There's probably a parasitic drain on your mower, or a connection issue. Have you verified what the battery voltage is when it cranks slow?
i always recommend the original battery tender, https://www.batterytender.com/weather-resistant-800
I have these on the boat(3), motor home(2), zero turn, tractor, snow blower, van. They work fine to maintain a charged battery.
I order them 5 at a time and sell them to customers.