LiPO cell an excellent power source for Wixel!

Just want to pass something along. I’ve an ongoing wixel project where only one side transmits, and the transmitter must be powered by battery. I’m transmitting a packet about ever 200mS and flashing a LED or two, and my normal average circuit draw is about 7mA.

Anyway, I want to mention that I’ve been having great success and some unexpected happy surprises powering the wixel with a 200mAh LiPO battery. The one I’m using I got from HobbyKing. (its much smaller than the photo)… … duct=23313

So fully charged, this battery will power my transmitter for a full 24 day. Now LiPO batteries don’t like to be drained too far, and you should NEVER let them get down below 1.8 V. But if you add code to put the wixel into sleep mode if unattended, and make sure to set up all the wixel pins to avoid driving anything before you go into sleep mode, it will draw less than 80uA and so protect the battery for days.

P0DIR = P1DIR = P2DIR = 0;
P0SEL = P1SEL = P2SEL = 0;
SLEEP = (SLEEP & ~3) | 2; // SLEEP.MODE = 2 : Selects Power Mode 2 (PM2).
__asm nop __endasm;
__asm nop __endasm;
__asm nop __endasm;
if (SLEEP & 3)
PCON |= 1; // PCON.IDLE = 1 : Actually go to sleep.

But it gets better! Once the battery gets down around 2 V, and you’re in sleep mode, the current drops even further, to the point where I don’t think it would ever damage the battery.

But who wants to mess with a special LIPO charger right? Well you won’t have to! The VALT pin of the wixel puts out 4.5 V when the USB is plugged in. If you add another diode and a 10 ohm resistor, you can run your battery through an SPDT switch so that one switch direction powers the wixel, and the other switch direction connects to VALT through this added diode and resistor. The diode blocks the battery from ever reverse powering the wixel, and along with the resistor this makes a nice current limited 4 volt source, which is a perfect charger for this little 200mAh LiPO cell!!! When the battery is pretty well drained and you plug in to a USB port a and make this connection, the circuit will initially supply a little more than 100mA (just a 1/2C charge rate) which will quickly begin to taper off. Since the voltage beyond the added diode and resistor will not exceed 4V, the current effectively drops to nothing as the battery reaches full charge. 4.2V is the safe limit for most LiPOs anyway, so there’s a safe margin and you won’t be able to leave it charging for too long.

Granted you shouldn’t leave it charging forever, and the above is not a FAST way to charge. In my case I’ve found it will take about 6 hours to approach a full charge. But it is a safe and cheap way to charge the LiPO cell, and a slow charge will actually allow it to have many more charge cycles than charging at a higher rate.

Anyway, I Hope this is useful for someone else. As ready made prototype boards go, the wixel is about as perfect a match for a single cell LiPO supply as I’ve ever seen!


Thanks for sharing your advice and experiences!


One update though, D2 in in your “Wixel power system schematic diagram” must be a germanium type?

I ask because I measure its typical voltage drop at about .3 v, not the usual .5- .6 I’d expect from silicon. So sure enough, if I leave the charge circuit I described above connected for a long time, say 20 hours or more, it will charge the Li-PO to a hair over 4.2 volts. This is actually even better as far as getting an absolute max charge out of the battery, because 4.2v is really the most ideal target charge voltage. However, since it does eventually go a “hair” over 4.2, I probably would not want to leave it charging this way indefinitely, or just forget that its plugged in.

The Wixel’s D2 is a Schottky diode and I think the voltage drop you are seeing is not that atypical for that kind of diode.