Power supply & battery sizing?


Senior Member
How many batteries will the Elk-P412 (12V 4A) system will support? I want a long runtime.

The Elk P624 says it can handle up to 10AH of battery, but that spec is missing from the P412.

Was there a "best practices" type document or posting for adding power to an M1? I cannot find it now. Something about isolating them to test the battery and such....
Was there a "best practices" type document or posting for adding power to an M1? I cannot find it now. Something about isolating them to test the battery and such....

I believe the posts refer to the importance of tying the negatives together for all auxiliary power supplies and main supply.
Both circuits use the same float charging circuit. There is a 5 ohm, 5 watt resistor that limits the battery charge if the battery is deeply discharged. By adding more batteries, the recharge time will be increased but they will recharge over time.

The main thing to maintain is a power supply voltage output of `13.8 to 13.9 volts coming out of the power supply. Without the current limiting resistor and recharging a deeply discharged battery, the recharge current could exceed the battery recommendations for a short period of time.

Recharge Note:
If you deeply discharge a sealed lead acid battery and leave it discharged for more than several days, the battery may not recharge using a standard float charger that is available in most security and home automation controls. You can force a recharge by placing the battery on a variable power supply and monitoring the current flow into the battery. On a dead battery, raise the voltage until you see current flow start going into the battery of around 100 - 250 milliamps on a 4 to 8 amp hour battery. This may be 16 to 20 volts depending on the state of the batteries plates and how they have been oxidized. Very quickly the current flow will start to increase and you will have to lower the voltage. Keep lowering the voltage until you reach 14.5 volts. Let it charge for a couple hours then move the battery back to a float charge system or reduce the power supply voltage to 13.9 volts. This will maintain a float voltage on the battery.

The downside of recharging a deeply discharged battery:
Once a battery is deeply discharged and left that way for days, an oxidation layer starts to build on the plates. By raising the voltage you are burning through the oxidation layer and forcing current into the battery. You never totally erase the oxidation layer, so the current delivering capacity of the battery will never be as good as it was before being deeply discharged. This is why the Elk M1 has a low battery cutoff circuit built in. The low battery cutoff circuit prevents the battery from being deeply discharged and ruined.

The same chemistry is true for your car battery. If you run it down and leave it discharged for a long period of time, it turns into a nice boat anchor!
Thank you for answering the question. I think I'll just stick with the 8AH battery, less chance of me messing anything up. :)
Is there any way to do a battery test on the P-412 unit? It appears that the only practical way to test & monitor the unit it to cut the A/C power with a relay and see if the DC output stays up.

From the M1 manual for zone definitions:
32 Power Supervisory 24 hr - For supervising an external power supply which provides alarm/trouble contacts. Wire the power supply contacts to open the zone in a Low Battery event and short the zone in an AC Failure event.
The A/C failure is easy with a relay, as documented in the P-412 manual. But how do I get a "Low Battery" trigger from the P412? Do any other Elk power supplies provide a "low battery" trigger? I know some of the Altronix models do. Do any brands perform a daily battery test like the M1 does?
What you want to do is monitor the battery voltage and see if it drops below a certain point, but without AC the battery could be bad and all the power turns off. Normally you can drop the power supply voltage to 10.5 volts and not cause any problems with any connected peripherials.

One way to test the battery on an ELK P412 is to put a string of 3 amp diodes ( each diode drops about .6 volts) in series so that when the diodes are present the voltage coming out of the power supply is about 10.5 to 11 volts. Put normally closed relay contacts across the diodes so that the diodes are essentially not in circuit. To test the battery, activate the relay so the diodes are present and the power supply voltage drops to 10.5 volts. The battery should maintain the voltage at above 11.5 volts. Monitor the power supply voltage and signal a low battery if the power supply voltage drops below 11.5 volts. You can build a battery monitor with an op amp or comparator circuit.