Wattage and ohms law. Decrease voltage, increase in amps to maintain wattage, increases resistance, heats up fuse. All assuming the batteries can still produce the amperage and those could probably do that enough to blow the fuse. Or not.
OK thank you everyone! This is good info for me for future. Sadly, I've decided to scrap the mower and recycle the batteries. Could not find replacement batteries online for cheaper than $200 so not worth it...
See my previous post #2. You can buy as set of new batteries for just $100, and they will be better than the original ones. I gave you a link to Amazon where you can buy them.