Arduino Project, LiPo powered

Hello there,

I'm creating a little arduino pro-mini project that will fit into a small box, be hand-held and run off a 1s 3.7v LiPo battery cell.

If I run the 1s LiPo through a 3.3v regulator to power the arduino, then measure the battery voltage using a voltage divider, how can I get the project to turn itself off using a software command to a component that will effectively 'cut' the power until it is turned on again. Something akin to a relay but the lower the power consumption the better.

Also another Q, I'm planning on making this device USB chargeable with a micro USB plug on the side. If I used a voltage divider or a 4.2v regulator on the 5v pin on the USB plug, is the most simple method of (safely) charging the battery to join that voltage in parallel with the cell (with a current limiting resistor of course)? I would assume that as the cell charges, the voltage will increase to its max of 4.2v then once there no more current will flow. Is that a feasible approach or would extra circuitry be required?

Many thanks.

No, you cannot charge a Li-Po battery like that, it would be dangerous. Modules to safely charge and prevent over-discharge are available and cheap on eBay etc. like this one.

The simplest way to maximize your battery life is to have the Pro Mini go into a deep sleep mode. You can also remove the power led. More battery life can be gained by using an external LDO regulator, bypassing the on-board regulator.

What would the danger be? If the current was limited to X amount, once the battery is fully charged both the battery voltage and voltage being applied to it from the USB would be both 4.2v, how is that any different from two battery cells being in parallel with each other. No PD so no current flow?

I may be missing something so apologies if I am!

Also should have said I’m trying to make this as light/cheap as possible so any extra modules I’d be slightly reluctant to add.

I am no expert in battery changing techniques, but I know that crude charging circuits can damage the battery, causing it to overheat and potentially explode. The modules I suggested do it all for you safely and conveniently.

Checking the Pro Mini schematic, the built-in regulator appears to be a mic5205, which is an LDO regulator, so not worth bypassing with an external regulator. But because the ATmega is ok up to 5V, you can simply connect the battery to the VCC pin and disconnect the on-board regulator and power led by disconnecting a jumper. You should be able to get the Pro Mini down to a few microamps in sleep mode.

jaddion82052:
If I run the 1s LiPo through a 3.3v regulator to power the arduino,

It would be a lot more efficient to run the Arduino directly from the LiPo. An Atmega 328 is perfectly happy up to 5v.

You can measure the supply battery voltage by comparing it to the internal 1.1v ADC reference voltage.

Also another Q, I'm planning on making this device USB chargeable with a micro USB plug on the side. If I used a voltage divider or a 4.2v regulator on the 5v pin on the USB plug,

The problem is not what happens when the battery voltage rises to 4.2v (though I think I would use a lower threshold myself), the problem is that the 4.2v supply may provide too much charge current when the battery is at its lowest voltage. The Microchip MCP73831/2 is a small and cheap LiPo charge controller and it does the job properly.

...R

If you’re running outdoors unattended you may want to consider thermal protection too. Most LiPo chargers include that functionality with an additional thermsistor.

jaddion82052:
If I used a voltage divider or a 4.2v regulator on the 5v pin on the USB plug, is the most simple method of (safely) charging the battery to join that voltage in parallel with the cell (with a current limiting resistor of course)? I would assume that as the cell charges, the voltage will increase to its max of 4.2v then once there no more current will flow. Is that a feasible approach or would extra circuitry be required?

You could use a regualtor that output not more than 4.2V to charge a lipo, assuming it also had a suitable current limiting mechanism as well.

A ‘voltage divider’ (I assume you mean a resistor divider network) to charge a LiPo would be dangerous, do not do it.