Wireless Electronics
The best batteries for Bluetooth beacons
For Bluetooth beacons, batteries are an essential design consideration, with their form factor and chemistries playing a big role in the design’s success. Tim Parker, product marketing manager – Batteries, EMEA, Avnet Abacus, explains more
B
luetooth beacons are essentially small electronic devices that sit in a particular place and communicate
with people’s mobile phones as they pass by that spot, transmitting small packets of information via Bluetooth low energy. This has a number of uses. Indoor navigation is one application where they are particularly valuable – in large indoor areas that GPS cannot reach, such as airports or shopping malls, beacons placed in particular spots broadcast their location to any enabled mobile phones nearby. As the location of the beacons is known, proximity to any particular one can help a navigation app on the phone determine where the user is. Another application that beacons are used in is in-store promotions. Beacons placed in various locations around a supermarket can transmit their packets to phones near those locations. For example, if a person spends a particularly long time looking at washing powder, receiving many packets from a beacon placed in the washing powder display may help the supermarket’s app on the user’s phone realise that the user is struggling to choose between washing powder brands. Delivering a coupon for a particular brand to the user’s phone at that time might help them make their purchase. Beacon devices are usually small standalone electronic systems, the size of
a mobile phone or smaller, in compact packaging. All parts of the system therefore have to be small, especially if the hardware is intended to be hidden from view. Beacons are also expected to work 24/7 for long periods of time without any human involvement whatsoever. While mains power would be ideal, outside of some specific retail applications (which already have power to each display shelf) in most places it would be costly and inefficient to provide mains power to all the locations that the beacons need to be placed in. They therefore have to be battery-powered. Selecting the right battery for a Bluetooth beacon can make or break the design. The ideal battery would provide a suitable amount of power from a small form factor, and operate over a suitably wide temperature range (some indoor applications, like shop window displays, can get very hot in the sun). They also need to be reliable enough for years of service, and provide adequate power density/energy density to last for a very long time on each charge to minimise any costly maintenance needed in terms of changing the batteries.
Batteries that meet these criteria fall into two camps: primary (non- rechargeable) or rechargeable. Batteries with primary chemistries are single use and cannot be recharged.
Image 1: Bobbin construction is the most robust for lithium thionyl chloride applications (image courtesy of Varta)
under extreme temperatures between -40 and +85°C. Self- discharge is extremely low, so they are suitable for remote applications. Tadiran’s series of lithium thionyl chloride batteries, which provide 3.6V with only two per cent self-discharge is ideal for Bluetooth beacon applications. In some
Alkaline batteries are the most common type of primary battery available today but they aren’t generally suitable for beacons as they don’t last long enough; replacing AA or AAA batteries in a group of hundreds of beacons in an airport indoor navigation system, even once a year, would be less than ideal. Other primary chemistries based on lithium, such as lithium iron disulphate (LiFeS2) and lithium manganese dioxide (LiMnO2) may be suitable if cost is a particular problem and the electronics can operate with a 3V battery, but the best match for this application is a battery based on 3.6V lithium thionyl chloride (LiSOCl2), which offers the highest energy density of all lithium-based batteries. Energy density refers to the amount of energy that can be stored per unit volume – for lithium thionyl chloride chemistries this is in the region of 1200 Wh/L (compared to alkaline batteries between around 250-434 Wh/L). Lithium thionyl chloride batteries also have a very long lifetime, in the range of 10-40 years, even
applications it will be necessary to add a capacitor or a Tadiran HLC product to support the high pulse current for transmission of RF signals. If rechargeable batteries are required,
there are also several options. Lithium ion, lithium polymer and nickel metal hydride (NiMH) chemistries are all widely used. While lithium ion and lithium polymer batteries can last up to five years if used correctly, their charging profiles must be relatively strictly controlled to avoid damaging the batteries, as this can reduce their capacity or even cause dangerous conditions like overheating. NiMH batteries, by contrast, have relatively simple charging profiles, so while their expected lifetime is similar to rechargeable lithium types, it’s much easier to actually achieve that lifetime in practice.
As an example, Varta has the Robust85C series of NiMH button cells which can withstand temperatures between -20 and 85°C. Self discharge is less than 10 per cent after one month at 20°C. While cell voltage is 1.2V, up to four can be combined to produce up to 4.8V.
Rechargeable batteries are ideal for applications that use energy harvesting, perhaps from a solar panel mounted on the beacon. In this case, it makes sense to check that the battery has the capability to accept a range of charging voltages and currents (at the tiny end of the scale). In summary, there are a wide variety of compact batteries available today. For primary batteries, lithium thionyl chloride represents the best option for Bluetooth beacons, while the best rechargeable option is NiMH, which comes in small button cell form factor. These types offer the best combination of energy density, longevity, reliability and cost effectiveness.
Image 2: The construction of a NiMH button cell (coin cell) battery (image courtesy of Varta)
www.cieonline.co.uk www.avnet-abacus.eu Components in Electronics July/August 2017 19
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