Elk-M1G Achieving 24-hours or Better for Smokes on Battery


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My inaugural post, thank you!
I'm considering replacing my end-of-life hardwired line-voltage smokes by adding smoke/CO sensor/sounders into my ELK-M1G system that currently does burglary and some Home Automation integration.
WRT smokes/CO I'm concerned about assuring at least the 24-hour on-battery availability required for smokes (UL), 1A max load on the main board (UL), and, not exceeding the useful capability of a 12AH maximum-allowed battery powering my P212S power supply. 
I've currently got 0.7A on the main board (0.5A of which is the direct-wired siren on Output 2), and 0.6A on the P212S which already exceeds the 24hr capability of its maximum recommended 12Ah battery in UL mode.  I can move some loads around if needed, like perhaps powering the siren with yet another power supply and battery.
Also, being here in Northern California, we could be without utility power for 3-4 days at a stretch thanks to our local bankrupt utility's woes.  (I *could* install generator-only outlets to which I'd move the panel upon a scheduled power outage if I can't get sufficient battery power to cover the outage duration. Note, So-far I only run my portable generator during the day using a hydra of extension cords to serve critical loads that haven't to date included the ELK-M1). 
For Smokes/CO, I'm looking at a System Sensor COSMOD4W module (.078A 4-wire) and seven COSMO-4W's (.280A combined) = ~0.4A worst case new load with the daisy chain of smokes alarming together.
I don't have central monitoring (a separate question for the future...)
In my looking at other configurations that include alarms, I'm not sure I've really seen enough battery capacity to meet code, particularly where non-fire-related powered sensors are significant (glass-break and motion in my case along with two M1XINs, 2 keypads, 1 M1XRF2H wireless, etc.).
1. Does anyone have any power-supply / battery capacity recommendations for this configuration?
2. Any thoughts about how to or if I can suppress Elk's on-battery sounding after a few seconds (but not suppress it on low-battery) when I know the utility-power's out until they restore it, and yet still maintain smoke/CO local alarm capability?
Otherwise, I'm liable to replace the smokes as is and pull their dry contacts back to the ELK. But, would a future central monitoring service  be OK with that?

Although you list the power requirement for the COSMO-4W and COSMOD4W as requiring a total of 0.4A worst case, that is the worst case for when they are in alarm mode.  Most of the time, when they are in standby, the COSMO-4W draws just 50uA each, and the COSMOM4W draws 2.4mA, for a total of 2.75 mA.   So you don't need much of a battery to keep them powered for 24 hours - 1 Ah would keep them powered for days, and still have enough power to sound an alarm.
The bigger challenge will be to keep the M1 and everything else attached to it powered for that long.
To keep the M1 powered for 24 hours at 0.7A would require a battery of around 18Ah.  But you say 0.5A of that is the siren, so a 5 Ah battery might be able to keep it running for 24 hours if it really only draws 0.2A in non-alarm state.
For the P212S, you need at least 14.4Ah to last 24 hours, but that assumes the 0.6A load is real, steady state, and not a max power number.  Easy enough to check with an ammeter.   You can put a battery larger than 12Ah on the P212S - it's just going to take longer to recharge the battery - and that gets into the next issue.
The problem I see is that if you have power outages that last 3-4 days, even a 24 hour battery capacity isn't going to be enough.   You say you can run on generator for a while in those cases, but you have to consider how long it's going to take to recharge the batteries.  Many power supplies use a charge current of less than 1A.  Altronix power supplies use 0.7A.  My guess is that the P212S uses 0.5A, but have never measured it.  So figure it's going to take 24 hours or more to fully recharge the batteries.   Can you keep the generator running that long or will that be a problem?
It might be best to leave your AC powered smoke alarms in place.  They can run on batteries for days and still sound an alarm when the power goes out.
Some AC powered smoke alarms have contacts that some folks think can be used to interface to an alarm panel.  But the ones I'm familiar with are not approved for that purpose (read the fine print).   Seems stupid, but that's the way it is.  There are devices that can connect to an alarm panel that listen for the sound of a smoke alarm sounding, and you could use one of those.
I was locked out of posting as a newbie for a "day" while my new account settles out. Only one post / day for a bit it seems...

RAR, thanks for the tip about standby versus peak power requirements. I should have figured that out and it changes everything.

I've now redone my loads spreadsheet to show both peak and standby power requirements split between the main board's several power supply points and the P212S. I had tried to move loads off the main panel to the P212S Aux power board. No more. I'm going to move most of them back.

When I separate the main board's peak and standby load (ampere) summaries by VAUX, SAUX, VKP, Output2, and also for the P212S, I'm better off unloading the P212S.  A careful reading of the installation manual provides the ampere limits for each main board power supply source and the constraining total limit across the Aux sources.

I will stay under these mainboard (Peak) limits as shown in the (USA) manual:
OUT2    (1A Limit)
SAUX    (1.1A PTC)
VAUX    (1.25A PTC)
VKP    (1.25A PTC)

and will stay under this Standby load limit as also shown in the manual:
SAUX + VAUX + VKP < 1.0A

An 18Ah battery should work for 24 hours for my main panel, with an 8Ah battery for the P212S.

It looks like the nominally 2-amp P212S is really only capable of about 0.3 amps of Standby load if one desires a 24-hour battery backup since the largest battery ELK specifies for the P212S is 8Ah.  (There's an older thread I saw here a few days ago with a quoted reply from ELK that the charging capability of the P212S won't support a larger battery, hence the ELK-1280 maximum spec in the P212S installation guide, versus 18Ah for the M1G itself).  

I'd stick an 800Ah deep cycle RV battery under the house for backup if the charger(s) would maintain it.
In that light, is there a way to disable the onboard chargers and use an external battery maintenance system?

Related question: When alarm panel battery Ah are specified, are those the usable limits for the batteries, or the kill-the-battery limits?  I know for typical wet deep-cycle batteries the recurring discharge level is often limited to 50 or even 25 percent of rated capacity to preserve battery life. (I understand that deep discharge events are supposed to be rare for alarm systems).
Thanks, RAR. From my browsing, you're a deep well of help around here.
I think you need to re-do your calculations a bit for the M1 power.

Although VAUX, SAUX and VKP have 1.25A PTCs, everything is sourced from a common power supply that Elk specs as being able to provide 1A max for connected devices, plus another 1A or so for the bell/siren output on OUT2, and about 0.2A for the M1 itself, for a grand total of 2.2A
It's important to do a maximum power calculation so that when the system is in alarm condition, with the siren sounding, keypads lit up, and all the sensors drawing power, you don't overload the power supply's output. 
But for battery hour capacity, I like to use the actual standby power the system requires.  That will be a bit on the low side if you consider that from time to time a keypad might be used, voice announcements made, relays activated, etc.   But I think it gives you a more accurate estimate than using the max power number.
The M1 will shut things down when the battery voltage drops below 10.2 Volts.   How long it takes the battery to get to that level depends on the capacity of the battery and the load that it is powering.  
When a battery is spec'd as being, say 8Ah, that does not mean it can deliver 8 amps for one hour.  Battery specs are set assuming a load on the battery of C/20 (where C is the Ah rating), or 5% of rated capacity.  That's a pretty small load, and when loaded to that level, the battery will provide power for about 20 hours.  If you load the battery with more than that, you will lose capacity in terms of the total Ah it can deliver. 
Battery discharge curves can help you figure things out.  You can find a set in the Power Sonic Technical Manual. See page 8.   As a general rule, if you load the battery with no more than 1/20 of it's capacity, a new battery should be able to run for 20 hours before it reaches 10.5 Volts.   But that time will decrease as the battery ages over the years, as well as with temperature, and the number of charge/discharge cycles the battery has been through.  After about 4 years, the battery will have only about 80% of its original capacity.  That's why it is recommended to replace them every 3-5 years.
I disagree with the statement you found that Elk says the P212S can not support a battery larger than 12Ah.   The P212S has no way to tell the capacity of the battery it is connected to.  The larger the battery, the longer it will take to charge, as the charger limits amount of current it delivers to avoid charging a small battery too quickly (which can damage it).   Since it doesn't know the capacity of the battery, it keeps the current to a small value
I believe Elk's max battery limit comes from the fact that they want to be able to recharge a 12Ah battery within 24 hours.   If you can live with it taking longer than that to charge a larger battery, then that's ok, as long as you understand the limitations that brings with it (such as the system going down if there is a second power failure soon after power is restored from the first outage).
I think your numbers look pretty good.  One additional thing to think about, though. 
You don't want the battery powering the P212S to die before the battery powering the M1 runs out and it shuts down.  If the Aux supply dies first, the M1 will notice the loss of the P212S itself, and possibly any sensors powered by it.   That will cause trouble reports at a minimum.
Your calculations show that you only need 4.3 Ah for 24 hours of standby on the P212S, and it is recommending a 1280 Ah battery.  That's a good margin and will give you almost 2x the required capacity.   That's probably enough, but if you want some added margin, you could go with a 1290 9Ah battery.  Same physical size, and costs just a couple dollars more.  Power Sonic makes a 1290 and some of the other manufacturers also make them.  I've had good experiences using the Power Sonic batteries.
Thanks, RAL. I'll look into the 1290.  But, Is it bad practice or may it possibly damage the ELK components to use a wet or GSM 12-Volt deep cycle battery in the hundreds of amp-hours and a, yes probably unsupervised, battery maintenance system for all my battery needs? That is, can the on-board chargers play well with an external charger on the same DC circuit, or be disabled?
Regardless, I don't want any chirps or other audible alerts when the system is on battery (at least until low-battery). Its very obvious when the power's out.
I would not use a wet cell battery indoors. Wet cell batteries produce hydrogen gas which is very explosive, even in small quantities.  You would need special ventilation to reduce the hazard. They also require maintenance to keep the water levels topped up.
The batteries like the Elk uses are Sealed Lead Acid (SLA) batteries, and do not vent hydrogen and don't require maintenance.  They are also called Valve-Regulated Lead–Acid (VRLA) batteries.
Some SLA batteries are a subtype called AGM (Absorbent Glass Mat).   Many SLA batteries are actually AGM batteries, although they may not advertise that fact.  Usually, higher capacity batteries are AGM.
The Elk charger should work with any SLA or AGM battery.