Hey guys,
i've just replaced my battery and now the battery discharge dash light comes on.
I didn't have this with the last battery, even when it died.....

Any suggestions?

Faulty alternator?

Possible. My mechanic didn't say anything about it when he checked the car out :(

Put a muilty meter on the battery while the car is running and see how much volts its got.
Also cheek your earths on the engine

It's too much of a coincidence, was the battery inadvertently connected backwards, ie. neg to pos when it was replaced?  Even just a quick splash of reversed polarity can kill the alternator diodes which would turn on the charge light.
 
Also, if your old battery went completely open circuit while the engine was running this can kill several components in the alternator.  I always shudder when I hear of people disconnecting the battery on a running engine to check if the alternator is working - it can cause damage, particularly if there IS another undiagnosed fault.

It could have been connected backwards. I've had a look and everything looks fine. The car runs, the dash, gauges and stereo works. The only thing i've found was that my headlights did't work (parkers did though)
i'm really getting ticked as i just want my car to run smoothly haha.

IIRC the light in the dash is the alternator light.
So with the engine running and a multimeter check the voltage from the post on the alternator to the body .
 
What is the voltage ?
 
 

I'm getting a multimeter today to find out.
The strange thing is that before my other battery died (still no idea how) the battery light was not on.
That was when i got it towed to the mechanics, and after they fixed the issue i drove home and around the beaches the next day with no warning lights on at all.
I've only seen the light on after the new battery was installed.
The car runs fine, i'm just worried that if i take it out i'm going to stop in the middle of nowhere

So you had a day's running around with the new battery BEFORE the charge light came on?  In that case it's probably unrelated to the battery replacement.
 
Batteries can die suddenly, it's no mystery - they just get old and the plates fall apart.  Alternator brushes also wear out, connectors work loose, electronic components fail, etc.  Forget about the new battery and just diagnose your current fault.
 
Are you sure it's the "battery discharge" light that's on?  I seem to remember that when the alternator light comes on it also turns on a couple of others, but I might also be thinking of a different car!  Some MR2s also have a battery electrolyte level light, could it be that coming on?
 
 

I had a day's running with the old battery, not the new one. The battery night came on after the new battery was connected.
I didn't have the fault until the new battery was put in