Martin probably has the right idea of a bigger battery.
The M1 will cutoff the battery when the power supply reaches about 8.5 volts to protect the battery from deep discharge and going bad.
To figure about how long the battery will run the system, go to keypad programming user menu 8, then diagnostics menu, right arrow, then up arrow until you see the voltage and current the system is drawing. Multiply the value in Amps times the number of hours you want to run with no AC power and the answer will be the Amp Hour rating of the battery, more or less. Pick the next higher Amp Hour rated 12 Volt battery available. The true calculation can get alot more involved but this is a good rule of thumb calculation.
Martins battery that he showed will run the M1 drawing 1 amp of current for about 26 hours. + or -.
I suggest if you are going to use a battery over 12 amp hours on the M1, add an auxiliary power supply. Like the
Elk P624 to assist the M1's power supply to recharge the battery. Add the auxiliary power supply across the bigger battery.
To make it even better, break the positive battery lead coming from the power supply through a relay on the M1 and write a rule to disconnect the auxiliary power supply before the 24 hour dynamic battery test. Let the M1 test the battery, then reconnect the power supply after the dynamic battery test. This way the test will only be testing the battery and the battery will not be aided by the power supply during the test.
To power up the M1 with no AC, you must short the two square traces on the PC Board at the extreme lower left side of the board next to the On/Off switch. This overrides the low battery cutoff. If the battery is at 8.5 volts or less it will cut back off.
I am sure you and many others in the path of the hurricane are having the same concern with systems shutting down with prolonged AC Power outage. This is the least expensive way to get prolonged up time on the M1.
Hang in there, our heart goes out to you all!!
The ELK Swat Team