Li-Ion polymer battery used in the Nuvi GPS needs replacement every 3-4 years as its internal (battery) resistance rises due to aging/oxidation. Replacing the unit with the correct battery type is important unless you are competent to make an equivalent type replacement.
Here is my experience on changing the Nuvi 300 Li-Ion Polymer 1250mA battery. Hope you enjoy the work.
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HERE. - Updated 20100718
Flip the Nuvi 300 Antenna up to expose the back cover screws.
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Remove the two screws
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Lift the back cover slightly to access the locking pin
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Insert a sharp prick/screwdriver into the slot to slide the spring lock that holds the antenna assembly to the housing. This mechanism is similar to the 'Toilet Roll Holder'
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Slide the latch back and lift the antenna assembly up.
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The locking pin.
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Remove the locking pin and spring
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Slide and remove the back cover... the antenna cable is coiled inside the hinge.
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Slide the cover away from the coiled flex cable of the antenna.
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Filp the antenna back to its position and secure it with tape. You need not remove the antenna for a battery change.
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... cheers!
Straight forward job. Sure can DIY and if the unit is damaged along the way, that's a good reason to upgrade 
