If the voltage has dropped below about 10 volts, you'll probably need to replace it, especially if it's more than 3 years old. Modern batteries don't like being discharged, and, in my experience, rarely recover
It's fairly easy to remove the 4 bolts, or nuts, holding the seat in, and lift it onto the passenger seat whilst you work on the battery. You can leave the seat belt attached if you do it this way, and don't take the seat fully out