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depends what was it there in the first place, look for voltage ratings on both units.
then your best bet is to contact these guys in wellington: Battery NZ

standard dry cell battery is 1.5V, alkalines 1.6V, nicad is 1.2V. although the enelop batteries are great you will be risking early shutdown of the gps due to low voltage even though the batteries...

some rechargeables are only 1.2 volts per cell. eg 4 of them =4.8volts not 6V
take this into account before you spend money on them