Terrible Makro service

I bought a Gentech Inverter from Makro in January for my parents in law to alleviate the loadshedding problem. 6 weeks later i had to organise a company to pick up the machine(it is heavy) and take it in because it would only power a tv for 10 min. They replaced the inverter as it was found to be faulty and then say i must pay for the batteries that the inverter has damaged. I paid R14000 for this item and have to replace the batteries that their inverter has damaged? The inverter is still with them and now i dont get any further replies after numerous requests. It is 10 months later