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.
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.
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.
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.
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.
We have created a table to show the relative difficulty of changing the battery in the beacons we currently have in stock. Generally, very easy means the beacon has a compartment opened with the edge of a coin, easy usually means the case prises open or uses screws and difficult means the case needs more force.
Sometimes you might need to put the battery in and need to know the battery orientation. For example, some of our beacons come with the battery separate or you might have taken the battery out and not taken a note of the orientation.
The following image shows the orientation of the battery. The -ve ‘button’ side of the battery fits against the printed circuit board. The side that faces away from the beacon circuit board is +ve. This is common to just about all beacons. Note that this is different to commonly used batteries, such as AA batteries, where the ‘nipple’ end is +ve.
If you put the battery in the other way around then you might damage the beacon or, more likely, short circuit the battery as most battery holders are metal wrap around types.
After project rollout, human effort used in regularly replacing batteries can be significant and the human resource cost of doing so can dwarf the actual cost of the beacons. Hence, unless it’s a temporary scenario it’s best to specify beacons with as large a battery capacity as possible. Beacons with smaller capacity batteries are only suitable for short trials, temporary events or use during development.
While BeaconZone stocks a very large range of beacons, we purposely haven’t stocked any beacons with batteries smaller than CR2032 because the battery life of CR2025 and CR2016 beacons is usually too short. One exception is the iB003M that requires a CR2016 in order to be slim but you can set it to motion triggered to save battery power. All our other beacons use either CR2032, CR2450, CR2477 or AA batteries.
Battery capacity is measured in mAh. The mA part (without the h) is the unit of current. As an example, a CR2477 battery typically has a capacity of 1000 mAh which means it can supply 1 mA for 1000 hours or 2mA for 500 hours. However, most beacons only use tens or hundreds of µA when transmitting, where 1 µA is 1000 times smaller than a 1 mA. Also Bluetooth beacons only transmit for a few milliseconds (1 ms = 1/1000 sec) at a time so you can see how a coin battery can last a long time.
Here are the main battery sizes and their typical mAh rating:
CR2032 = 250 mAh
CR2450 = 500 mAh
CR2477 = 1000 mAh
2 x AA = 2200mAh (Alkaline), 3000 mAh (Li)
4 x AA = 4400mAh (Alkaline) or 6000 mAh (Li)
You can see that a beacon such as the SmartBeacon-AA containing 4 Lithium batteries can last 6x one with a CR2477 battery and 24x one with a CR2032 battery. This gives a battery life of up to 7 years depending on other configuration parameters.
Lithium AA batteries such as the Duracell Ultra Lithium and Energizer Ultimate don’t just last longer than Alkaline AA batteries. Their voltage also doesn’t vary so much with temperature which might be a consideration if your rollout is outdoors.