Sunday, October 11, 2020

Airball probe v8 battery charging

To match the battery drain test, I timed a battery charge cycle starting from the completely discharged state. Here is what we have:

You can see that my charger is charging at 270 mA, no more no less. It takes about 12 hours to get to a full charge. Can we do better? Let's think this through --

  1. I am configuring the charger chip (a TI BQ24075) to draw at most 500 mA from the USB input. This is so that I am safely a "USB 500" device and am compatible with most computer USB ports and charger bricks.
  2. I am also configuring the chip to charge the battery at whatever current it can get away with, basically, given whatever the input provides.
  3. I am using a "power" switch connected to the SYSOFF signal on the BQ24075 to control whether the entire system is on or off. When SYSOFF indicates the system should be "off", but the USB input is connected, the BQ24075 powers up the whole system anyway. This means that, any time it is charging, regardless of the value of SYSOFF, the circuitry is operating and drawing the usual 180 mA.
  4. For 500 mA of input, we expect the maximum battery current to be (500 - 180) = 320 mA.
  5. We are getting 270 mA, which is (270 / 320) = 84% of the theoretical maximum.

Based on all this, it seems like the circuit is operating the way I asked it to, and indeed, because of the enormous size of my battery (3700 mAh), and the degree to which I am being "conservative" about the charging, I should not be surprised that it's taking 12 hours to charge.

So how would we "improve" this, today or in the future?

The reason I am "conservative" with the charging is that, as a hobby device, this thing will be connected to people's computers. If it draws too much power, the computer (or cheapy USB hub or whatever) will bump it off USB. This kills the ability to program the thing from the same USB port I'm using for charging, and also makes it hard for people to charge the thing simply by plugging it into their PC because they don't happen to have a power brick laying around at any given moment.

I could create two USB ports -- one for programming and one for charging. The programming port would draw zero current, and the charging port would not do any comms. But then I'd have to explain all this, and mess around.

Another way to think about it is this. I run at -180 mA and charge at +270 mA. Since (180 / 270) = 0.67, what this means is, for 1 hour of operation, you need 40 minutes of charging. Is that great? Is it a "fast charge" monster? Is it a consumer device for the ages? No. But ... does it work? Yes.

My inclination is to leave it as-is and call it a measured success.

No comments :

Post a Comment