Author Topic: Bluetooth transmitter not getting enough current from CR2032 coin cell?  (Read 825 times)

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Offline flubba86

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I got one of these jobbies from Ebay last week:
It is a combination pedal cadence and wheel speed sensor for my bike and it communicates via Bluetooth 4.0 to my smartphone. I paid less than $20 including shipping from an Australian seller, and they can be had cheaper from Chinese sellers.

When it arrived, I installed a CR2032 battery into the battery compartment, mounted it on my bike, and tried to connect it with my phone as per the instructions. It simply did't work. It was like it was dead, my phone detected no bluetooth device at all.
I took the battery out and tested it with my meter. It showed 3.3v, so it was a nearly full battery (but had been used in a LED light before). I put the battery back in again, and tried to connect as per the instructions again, still no go.

The next day, I was playing with it again, and I decided to try a different CR2032, this one read 3.3v on my meter, and it came straight from a working LED light so I knew it was good. While swapping the batteries over, on a whim I tested the first battery again. It was at 1.8v. It had completely drained in the device over night for some reason, so the unit wasn't completely dead, but was doing something.

With the new battery in there, I tried to connect again, but I got the same problem. It was still not showing any bluetooth device. I decided it was teardown and repair time. I thought there might be a defect on the board causing a short or something, plus I wanted to measure what voltage the battery was giving in-circuit.

After I cracked it open (kinda destroyed the case, but I can probably glue it back together), I installed the good battery, and measured the voltage in circuit across the input capacitor. It was reading 2.7v. So it was going from 3.3v out of cirtuit to 2.7v  as soon as I put it in the circuit. I deduced this thing must be pulling serious current to have that effect on the battery, and the cheap CR2032 I was using was simply not able to give it enough juice.

I went searching through my drawers and found a new-ish CR2032 unopened. I tore open the packet and it measured 3.2v on my meter. I dropped this one in the unit and it showed 2.9v in circuit. I figured thats still a big drop for a new battery, but better than the other one. I checked my phone and BOOM, I could see the "Combo Bike Meter" bluetooth device appeared. I tried to pair it, but everytime I tried to pair the device the pairing procedure would time out and the "Combo Bike Meter" name would drop off the list. Taking the battery out and putting it back in again would make it appear again, but I still couldn't pair it.

I measured the voltage across the capacitor while I initiated the pairing procedure. The voltage goes from 2.9v down to 2.7v as soon as I press the pair button on my phone, then after a few seconds the voltage goes up to 3.0v and sits there doing nothing. I'm guessing when it drops down to 2.7v the processor is resetting and interrupting the pairing procedure or something like that.

So here is my question... is it unreasonable to expect a single CR2032 to power a device with a processor and a bluetooth transmitter? Im guessing its not using BLE technologies. I can't be the only one this is happening to, if this is a fundamental design flaw. I thought about trying to run it on two CR2032 batteries at once, but they only way I can think of doing that would be to stack them, but that would put them in series rather than parrallel, and would give 6v to the circuit.
I could bodge in a second CR2032 holder on the outside of the case and wire it across the input cap, to give a second battery in parallel, but I don't know if it is worth the trouble. The way I see it, if it is drawing too much current for one battery to deliver on its own, then running it on two batteries is probably going to see both batteries lasting only about 24 hours in the device before depleting, is that right? Should I just simply find a really good brand brand-new CR2032 and try that first?
« Last Edit: June 09, 2016, 09:37:22 pm by flubba86 »

Offline SL4P

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Step 1.  Stop testing with batteries of dubious heritage.
You've admitted that all the cells you tried are 'almost new', or from a LED light.

The device may not be perfect, but give it a chance.
You may discover it works a while - and you really need 2xAA batteries to get decent life, but starting from dead batteries is not a good reference.
Don't ask a question if you aren't willing to listen to the answer.

Offline System Error Message

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It takes more accuracy to actually measure a lithium/NIMH battery capacity because they have a much more constant discharge profile compared to alkaline batteries.

The other thing you should do, set your meter to continuity mode and test various points on the device. Is the bluetooth emitter connected? Is there a fault or a short? For small batteries if you connect positive to negative on the same battery directly they dont instantly drain, they take a few hours to drain.

You can test this if properly, use a bench PSU set to 3.3V and see if it works. Is it the amps that it needs? Is there a short?

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