Few days ago driving my Volvo 98 V40 T4 and I noticed the A/c was on
permanently whether the switch was on or off and car felt very sluggish.
When I got home, opened the fuse box/which also holds relays and trying
to check the compressor relay, noticed it clicked constantly when
removing it. Figured it was faulty but had other uregent things to do so
left it to look at later. Yesterday, I go to start the car and battery
is as dead as dead - no lights, nothing. Called local road service who
started it and when I pointed out the A/c was on permanently suggested I
get it checked. I removed relay and swapped with headlight relay which
is the same and lights went on with switch off so the relay is
definately faulty. Should I check anything else?

My question is could the relay  be the reason the battery was drained?
It (the battery) is just over a year old and have just returned from a
900km round trip so should be quite charged.

thanks in advance..