Getting the Most Out of Coin Cell Batteries

Nordic Semiconductor, the manufacturer of the system on a chip (SoC) inside most beacons, has a useful recent post by Jon Gunnar Sponås on Improve battery life in Ultra Low Power wireless applications.

It explains that while a battery has a fixed initial capacity, how you draw current from the battery affects how much of that capacity you get to use. At a relatively low constant current of 0.5mA you get most of the capacity while at 3mA you only get 60%.

For Bluetooth LE the current isn’t usually constant. Instead, it advertises at up to 7mA, for of the order of a milliseconds followed by a pre-set inter-advertising period between 100ms and 10 secs. This gives the battery time to recover.

The article explains how Bluetooth LE firmware should be designed to not turn everything on at initial startup so as to not stress the Battery unduly. It also mentions how it’s also wise to test the battery in the actual situation rather than relying on the battery mAh rating to calculate expected battery life.

Battery Power Use When Advertising Multiple Bluetooth LE Channels

Most beacons can transmit more than one type of advertising , for example iBeacon, Eddystone and sensor data. In practice, no beacon can send more than one kind of data simultaneously. Instead, they send the different data sequentially, one transmission very shortly, milliseconds, after the other. Many manufacturers describe this as sending data in different channels which shouldn’t be confused with different Bluetooth LE frequency channels used to reduce the affects of wireless interference.

Some devices such as Minew and Sato can send 6 channels that can include iBeacon, Eddystone UID, Eddystone URL, Eddystone TLM, sensor, acceleration and device info:

Sato setup: Channel types are shown at the bottom

Transmitting one type of data takes of the order of 1 millisecond (ms) every configurable 100ms to 10secs period. It’s during the sending that the majority of the battery power is used with the beacon sleeping between transmissions. The following oscilloscope trace shows the battery power used, over time, with one channel:

Care should be taken to configure only those types of data that are required. If you configure more than one channel then there’s a corresponding, almost linear, increase in use of battery power for every extra channel.

New Beacon Power Measurement Service

Since our original post on power measurement in 2016, we have been occasionally providing power measurement services as part of consultancy. Last week we started offering it as separate service orderable via our web store.

Our in-house designed power analyser

We take a beacon the same as yours, or one you send to us, and measure the actual power use with your specific settings.

Note, however, that if you will be using batteries that have been included with beacons, those batteries will have been used for an indeterminate time in the factory for soak testing the beacon. You will need to use new batteries to obtain the maximum battery life.

How Power and Advertising Interval Affect Battery Use

Nordic Semiconductor, the manufacturer of the System on a Chip (SoC) in most beacons has a useful online calculator that helps work out the battery current used when advertising or when connected.

You need to set the SoC chip type (see the specification for the beacon you are using), voltage (3v as it’s usually a coin cell), DCDC (usually off), clock (usually external) and tx payload (set to 31 bytes). You can then vary the role (advertising or connected), power and advertising interval to see the affect on the battery current.

Dividing the battery capacity by the current will gives the approximate battery life. The resultant battery life calculation will be a very rough approximation and will be less if the manufacturer has added extra circuitry such as sensors. The online calculator is best used to get an appreciation of how changing parameters or the SoC type affects battery life rather than being a definitive value. For more accurate battery use it’s necessary to measure the actual battery current.

Wiliot To Enable New Beacon Usecases

We mentioned Wiliot last March and since then their R&D team has created early engineering samples that prove it’s possible to create a battery-less Bluetooth LE beacon harvesting energy from radio frequencies (RF).

The Wiliot device looks more like a RFID tag than a traditional beacon in that it’s supplied as a very thin PVC inlay sheet containing the chip and wire antenna together. The thin form factor, no battery and the relatively low cost will allow it to be manufactured into or stuck onto clothing and packaging that will provide for many new usecases.

Producing such a device isn’t easy as it can’t use existing System On a Chip (SoC) devices as produced by Nordic, Dialog and Texas Instruments (TI) because they are too large and use too much power. Wiliot has had to create their own SoC from the ground up, including software tools to develop and program the devices. We have been told it will be a year before Wilot has all the components in place for commercial rollout. Meanwhile, selected organisations can join the Early Advantage Program (EAP). There’s a new a product overview (PDF below) that explains the EAP and the main usecases, connected packaging, connected apparel, logistics and asset tracking:

Wiliot already have Early Advantage Program (EAP) agreements in place with over a dozen brands including top fashion brands, a telco, appliance companies, a furniture brand and packaging companies.

Beacon Battery Level Observations

Most beacons provide a battery level % indication that’s visible in advertising and/or the manufacturer configuration app. It’s also usually visible via a Bluetooth Service Characteristic.

Some observations:

  • Lithium batteries (if you are using them) have a very flat voltage profile with a sudden drop off towards the end of their life.

Here’s an example for Energizer Lithium AA:

For a typical CR2032 Lithium coin cell:

  • The beacons use very little power over time. If you are measuring over days when batteries last years, you will see very little difference.
  • The firmware in the beacon and/or app need to determine what voltage signifies 100%. This can vary by battery type. Some beacons/apps simplify things by using a fixed voltage for 100% such that it’s possible that the voltage is higher than this at the start of the life of the battery. The level will appear to stay at 100% for a long time.

A consequence of the above factors is that you can’t estimate battery life by looking at battery percentage over time. You need to measure current use. We have a previous blog post on this topic.

Battery level can only be used as an indication that the battery is low and should be changed.

New Minew E2 in Stock

We have the new Minew E2 in stock. It’s a waterproof beacon advertising iBeacon and Eddystone.

What’s special about this beacon is the 10+ year battery life that comes through using the power efficient Nordic nRF52 SoC and 4 AA batteries. The in-use battery life will depend on settings.

This beacon also has a power amplifier that provides for an exceptional range of up to 500m. However, as with any long range beacon, the maximum range is achieved only when there’s line of sight.

What’s the Best iBeacon?

We often get asked what’s the best iBeacon? Unfortunately, there is no one best beacon for all scenarios. It depends on your particular project and business requirements. Having said this we have some favourites based on specific characteristics:

Best for Price: TON9108 – Well built, Apple MFi certified beacon.

Best for Features: iB003N – Waterproof, large easy replaceable battery, long range, accelerometer, ultra long range and sensor variants.

Best for Battery Life: SmartBeacon-AA and SmartBeacon-AA Pro – Allow use of 4x AA batteries. Use lithium AA batteries for 7+ year battery life (also depends on settings).

Best for Setup App: Minew range – Minew’s latest BeaconPlus range (those supporting both iBeacon and Eddystone) provides the best in class app.

View our complete range.

New Long Range 210L Beacon

We just received the 210L ultra-long 200m range beacon into stock.

Most beacons tend to have a range of 30m, 50m or 100m. The normal output is 0dBm but they can be boosted to +4dBm to achieve the maximum ranges. Read our article on Choosing the Transmitted Power for more information.

The 210L beacon transmits at +10dBM which is the maximum allowable for this class of Bluetooth 4 device. This is just over 3x the power of a beacon transmitting at 0dBm. Hence, there’s respective reduction in battery life.

View our ultra-long range beacons

What’s the Best Beacon?

Too many potential customers contact us asking what’s the least expensive beacon that provides the best range, the best battery life and the smallest size. Unfortunately, all these things are related. You need a larger battery to provide enough power for a longer range. A large battery implies a larger beacon size. A larger battery and case implies a more expensive beacon.

Instead, you should take a look at your project/scenario and determine what really is the most important factor and use that as a starting point.