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Measuring/Calcuating Battery life of a wearable device

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safetylink

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We are a developing a wearable device with BLE technology (BT Smart). We are not able to accurately measure battery life our device. The battery dies faster (about 30 days) than expected/calculated time of over 300 days. We can improve the performance once we have better measurement accuracy. Following issues make this measurement and calculation complex:
1) Current is low 2-9 nA and
2) Current use is in spikes - spike width is in ms (since higher current is used each time a packet is sent wirelessly)

We are looking for a cost effective tool which is able to connect to our wearable device (between PCB and Battery) to measure current for a period of time and provide a profile of current used. This profile or average current can be then used to calculate the battery life.

Ideally, we are looking for a USB/Windows/Linux/RPI based tools which gives us flexibility in measuring.
 

MikeMl

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Most Helpful Member
Your battery is likely unable to deliver the peak current required at a the low duty cycle. Do you have an energy-storing capacitor connected across the battery. That way, the battery can pre-charge the capacitor during off times, and the capacitor can help hold up the voltage during the time the data is sent. You get a longer net utilization of the battery...
 

safetylink

New Member
.... Do you have an energy-storing capacitor connected across the battery. That way, the battery can pre-charge the capacitor during off times, and the capacitor can help hold up the voltage during the time the data is sent. You get a longer net utilization of the battery...
Is there a literature which we can refer to find things like - battery life extension because of the capacitor, calculating the capacitor value etc. Naively, assuming energy conservation, the battery life would remain unchanged. I would appreciate if you would shed light on how the battery life improves by having a parallel capacitor.
 

ronsimpson

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There are many things we don't know.
We should know the battery resistance. Knowing the peak current, during transmission, will help you know if the capacitor will help.
Some of the little watch batteries have 100s of ohms of resistance. During peak current there might be too much voltage drop due to resistance.
The resistance goes up with battery age.
 

bountyhunter

Well-Known Member
Is there a literature which we can refer to find things like - battery life extension because of the capacitor, calculating the capacitor value etc. Naively, assuming energy conservation, the battery life would remain unchanged. I would appreciate if you would shed light on how the battery life improves by having a parallel capacitor.
You need to be talking to the battery manufacturer. The critical thing here is the battery's internal impedance which causes the voltage to collapse when you try to draw a peak current. The battery maker should have data on this and as stated above, the internal impedance increases with age (it also varies with temperature). It's a critical spec for your app.

High peaks also increase internal power dissipation (because of IsquaredR power loss) which is why your battery has a much shorter life than you calculated it would. The mA-hr rating for a battery is typically onlt valid for a DC drain of about 0.1c. Bottom line: I agree you must have enough bypass capacitance to protect the battery from the peak currents.
 

bountyhunter

Well-Known Member
We are a developing a wearable device with BLE technology (BT Smart). We are not able to accurately measure battery life our device. The battery dies faster (about 30 days) than expected/calculated time of over 300 days. We can improve the performance once we have better measurement accuracy. Following issues make this measurement and calculation complex:
1) Current is low 2-9 nA and
2) Current use is in spikes - spike width is in ms (since higher current is used each time a packet is sent wirelessly)
But you must know exactly how much current and if possible, what the rise time is. You'll need to know that to define the cap.
 

spec

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Most Helpful Member
We are a developing a wearable device with BLE technology (BT Smart). We are not able to accurately measure battery life our device. The battery dies faster (about 30 days) than expected/calculated time of over 300 days. We can improve the performance once we have better measurement accuracy. Following issues make this measurement and calculation complex:
1) Current is low 2-9 nA and
2) Current use is in spikes - spike width is in ms (since higher current is used each time a packet is sent wirelessly)

We are looking for a cost effective tool which is able to connect to our wearable device (between PCB and Battery) to measure current for a period of time and provide a profile of current used. This profile or average current can be then used to calculate the battery life.

Ideally, we are looking for a USB/Windows/Linux/RPI based tools which gives us flexibility in measuring.
Hi safety,

Welcome to ETO,

To answer your question, I would suggest the best solution would be to use a data logging multimeter, set to current, to monitor the profile of the current consumed by your system over time. Here is a link that you might find informative. http://www.eevblog.com/forum/testgear/datalogging-multimeter-yes-or-no/

I know the principle of this aproach but have never done it myself but I would connect the output from a suitable multimeter to a personal computer (PC), ideally via a USB interface, and do all the processing in the PC.

Alternatively something like this may do your job: http://www.ebay.co.uk/itm/like/231664282385?limghlpsr=true&hlpv=2&ops=true&viphx=1&hlpht=true&lpid=122&chn=ps&googleloc=1006567&poi=&campaignid=207297426&device=c&adgroupid=13585920426&rlsatarget=aud-133395220626:pla-142405612386&adtype=pla&crdt=0&ff3=1&ff11=ICEP3.0.0-L&ff12=67&ff13=80&ff14=122&ff19=0&ul_noapp=true

Or this: http://www.ebay.co.uk/itm/VICI-Digi...7-/131138976865?&_trksid=p2056016.m2518.l4276

The majority of really low cost data loggers are for temperature but with a simple modification I would think these could be converted to read current, but I am thinking out loud here: http://www.ebay.co.uk/bhp/usb-data-logger

Also for the Arduino: http://www.ebay.co.uk/sch/i.html?_o...&_trksid=m570.l1313&_nkw=data+logger&_sacat=0

And another: http://www.ebay.co.uk/cln/igor_gele...2274099&collectionid=257733141014&pos=1&pid=1


spec
 
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