Battery dies too quickly

Hello there! I am using arduino to measure bike's speed. Idea is simple: wheel spins, magnet is flying over Hall sensor, Arduino is constantly checking for Hall sensor response (digitalRead, pullup), then it counts time between rotations and sends bike's speed to 1.3" oled.
It is powered by pack of 3 3V coin batteries (CR2032) in series through Vin pin and GND pin.
When I measure battery pack with voltmeter alone, it shows ~9V, when I plug in batteries to circuit and measure voltage across battery, it shows ~5.5V and the gradually drops to 3.5V where Arduino dies.
According to Mr.Google CR2032 has capacity ~210mAh. Ampermeter shows that circuit is using ~33.5 mA, so theoretically it has to work for several hours. But for some unknown to me reason it dies after several minutes.
Can someone, please, explain what is going on?
Here is my connection diagram, if needed.

It sounds like one or both of the board voltage regulators in the Nano is overheating and shutting down.

That setup is a very poorly conceived mixture of 5V and 3.3V parts, with two very inefficient linear voltage regulators, wasting over half the battery power as heat (until they overheat).

Buy an efficient switching voltage regulator, like this one, use a 3.3V Arduino, and power the entire setup with 3.3V.

Hi, if it is due to voltage regulators, why then this circuit works fine when powered by usb? USB hub and Vin is the same when it comes to powering, isnt it?

No.

Why there is Vin pin then, if it cant handle voltage in allowed range?
And can you, please, explain, what exactly is poorly conceived?

The data supplied indicates that the 'battery pack' has a series resistance of > 100 ohms. Those batteries are designed to supply a small current (~1 mA) for a long time. I think you would stop working soon if someone made you work >30 times harder than you should!

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How much current does the Arduino need? The OLED? The Hall effect?
The 5V regulator drops 9V to 5V, a 4V drop times total current = Watts dissipated (wasted) as heat.

Mixing 5V and 3.3V parts, with energy-wasting linear voltage regulators to provide the operating voltages, from tiny batteries.

Why there is Vin pin then, if it cant handle voltage in allowed range?

For powering JUST the Nano, from a voltage source of up to about 12V. Nothing else, except maybe an LED or two. Certainly not a display.

The solution is to buy a 3.3V Arduino Pro Mini, a 3.3V switching regulator, and get on with your project.

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It can handle the voltage but can't handle the amount of current you want to draw from it.

When you referrer to the "allowed range", I am assuming you mean current. The allowed current from the internal regulators is only what you need to supply the Arduino with a few LEDs. It is not designed to supply current to any amount of extra stuff you want to add to it.

I can't believe that your screen only takes 9mA, what sort is it?

It looks like a small OLED. They only use about 20mA even if every pixel is lit, so 9mA to display some text on a black background sounds normal.

@smilife_cactus I agree with @jremington suggestions, except maybe the switching regulator. I would at least try with an LDO linear regulator first. Pro Mini may already have a good one, I can't remember, but if not, mcp1700 or similar. Put the arduino into a sleep mode for as much of the time as possible, waking when it receives the signal from the hall sensor. But don't use a sleep mode where the main crystal clock is powered down. You need it's accurate timing to calculate bike speed accurately.

I would also recommend forgetting about the button cells. Get a small 3.7V li-po pack. They come in all shapes, sizes and capacities and will perform far better than your button cells.

Not a valid assumption.

210mAhr is the capacity at a very low load current, typically 100uA to 200uA.

Your drawing many many times the normal design current for these batteries, expect the battery life to be very short indeed.

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