I found a pad on the 436 front circuit board that is connected to the USB jack 5V power. It's labeled V_USB, the rightmost of the 3 pads in a row near the center of the photo.
The 436's inability to charge the batteries while it is powered on is one of its greatest design shortcomings. (I know, it's because of a BS California environmental regulation.)
Anyway, I realized that a simple trickle charge capability could be added to the 436 by connecting a 20Ω resistor and a SD103C diode between the USB power pad and the positive battery terminal. Given a USB voltage of 5.1V and a battery voltage between 3.1V and 4.1V, that particular set of components will result in a charge current of about 80mA when the battery voltage is 3.1V, tapering off to about 30mA when the batteries reach 4.1V. The idea is to fully charge a set of dead batteries over 36 hours or so, and keep them topped off while the scanner is plugged into USB power without frying them, so that a unit normally plugged into USB power will always have a fully-charged set of batteries.
The 436's inability to charge the batteries while it is powered on is one of its greatest design shortcomings. (I know, it's because of a BS California environmental regulation.)
Anyway, I realized that a simple trickle charge capability could be added to the 436 by connecting a 20Ω resistor and a SD103C diode between the USB power pad and the positive battery terminal. Given a USB voltage of 5.1V and a battery voltage between 3.1V and 4.1V, that particular set of components will result in a charge current of about 80mA when the battery voltage is 3.1V, tapering off to about 30mA when the batteries reach 4.1V. The idea is to fully charge a set of dead batteries over 36 hours or so, and keep them topped off while the scanner is plugged into USB power without frying them, so that a unit normally plugged into USB power will always have a fully-charged set of batteries.