> It's a very neat little
> toy, with the exception of the battery charging circuit - leaving it
> connected to a USB port for too long with the batteries connected (via a
> jumper) gets the batteries so hot you can hardly touch them.

If there is current limiting between battery and source then a simple 
voltage clamp can suffice to greatly reduce this effect. Adding a series R 
in the clamp makes it soemwhat current dependant. Adding temperature 
compensation didoes ... (aaaagh) ..

  R

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