Short Lifespan of Harbor Freight 800 watt Generators

i assume your getting an exchange
or
you could have taken that money bought a better generator

you say 'charging the batteries'

do you mean directly with 12v from the generator?

or do you mean thru a charger with 120v use

any dc 12v charge is intended as an emg rescue start your vehicle
Not
for full time battery charging,
you are over working them

If its with 120v and a charger, i have no idea,
other than over heating the core and causing the armature to 'demagnetize'