Coin cell battery for Bluetooth?

Question

I'm trying to determine which battery to use for my application. I'll be using a class 1 or class 2 Bluetooth (not low energy) module, which requires about ~50/60mA of RX/TX current. In addition, there is a microcontroller and some low power analog IC's. I require a continuous stream of data from the device for 1-10 minutes.

Are coin cells completely out of the question? I'm talking about CR2450 Lithium-ions. I don't really mind if they don't last long, but will they work? Because in my application, size is key, and battery life is a trade off that I'm willing to make.

Answer 1

No, they won't work. CR cells are designed to deliver maximum a few mA continuously, so a low power microcontroller like an MSP430, for instance running off a 32.768 kHz crystal, may work, but the power for an RF transceiver is much too high.

The datasheet for this battery even specifies only 0.2 mA as "continuous standard load".

If you would need your RF in short bursts then you could get the power from a large electrolytic capacitor, for say 100 ms, and then recharge it over a longer time, so that the controller still gets enough current. For continuous streaming this won't do.

Answer 2

A CR2450 has quite a large capacity for a coin cell, but is not capable of providing enough continuous current for this application.

Here is a typical CR2450 datasheet

It says for continuous dischage, it is rated at less than 1mA, which is nowhere near what your BT module requires.
Even for a brief pulse, the rating is only 9mA for 2 seconds. This graph from the datasheet is quite informative:

You can see the IR (internal resistance) starts out at ~15 ohms and rises quite quickly. Given this info you can calculate the voltage drop at different currents. For instance if you want just 10mA, then you will get a voltage drop of 0.010 * 15 = 150mV. At 100mA this is 1.5V!
Steven mentions a capacitor and short bursts of RF. This is certainly possible (one of our products uses a coin cell and does a similar thing) but may not be suitable for you application.

So, you will have to select a different battery. Nowadays you can get very small Li-Ion batteries that would be good for this application. Have a look on Mouser, Farnell or maybe eBay - I recently bought one of these for a prototype:

For 500mAh (not sure if this is accurate, haven't tested it yet. I expect it to be between 300-450mAh) it's very small. The dimensions are given as:

Size: 58mm(L)*22mm(W)*4.4mm(H)

EDIT - what about an AAA with step-up?

This would work okay. It would drain quite quickly though as the battery would be providing more current due to the step-up and efficiency. For example if we have a 1.2V nominal NiHH AAA, and your BT module requires 3.3V at around 50mA:

50mA * 3.3V = 165mW.

If we assume an 80% efficient boost converter then:

(1 / 0.8) * (0.165W / 1.2V) = ~171mA required from the battery.

A typical AAA is about 850mAh, so you can run your module continuously for around 850 / 171 = 5 hours.

Answer 3

Zinc air is the best choice for small capacity high current batteries. The air forms half the electrolyte so they are high density. Try hearing aid batteries like a Duracell da675. Use 3 to get more voltage. Will still be tiny.

The other question is why use Bluetooth it is far too power hungry unless you are sending long long packets. Zigbee or a custom 433mhz solution would be better. Look at what TPMS guys use. 433 MHz with 80 bit Manchester coded frames.