Wednesday, October 14, 2020

More about Airball probe v8 charging

Fancy aerospace R&D requires fancy instrumentation. No expenses spared. So you will be delighted to know that the Airball project has acquired a top-of-the-line Eversame USB Digital Power Meter (Amazon's Choice) instrument. Nothing but the best.

With the help of this device, and accounting for milliamps here and there, it seems that our charging circuit is "losing" maybe 20 mA somewhere, but otherwise, the current that gets soaked up by the running system plus the current dumped in to charge the battery more or less equals the 470 mA that we are drawing from USB, which makes sense given that we are a "USB500" device that promises only to draw 500 mA. So all is good on that front.

The next step is -- when I know that I am charging, I can put the ESP32 into a sleep mode to minimize the amount of current I'm drawing for stuff like Wi-Fi transmission. This should hopefully let me charge at a somewhat faster rate. It is probably possible to keep my TCP connections going and transmit battery status only, once a second or something, while in charge mode, but that's fancy. For now, I know I can accomplish what I want to accomplish.

But. There is always a "but", isn't there?

Remember how the circuit has this nice blue LED blinkenlight that blinkens when it's charging? In case you forgot, it's this one:

Ya well, so when the power switch is OFF, the TI BQ24075 chip does NOT charge the battery (weird, but mmkay, whatevs), however it also keeps the "charging" LED ON!

In other words, you plug your probe into USB with the power switch OFF; it tells you it is charging; you go to sleep and come back the next morning; and it has lied to you! It has not been charging the battery.

This is bad. Wrong feedback is strictly worse than no feedback. :P

So a very kind EE friend looked over the data sheet and my design, and confirmed that yes, the data sheet is SUPER misleading. According to their design, this behavior is "allowed". They even give you a sample design with a little "charging" LED just the way I've done it, and it's useless.

So basically I'm going to blow away the "charging" LED and instead use the signaling LED that I have access to from software. I will detect whether or not I'm charging, and based on that, blink the LED appropriately. And also do the sleep thing to save current.

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