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my 1 year old 305 seems to be losing batt life, its hard to judge but its lasting approx 4/5 hours, any suggestions
Run faster then you get home in time!
If it is a real problem, return it to Garmin, and they will send you a refrubished model (to all intents and purposes brand new) - costs about £50.
Other option is a guy in the states who advertises on ebay - cheaper at about £30, and a better battery, but don't get the guarantees and there is a chance he'll break the beeper - quite upfront about it on his page.
Thats what I was getting with mine after around 2 years. Be honest and sell it on Ebay then buy a 310xt with 20 hours battery life. Sorted! You will get around £100 for your 305.