So work is continuing on the glow fans. I find myself wanting to run the next version off lithium batteries, but I’m a bit confused. Ok, overcharging them is bad, so I’ve a design for a proper charging circuit, based around the Linear LTC1734. And that’s all well and good.
Discharging them is another matter, with dire warnings for letting the battery voltage get too low. However, here is where I get confussed. Surely whatever circuitry that is responsible for monitoring the shutdown is going to draw a load? Is it the case that so long as the load is low, then the battery won’t be damaged? How low?
Current plan for the low battery behaviour is to have the Picaxe wake up once every umm.. few seconds, check that charging hasn’t started, make sure that everything else is off, and then snooze again for a while. The Picaxe gets its 5V from a charge pump converter (a MAX1595) with a 0.2 mA draw when the Picaxe isn’t drawing power.
(I think this is the main power draw when off. There’s other gubbins – the other step-up converter to power the LEDs draws only 5 uA when shutdown, but needs a pull-down resistor to keep it shutdown, which will have it’s own current needs. But I think all that should be much smaller than the 200 uA from the charge pump.)
Is that all I need to do? Just have the Picaxe mostly off and a 0.2 mA draw running on the battery? Or are there better ways to do this? And yes, some lithium batteries come with under-voltage protection built in. If under-voltage is so bad, why don’t they all? And why can’t I find an undervoltage protection chip for just one lithium cell, instead of the 2-10 that TI offer.
Brain all hurty now.