I'm assuming you're talking about the AA or whatever batteries you put in it? I don't know that those will last all summer unless you rarely use the GPS. Are they going to be rechargeable?
Like most computers, the physical memory in the GPS unit probably has some kind of internal battery that will keep enough power to the memory chips to keep the information stored on them for years.
That is how settings aren't lost in computer chips in most cases when they aren't plugged in or running. I wouldn't worry unless your unit's documentation warns against this. You did read the documentation, right?
What brand and model ? These little hand helds eat batteries like candy I've got an e-trex legend and I use 2 aa's about every 12 hrs I have it on. Depending on how old it is I'd say dont worry about it.