I'm a field service engineer and have been for over 30 years and cannot understand why engineers won't touch battery repairs? I do and so do plenty more I know the onky repairs we don't do us warranty work.
As for brands fitting tgeur own batteries? Yes it happens but most still use a battery company maubky GNB or Ernersys in the UK
With Lithium things are changing most manufacturers suooky with their own batteries in the machines and field service engineers are being trained to repair them particularly the larger brands Linde etc