I have one of the early Lidl cordless multi-tools - the cells began to fail after a good while and I managed to open the battery and replace them - they are AA size, so it wasn't too difficult. It works very well and I have hardly ever felt the need to plug the mains one in since then. I did also replace the collets with a chuck and that has been a useful mod, I feel.
I can't vouch for whether the later batteries are openable or not - anything can be done, I suppose, but there can be a tendency to impede the consumer these days...