I found out some cool things today, while I had the voltmeter hooked up to the battery in one of the key fobs (that came from a diamond key). EDIT - I turned on the voltmeter, measuring battery voltage on the fob, without doing anything at all, no buttons pressed, etc., so the battery was at a very steady state. It was holding steady to the mV. So I could then just move it to different locations, such as on the charging pad, etc. and watch the mV click up for example at about 1 mV per minute while on the charging pad.

I confirmed 100% that my Braun / Oral-B electric toothbrush charger does nothing to charge the battery.

My son's iPhone charging pad thing DOES work to charge the key. Will keep that in mind for the future.

When holding the key right where it would be if it were still part of a diamond key in the ignition, it charged pretty well, a little faster than the charging pad. Then I tested by holding it a couple of inches lower, exactly where it would be if it were on a key ring connected to a valet key, and there was hardly any charging. Surprising to me. So the charging region is very localized, aligned with the key cylinder.

I think I found that the one fob I tested extensively was a little flaky still, even with 3+V applied from the 2x AA batteries. And it seemed that when I'd pull up on the right corner of the circuit board while pressing on the button it would work consistently. Then I reflowed the solder joints on the buttons and coil, which seemed to make it work consistently all the time.

I keep reminding myself that I bought these keys off eBay for $15 each, but it's a fun project to figure things out. And the knowledge on keeping them charged going forward will help, as will the new batteries when they arrive.

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2001 X5 3.0i, 203k miles, AT, owned since 2014