Changing the agreement

I never had issues with the security company. I am renting the alarm system from them but since loadshedding starting I have been receiving sms's to say my battery is low. The original agreement was they will fix whatever is not working on the alarm as I am renting the system and paying more. Now they said they can't replace the battery and I must pay for my own battery but it is their system. Their reason is because they are not covered with their suppliers on these batteries which is not my problem and this was never communicated at all. I suggest that you follow up with them as this was never the agreement.