I owned a MythoS 125 a few years back and never had the issue you describe. I ran it on 8S (as most did) and had to put the batteries as far back as I could.
Perhaps you ran it on 6S ?? - that was what Sebart originally stated for some bizarre reason. this was a 1.9M long model, 6S would never fly a schedule comfortably. I found 2200W was the sweet spot for this plane, on 6S that equates to a current draw of around 100A !! Even more bizarre when you consider the MythoS 125 predecessor, the Wind 110 was designed for 8S from the very start.

Steve