Edge batteries are readily available online & not hard to fit. Bought one for my 705 in September, under a tenner including shipping from the US and all the necessary tools. I've not actually got round to fitting it yet but all it takes is undoing a few screws to get in the case, pulling a couple of connectors off, prising the old battery out and reversing the procedure to get the new one in. If there was an AA-powered Edge, I'd have probably gone for that, but it's by no means a hard problem to deal with. Out on longer rides, I have a couple of external AA packs that (in the case of the 3xAA one) can keep the 705 fully charged and running for 12+ hours (depending on temperature and usage).