How do i charge this battery pack from an imax RC charger ?
I have put 7..5 v on it for a few minutes to get the voltage back up but I keep getting an error message on the charger .

 

After you've applied 7.5V for a while, disconnect the charger and measure the pack voltage. If it's well below 7.5, it's possible a cell has failed and it's not taking a charge anymore.

Sometimes you can get some more life out of the pack by cutting it open to replace the bad cell, but you my be better off just building yourself a new one or of course, replacing the pack.

 

Are you able to measure the pack voltage after the charging attempt??
You can keep trying to use that charger, but feel the battery pack and stop if it gets more than slightly warm.

 

Or - is the charger faulty? Can it charge other batteries and not just this one?

 

What is the error message?
I mean, you should provide as much information as possible to prevent us from making wild guesses about your exact problem.

 

Without having any hint about that error message, I am making a guess, which is that the battery pack is not the model that the charger is designed to charge. In that case, you might "be out of luck."
A very common rechargable battery failure is being unable to accept a charge, in which case the current will not be what the charger expects to see.
So we need two answers:
First: What is the error message?
Second: is this the model of battery that the charger is intended to charge??