This is the way I modified the battery tray to allow rechargeable NiMh cells to be charged inside the newton.

I used an 10 Kohm radial thermistor. You just have to make a couple of very thin holes where the photo shows.

Upper view:
The termistor is bended up at an angle enough to be in contact with the batteries

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I cut two pieces of tin to make the added battery contacts.
These pieces are soldered to the thermistor.

Bottom view:

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The longer one is also bent to make contact with the negative pole of the last cell.

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A couple of superglue drops will maintain the contacts in their place.