Not sure if this has been posted before, but does anyone has any idea where I can send my old i-phones 6 and 6+ for complete refurbishment, at a reasonable cost?

I have sent them for change of battery but its still drains a lot of battery life, I was told its due to the chipset or electricals, and I have to change them too. The total cost may well be buying an old iphone 7.

Any ideas anyone, or shall i just junk my old phones ?