When we had a hurricane and NJ flooded some years back, my AIO sump pump died: lead-acid car battery, fully discharged, would not allow recharging.
I decided a replacement was in order. I went to Wal-Mart, ordered separate backup pump-charger and marine deep-discharge battery.
We have not had subsequent flooding, so my house current-operated sump pump works recurrently. But the backup sump pump is waiting. The battery is sealed, so there will be no water losses.
They cost more than an AIO but buying the units separately allowed me to buy an install-and-forget system. I recommend you spend the extra money when you buy a backup sump pump and buy separate pump-charger and Marine battery.