, but the accuracy depends on many factors. We highly recommend getting proper measurements and modeling done for the device.

There are several keys to reducing power consumption in BLE and we’ll go over each one.

Reducing BLE peak currents

Peak currents in BLE are usually dominated by the BLE radio. Choosing a low power BLE chipset with low current peaks can help extend the lifetime of a coin cell battery and others as well.

It’s also important to control the output power of the system if possible. Lowering the output power of the system during a connection if the performance is good can reduce power consumption. Look at this for the nRF52832 chipset from Nordic:

The table above is specific to this chipset, but the results are very similar in other devices.

The first important thing to note is that using a DC/DC regulator in BLE chipsets reduces the current consumption significantly. But, this has a tradeoff of reducing the sensitivity of the BLE radio (and the range) by around 2dB.

It’s also apparent that the real gains in reducing the output power are in going from +4dBm to -4dBm. Beyond that there are diminishing returns.

As we mentioned before, the peak currents affect the coin cell batteries significantly because they are over 10x to 30x as high as the average current that can be drawn from the coin cell. Peak currents require significant energy for a short period of time. One approach to avoid drawing so much current from the battery directly is to use a large capacitor to provide storage. By using a capacitor in parallel with the Coin cell, the capacitor can provide the peak current when it’s needed instead of the coin cell (the capacitor has much lower effective series resistance).

However, the large capacitors also have increased leakage and cost, meaning that their value can be limited and needs to be evaluated in a particular product.

Reducing BLE Average Currents

The average current of BLE devices or any wireless radio takes into account the peaks but also the sleep current of the device (if in sleep). To extend the battery lifetime it’s critical to reduce the number of transmissions done by the device. If the device is in advertisement mode, then increasing the advertising interval to 500ms or higher will reduce the power consumption drastically. Similarly, if the device is in a connection, increasing the interval and slave latency will help reduce the current consumption.

As an example, let’s look at the average power consumption for Nordic’s nRF51 chipset during advertisement, assuming it goes to sleep and the advertisement, transmits at 0dBm with a 20 byte advertisement packet:

As you can see above, increasing the interval to 500ms helps the system runs 3x as long, while increasing it to 1000ms helps the system run almost 5x as long. If your device spends much of its time advertising, increasing the interval has significant gains.

Note that the DC/DC operation mode in this case makes relatively little difference.

Shipping Batteries in Products

Once you’re design is ready and you’re shipping your products, it’s common to ship batteries inside or with the product. The customer can quickly turn on the system and use it. Reducing the steps to get a product working always makes customers happy, but it can create challenges.

It’s important for the battery to be as charged as possible. Shipping and storage time does count for the battery expiration and longevity, so this needs to be taken into account. If the battery is installed inside the product, it’s important to avoid drawing any significant power or else it will be too discharged for customers. There are multiple solutions to this:

• Physically preventing the battery from forming a circuit – Either placing the battery outside of the product, or using an isolating material such as plastic pull-tab prevent the battery contacts from touching the product contacts can work.
• Ultra low Power – For products where inserting the battery requires more effort, doing an ultra-low power design where the battery is in place can help. Usually designing a system that draws 500nA or less means the battery will be good even if the product takes months to ship.

Battery Design Care

Have you ever looked at the instructions manual of a battery and been told to avoid using batteries that are different and not fresh? There’s a good reason for it. When batteries of different conditions are inserted into a product, a lot can happen.

Imagine that a customer inserts an old and a new battery in parallel in a device. The batteries likely have different voltages given their different discharge states. The battery with the higher voltage will attempt to charge the lower voltage battery. But alkaline batteries aren’t rechargeable and therefore this can damage the battery and even cause it to leak and be damaged.

Lithium coin cell batteries are very affected by this and it’s a bad idea to put them in parallel.

Battery Lifetime

It’s important to note that batteries do age and expire. Even if a system is designed to work 10 years off of a battery, it’s unlikely it will, given the aging process that’s happening internally. It’s very important to take this into account and if needed to choose batteries that are designed for long life.

Most vendors don’t guarantee any kind of operation after 10 years, since the battery can leak and corrode.

References

1. Aside from our experience, the following resources were used as a source for the information presented.

Nordic Semiconductor. High pulse drain impact on CR2032 coin cell battery capacity

2.  ENERGIZER CR2032 Product Datasheet
3.  Manganese Dioxide Lithium Coin Batteries: Individual Specifications
4.  ENERGIZER EN91 Product Datasheet
5.  ENERGIZER L91 Product Datasheet

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