For the benefit of those of us who can't take apart a Nuvi 42 and look
inside...
Was it working then broke? Or did you buy it used broke?
How do you remove the battery? unsolder wires? unplug? how many
conductors?
What's the battery chemistry? How many cells? I'm gonna assume some
form of lithium,
but I have no idea how old a 42 is.
Where is the battery protection circuit? Integral with the battery? on the
GPS circuit board? Which side of the protection circuit are you measuring?
What's your definiition of good? Battery voltage is determined by
numbers. "Good" is not a number, and it can matter.
What's the voltage on the battery connection where you removed
the battery with the usb plugged in?
Fuses on the board?