You might as well understand that lawn equipment batteries have practically no warranty.
One thing I suggest that help is purchase a solar/sunlight maintainer and leave it connected to the battery and let it keep the battery alive when not in use.
If you park the mower inside, use a motorcycle battery trickle charger. A trickle charger will extend the battery life very well for a few years at least.
The whole idea is to keep the electrolyte in the battery active so sediment sulfation does not happen as fast.