NiMH rechargeables are 1.2 volts, while disposables are 1.5 volts*. Any rechargeable AA batteries will be the same. The light is a dumb thing which doesn't care, but obviously the satnav does. I trust Kim on things like this, so I'd take her advice on changing the settings on the Etrex.
*In practice, both will start out a little higher & gradually drop off, but by the time a disposable drops to the voltage of a rechargeable it will be severely depleted, & the amperage will also be significantly reduced.