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I've been investigating the backlight flickering that I showed in my previous log entry. My main hypothesis was that the long wires between my breakout board and my main PCB were adding too much inductance, causing increased ripple. I had moved an input decoupling capacitor to the breakout board, which helped some but didn't entirely solve the problem. Adding more capacitance on the input side didn't help much more.
One thing I noticed is that the problem drops entirely when the tablet is connected to the charger. This made me suspect that maybe my battery is the problem.
Looks like there isn't much difference between the VBAT measured at the battery terminals and VBAT measured on the breakout board (at least at speeds the Saleae can measure; it's possible that the breakout board sees switching transients.)
But yikes, the battery is dipping ~400mV under load? That seems like a lot. Assuming that's happening while the board is drawing 2 to 4 amps, that's 100 to 200 milliohms of internal resistance on the battery. I dunno what the internal resistance of a LiPo battery is supposed to be, but this feels high. Perhaps my test battery is aged - internal resistance goes up with use + abuse. (I'm using a separate battery for testing; just a random 2000mAh cell I had lying around, because the iPad battery is glued to the back of the case and doesn't have wire leads, just spring terminals.)
On the topic of battery health...
I've heard one way to keep lithium ion/polymer batteries healthy is to limit the maximum charge voltage to something lower than 4.2V, like 4.0-4.1V. The battery charge IC I'm using (TI's bq25895) supports this functionality, so I've added a device tree overlay which sets this parameter. This also lets us read the battery voltage and charging current (but not discharge current?) easily in Linux:
meatmanek@cm4:/sys/class/power_supply/bq25890-charger$ cat status Charging meatmanek@cm4:/sys/class/power_supply/bq25890-charger$ cat voltage_now 3924000 meatmanek@cm4:/sys/class/power_supply/bq25890-charger$ cat current_now -400000 meatmanek@cm4:/sys/class/power_supply/bq25890-charger$ cat constant_charge_voltage_max 4096000
Hopefully this helps keep the 12-year-old iPad battery in good working condition.