I had 2 batteries in my mk3. I just replaced the standard batteries. I didn't open the packs up but looked like a number of button cells together in each pack, Personally for a 10 min job I would just change like for like. Its likely to be at least 5 years before the next change. Possibly a lot longer Only ever charged mine once a fortnight/month after I replaced them which I was using regularly at the time.