Power. Faced with the design challenges of keeping things that don’t sleep awake as long as possible. One of the more intriguing aspects of the Flavonoid project has been to find ways that it can stay active and recording without it requiring a large, heavy battery. This battery here — the “Powerizer” — is about as big as I want to get. I also have some “flat-pack” style LiPo cells that might work well. The Powerizer is an industrial 3.6v 650mAh cell. Here Flavonoid is drawing about 18mA while recording data — the A2D converter is on and pretty actively sampling, the I2C/TWI bus is running frequently reading the time from the real-time clock. And every 15 seconds the LED blinks for a moment just so I know its alive. It’s also running a command line console interface over USB, so the FT232RL RS-232<->USB chip is spun up.
One problem I faced — still facing — is how to recharge the battery, which is basically “fixed” (non-removable, or only removable by removing soldered connections.) The design presently uses the Maxim MAX1555, which a chip that will charge a single LiPo cell over USB or from a +5 volt, higher current source, about 200mA, whereas USB will max out at 100mA. My laptop’s USB port won’t charge at more than 80mA at best. The idea here is that USB charging would be more convenient than having another wall plug and wire running around. Since the Flavonoid uses USB for configuration and downloading data, you’d have it hooked up over USB anyway. I assumed USB would be fine, and generally it is. But the times that I’ve tested the Flavonoid’s ability to run for a couple days, the battery went way low — 1.8v — and I had to recharge it from the higher current +5v source. Getting the Flavonoid to charge in the flat-battery circumstance is tricky. Presumably, since there’s no on-off switch, the Flavonoid is trying to start up, which draws a modicum of power, making it more difficult for the battery to just chill and charge.
What’s the alternative? Well, the immediate fix is to keep the Flavonoid plugged in at night so it maintains its charge. And I’m looking at adding a battery monitor (Maxim MAX6439) that will be able to let the Flavonoid microcontroller know when the battery voltage drops to a certain level. There’s built-in hysterisis so the battery needs to go above a specified voltage before the MAX6439 will let it know that its back in some kind of nominally revived state. The thinking here is that the battery monitor could trigger a pin-change interrupt that would put the Flavonoid to sleep so it draws barely any power at all. When the battery is back to normal, it’ll come back to and carry on as usual.