Andres Rodriguez
New Member
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
No clash Phil, you made some additional valuable points.(This clashed with @inglewoodpete 's message - but, what the hell, I've typed it in now )
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.What can I expect to happen when the voltage of the LIPO drops below 3V
I have been mulling over some battery protection to add to a 14500 battery powered (recharged by a solar panel approx 50mm x 50mm) circuit that flashes red and blue LEDs to tell me the temperature at the bottom of my garden. My existing circuit is identical to that of the attached schematic but has no MOSFET between the solar panel and battery (only a diode). It has been running well since last December but the 14500 battery does not now hold a charge. I wonder if I have damaged it by exposing it to the approx 5.5V the solar panel can generate (less a diode drop).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:
I've implemented the 150 ohm shunt resistor solution for now with routines for high and low voltage protection. I have quite a few 18650 lithium ion batteries (I don't know the exact chemistry) available so will use those for now and monitor how the battery varies over the year (I have the device 'flashing' the battery voltage as well as temperature now). I see that LiFePO4 batteries seem to be capable of many more charge / discharge cycles so perhaps I'll try those in the future.For higher currents, a resistor of about 150 ohms could be connected between an available output pin and the supply, which can drain up to 25 mA, or 100 mW. For higher currents still, you don't need much more than a simple shunt NPN transistor (e.g. BC337) or FET (2N7000 or BS170). An advantage of the PICaxe is that you can set your own limits and I'd probably try a conservative range of 3.6 v - 4.0 v.
That's probably enough to have prematurely "killed" an unprotected Lithium Ion cell, although it's normally over-discharge that causes the problems. It might be worthwhile to set the "overvoltage" level at (say) 4.0 volts (at least in summer) since you might not be able to pull any excess voltage rise back until the following night. Even running all the PICaxe's internal hardware "flat out", I can't see how to make it drain as much as 10 mA.My small solar panel was generating 32mA at 16:50 BST yesterday in direct sun .... it should never generate more than 44mA ...