The day before yesterday I played a gig with my band Frozen Realm.

Unfortunately the battery in the guitar was dead so I had to change it before the show. The changing of batteries went smooth. However, when we started the soundcheck problems occurred.
For some reason the guitar sounded "less gainy". The output of the guitar sounded like it was in need of a new battery. The battery didn't sound completely dead though. But the output definately didn't sound the way it's supposed to do.

Any suggestions of what might be the problem? Did I manage to break a cable in the guitar or something?
There's nothing you can break in an active system which will lower the output. Active systems are pretty all-or-nothing, it'll either work perfectly or it won't produce any output at all.

The exceptions are a half-used or flat battery, or not conecting the battery cleanly, resulting in the loss of some power (essentially the same as using a slightly flat battery).

Try another battery and see what happens. Really you should only ever swap in batteries which you know are new and haven't been sat around for ages. Remember that batteries do lose some charge over time, even if they are not being used.
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Well, very weird in that case.

The first battery I changed to was straight from the package as well as the other battery which I changed to now. Still doesn't work properly with a battery guaranteed 100% fresh.

Thanks anyway!
Haha, this just made it even more weird!

I just plugged in the guitar again and it sounded the way it was supposed to sound!

However, suddenly the gain level slowly started to lower itself finally reaching the level I spoke of above. After a couple of minutes it was back at sounding great again!

(This cannot be any pedal since I only used guitar to amp for this. It cannot be the amp either since I've tried it on two amps and got the same problem twice)
Huh. Yes, that is bizarre. Sounds like there is enough of a power leak to interrupt the signal, but not enough to kill it completely. I've never heard of this happening before, and I must have installed active pickups in a hundred guitars by now.

My only suggestion, without being able to see the guitar, is to open it up and look for anything which looks like it could be causing any kind of power leak at all. This could be a cold solder join, the jack could be a tiny bit loose, a few wires touching things they shouldn't, etc. Basically, double-check that everything looks like it is how it should be. Once you've ruled out obvious wire clashes/loose jacks/loose switch/bad solder/etc, you can then start testing each part one at a time; essentially, you just take a part out of the circuit (e.g. disconnect a pot, use a different cable, whatever) one-by-one until the guitars sounds normal. Then you know that whatever it was you just removed was the cause of the interrupted signal.

I'm afraid, like I said, active pickups are pretty all-or-nothing usually, so there's not an easy way to tell what is going wrong with a set without just going through each element of the guitar, testing everything fully.
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