thanks for the suggestions. the security system is a simplisafe 2 base station, and it has replaceable backup batteries located in the bottom of the unit: 4 1000ma 1.2v NImh double-As. yes, the power supply is 7.5v.. I can only assume that when power goes out it is able to perform it's duties using 4.8v from the backup batteries, perhaps at reduced consumption. during a blackout, the company texts us to let us know the base station sensed an outage, and those AA's are supposed to last about a day.
I thought of jumping in a bunch more identical AA's in parallel banks of 4, but I decided against it. It would look terrible in the pantry, and cost as much money for 5 days of backup as an AGM would for 20-30. I would like to have about a month of backup. our normally safe neighborhood got pretty dicey during the last hurricane.
chosun: since the base station has an integrated battery for power outages, i'm not worried about the long-ish trigger time of a conventional relay. it will "see" something similar to a brownout, and probably won't even text us.
what I am worried about though are the voltage spikes that I read about online while the coil in a relay de-energizes, and that goes over my head (I'm a DIYer, not an engineer!)
kiwi: i thought of running the security system just off a charger. I appreciate the elegance of doing it that way, but this setup is at my parent’s house, not mine, so if anything goes wrong, I have to drag myself down there to fix it. This is what led me to think it best to use a relay. that way, when the battery fails (or the dc converter), the system will still work normally.
I have several concerns, all of which are unfounded (I made them up!).
1: how reliable (for continuous use) are cheap $7 dc converters? And how well do they regulate voltage? if the voltage isn’t as stable as the stock switching power supply, could that effect the longevity of my base station?
2: if (when!) the battery fails, what effect will that have on the charger? will it revert to just being a power supply that outputs the float voltage? will the current demands of the base station cause voltage fluctuations from the battery and confuse the charger into switching back and forth between ~14v charge and ~12v float?
3: if (when!) the battery fails, what effect will that have on the base station? during sporadic periods of maximum current demand (like when it calls the cops), what will happen between the failed battery and confused charger?
These seem like unknowns to me. Even if they are very likely to be moot.. this is a mission-critical piece of equipment, not a stereo or whatever.