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Yet something else to think about when selecting an amp. Thanks to the OP for bringing this up!
Man I was almost set on buying a RSA P-51 Mustang but this issue concerns me as well. I want to buy an amp that later on could be considered vintage or classic etc. The other amp I was considering due to this issue was the RSA SR71A but it's definitely a larger setup considering I would want to use it in a portable setting once in a while. In the end I think it would be great to have this amp for 10+ years and still be enjoying a well crafted piece of engineering.
The Shadow and perhaps other RSA amps has a 3 year warranty that includes the battery. That gives me some confidence. I suspect RSA and others will have a solution for this in the future.
Of course, let's just hope that the next gen of amps has a manual hand crank so we don't have to worry about all these battery questions!
a coupl;e portables that have an easily replaceable LIon battery are the offerings form Leckerton Audio. It comes out just like a AA battery and cells can be purchased for relatively cheap on e-bay