I'm getting input voltage error on my IMAX b6. I am using an old computer psu that has 15a on the 12v lines. I am charging a 3600MaH NiCad battery from a few years ago and I can't charge it at ove 2.5 amps because I get that error. The battery is holding a charge fine.

Most likely your psu is too weak and its output voltage drops under load. You can try lowering the Input power low cutoff in the User set program menu.

Your probably right it's only a 300W psu.

300W would have been enough, it could be either old age or not the construction of the psu itself; do you have any load on the 5V rail? Many computer psus require it.