Using a single cell LIPO to power an 8M2 or 14M2

I am planning to use an 8M2 or a 14M2 on a project to drive 8 UV LEDs. Considering using a 3.7V LIPO. What can I expect to happen when the voltage of the LIPO drops below 3V
 

PhilHornby

Senior Member
The Picaxe will keep running - potentially down to as low as 1.8V (I believe), depending on what internal hardware is configured. I'd assume you'd need to monitor the supply voltage and 'sleep' when you get down to some minimum safe voltage, to avoid damaging the LIPO.

It would probably stop being able to drive the LEDs at some point before this - whether this 'point' would be ok for the LIPO, I don't know.

(I'm not a LIPO expert ;) )

(This clashed with @inglewoodpete 's message - but, what the hell, I've typed it in now :) )
 

AllyCat

Senior Member
Hi,
What can I expect to happen when the voltage of the LIPO drops below 3V
As already said, the main "concerns" should be with the LiPO and the LEDs, not the PICaxe. As far as I can see, UV LEDs require a forward voltage of more than 3v (as do white and blue LEDs); also, LiPO cells should not be discharged below about 3 volts (and might/should contain a "protection" chip to switch off any output drain below this voltage). But an 08M2 will keep "working" down to about 2.2 volts and the 14M2 to about 1.8 volts. Note that the PICaxe can use its CALIBADC instruction to measure the (battery) supply voltage without any additional components or pin connections.

I don't know how you plan to drive 8 LEDs from a (4 output pins) 08M2 but don't expect the PICaxe to deliver anything near to 20 mA through each pin at 3 volts. The graphs (e.g. FIGURES 31-43/44) towards the end of Section 31 of the relevant base Microchip Data Sheet shows that at 3 volts supply, you can expect about 4 mA pull-up or 10 mA pull-down current for each pin, with a 0.5 volt drop.

Cheers, Alan.
 

geezer88

Senior Member
If you look on the popular online seller sites you can find tiny boards about the size of a postage stamp that provide a micro usb charging port and a cutoff to protect the lion cell. I've used these on several projects, and they are usually priced around $2.
tom
 

premelec

Senior Member
Look in Ebay for "USB Power Bank Case 18650 Battery Charger DIY Kit" - there are many varieties... usually holding one or 2 18650 cells; the key is that they are Do It Yourself "battery bank" units... have fun... If you want just the protection circuits rather than up converter part they are under cell protector..."BMS Protection Board " for one to many cells in series... [Battery Management System = BMS]
 
Last edited:

AllyCat

Senior Member
Hi,

For low currents you can do much the same with a few lines of code in the PICaxe, or try the following descriptions for your "local" website:



Overcharge detection voltage: 4.28 ± 0.05V
Overcharge release voltage: 4.26 ± 0.05V
Over discharge detection voltage: 2.75 ± 0.1V
Overcurrent detection current: 1-3A
The main function: Overcharge protection, over-discharge protection, short circuit protection, overcurrent protection.


Cheers, Alan.
 
Top