What, your clippers work just as well on batteries as they do mains at the moment?
So, how have you tested the batteries to think it could be that? Or are you saying you haven't because they act the same if plugged into the mains or just on battery (and I believe you are).
Well, you implied they could be run from mains but didn't say they weren't working ok on mains?
Quite, so you can still use them on mains, or are you saying the 'cut out' on either power source?
Ok (not on any I have and why I took your initial mention of 'they can be run on mains too' to suggest they were still working on mains). ;-)
However, is it possible that whilst they may 'work on mains too', they need to battery to be 'fit' and are you sure yours actually are? By that I mean they may indicate a reasonable off-load terminal voltage but don't have any real capacity. Once the startup load suppressed the terminal voltage, the BMS cuts the output (which may be the same be it on mains or battery)?
Cheers, T i m