Adding to that: if under those conditions the panel cannot supply the lowest current the mower ever draws then the panel voltage won't exceed the maximum input voltage to the mower. I would want to know what margin of safety there was, which would mean measuring how much of the current came from the panel and how much from the battery: if the panel can supply 99% of the current I'd consider that very risky, if it can only supply, say, 75% I'd consider that safe.