A good battery backup unit / BBU will normally have surge protection as aprt of the unit. The surge protection may save your equipment from the reason you have the power failure. Like a close by lighting attack. 

The battary backup vendor APC does not recommend stacking battery backups to extend the power. Two 4 hour backups do not give you 8 hours or backup.

Also remember when the battery backup no longer charges at the end of its normal life it is the customers responsibility to repace the battery.