Warranty claim - battery falure

I have a UPS system purchased from Cooper Power. I recently had a warranty claim as the UPS only lasted for 40 minutes during load shedding. They tested and confirmed that 3 out of the 6 batteries were faulty. They agreed to replace 3 of the 6 batteries under warranty. Previously Cooper Power notified me, that if there was a fault with a battery, I should replace all of the batteries and not just the faulty battery. My expectation, in this case, is that they must replace all 6 batteries - not just 3 of the 6. They are prepared to give me a discount on replacing the other 3 batteries still covered by the warranty. In my opinion - that is not good enough. Why should they still make a profit to resolve an issue that should be covered under the warranty claim? In my opinion, the warranty was part of the purchase price when I bought the system.