IO450: Yes, a qualified BGA & EASA approved inspector looks at the glider every year and should spot non-EASA installations / modifications. Since we have only just come under that umbrella I don't know what would happen if they spotted a non-compliant installation / modification. Yes, we take everything out we can first - parachutes, pee bags, charts, camelbacks, food crumbs, flight loggers, Flarm, O2 bottles - and as can easily be removed is not an issue for EASA, PCAS could probably go in just about any glider, ModeS not.
You don't need Mode S - not relevant to any gliding context. Mode C is all you need and they are cheap enough on US Ebay. DIY installation is trivial, and so it removal when needed.
If I was gliding, that's what I would do. It makes me visible to many more planes, directly and via ATC radar services.
Modern lithium batteries - URL please, I'm not sure what they are, what sizes they come in and so on. But remember the only kind of transponder that EASA would do a scheme for now would be Mode-S which uses a lot more power than Mode-C.
True but I am talking about an unofficial install.
Here is one of many shops doing the batteries. You need a 3S or 4S variant.
Someone at my club said that in 10 years or so ADS-B will be side-stepping the issue. Can't comment except to say that he is very well connected in both the gliding world and the CAT world and is nobodies fool.
In 10 years' time, possibly. But IMHO there will never be a "removable" ADS-B product, due to certification issues that get stuck to anything that transmits on these frequency bands. And it is highly likely that ADS-B will be implemented using Mode S transponders, using the 1090ES feature (a kind of data back-channel).