Rough battery life for continuous active accelerometer

sonalideo

Hi.

We are evaluating the metawear R with only measuring acceleration and battery level once every minute. The battery is then drained within a few days. Is this normal?

Does someone have rough figures on battery life and how to optimize it when continuously measuring acceleration only.

We are doing the following on the board.

1. Acceleration 10Hz measure with split in:

1a. stream mode but having triggers so only Bluetooth sending should be needed ~2 times a day.

1b. In log mode for the raw acceleration data and emptying the logs once every minute.

2. Measuring battery level once every minute.

What is optimal to extend battery life in these cases? Disconnect the board during wait times or does it go into some sleep mode by itself when not sending data?

With metawear C will the battery life be increased/decreased and roughly by how much?

Laura

Hey sonalideo,

If you constantly stream raw accelerometer data at a fast rate the battery can die in a few hours. At 10Hz, it will die after a few days (this is normal).

A few options:

1) Replace the 100mAh battery with a larger battery such as 200mAh.

2) Only use the logging feature (no streaming) to conserve battery. 

The biggest battery drain is always the Bluetooth connection in the MetaWear; if you log the data and download it once a day, you will greatly conserve the battery. 

At 10Hz you can log for a very long time before you fill up the 10K+ entries in the logger.

We are working on a new firmware/API feature that will allow you to auto-sync to a paired device. You will also be able to setup the device to stream when nearby (connected) and then log when disconnected. This is similar behavior to what you would see on a Fitbit style device. This will greatly change the battery life of the MetaWear. 

We will release the new firmware at the same time as the MetaWear C. The MetaWearR and the MetaWearC run the same firmware base and both benefit from the same power optimizations at the firmware level. However, the different batteries and their chemistry will affect the battery life.

To answer your remaining questions:

What is optimal to extend battery life in these cases? Log, don't stream. Disconnect the board during wait times or does it go into some sleep mode by itself when not sending data? You should disconnect the board, the BLE connection is the biggest battery drain. The device does go to sleep automatically when it is not doing anything.

With metawear C will the battery life be increased/decreased and roughly by how much? Assuming they are running the same firmware, the MetaWearC will have the longer battery life because it has lower power sensors on board and comes with a coin cell battery. The typical CR2032 lithium cell has on average 200mAH capacity which is twice as much as the battery that comes with the MetaWearR.

sonalideo

Hi.

Thanks for the reply.

"At 10Hz you can log for a very long time before you fill up the 10K+ entries in the logger."

Raw accelerometer data at 10 Hz will just be logged for 1000 seconds, i.e. 16 minutes or? For me long time is more like 24 hours.. Will the metawear C have extended memory on board?

Does the device go to sleep/low power bluetooth mode in case the board is in connected state but not currently sending any data?

Laura

sonalideo,

We do offer extra FLASH as a service if you are interested.

You can also use the filters and data processor to log optimally; logging the raw data is not optimal.

JpEncausse

How would you handle this use case for battery:

An elder have a necklace with a MetaWare configured for Free Fall. If he fall I want to trigger something on the mobile app to call for help.

- I can't stay connected to bluetooth otherwise I will kill the battery ?

- I can log free fall offline but it's useless if I want to do something fast

What suck the battery ? Streaming of data throught the BLE or just stay connected to BLE ? Is there a trick to ping the mobile app (I don't think) ?

Laura

BLE streaming/connections depletes the battery.

You could set up the necklace to wake up on motion events to conserve battery.

Freefall motion detected -> wake up -> stream BLE -> go back to sleep

sonalideo

What android SDK method should be called for wake up and sleep?

Laura

@sonalideo,

I don't think this feature is available in the latest Android SDK yet, I will add it as a feature request.

sonalideo

Any rough time estimate when this feature is available in the android SDK?

Eric

I think there is some confusion with the previous posts.  The accelerometer on the MetaWear R has a mode where it will enter a sleep state and wake up when there is an interrupt.  This mode does not exist on the RG and RPro accelerometers.

The use case outlined by @JpEncausse requires a BTLE connection to be maintained.  In the next firmware update, connection parameters will be exposed so users can configure a low power connection mode if they do not require high frequency data streaming. 

sonalideo

If I want to achieve the following behaviour "Freefall motion detected -> wake up -> stream BLE -> go back to sleep" how do I achieve it on Metawear C in Android SDK?

Eric

The accelerometer on C is the same as RG and RPro.  It does not have a "wake on event" mode.

sonalideo

i.e. Lauras post is not something that is possible to achieve currently?

Eric

i.e. MetaWear R uses a different accelerometer than RG, RPro, and C and the two accelerometers have different power features.