Larger battery pack for extended base station operation in power outage

I have seen the occasional topic about adding a UPS to extend the backup power, but why not add a connection to the base for an external battery pack to extend the power?  There could be a proprietary connector and an external pack of rechargeable AA batteries that can be attached to the base.  Maybe a slot  running top to bottom on each side of the base- one for positive and one for negative, of different sizes to prevent reversing the polarity.  Then a "donut" ring of batteries that can slide down the outside of the base, keeping it balanced and maintaining some of the aesthetics of the design. Or maybe an extra pack built into the wire from the power supply as one unit, let it switch over on line undervoltage.  Let your engineers handle the wiring to keep the voltage the same and increase the capacity, maybe modify the power supply.  Sell it as a kit to those that want to add the extra capacity.  You could price it lower than a standard UPS (and even provide comparison run times).  I'm sure businesses and homeowners with residences that they don't visit frequently would be interested.