Battery calculations for a fire alarm system

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Weaver Road

Weaver Road

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Willington, CT
I have inherited dozens of existing buildings to determine if the current f/a systems have enough battery capacity to carry the NFPA 72 requirements. I can do all the math for the 24 hours at idle and 5 minutes at alarm state. What I can't seem to get a good answer on is how to accurately measure the actual current draw of the system so I can then do that math.

I have measured Ah on the 120v side and then converted to the 24v by math, but that seems to be wrong, I have tried measuring Ah the 24v side with the A/C off, but that doesn't match the previous math. I assume I need to find Ah on the load side of the power supply, but that is not always easy with integrated power supply units.

I of course, do not have any of the original battery calculations or other design info.

I am working typically with Simplex 4100u and Siemens MXL-IQ.
:-?
 
I don't think measured values are usually used in FA calculations. Each device on the system should have specs. that tell you how much the max. current draw is. I think you should be using those numbers.

I'm not sure measured values take into account all the variables. For example, how are you calculating the increase in current that will happen as the battery voltage drops during discharge?

Steve
 
Steve66:
Thanks for making that additional point about the voltage drop. I have been coming closer to the conclusion that this really has to come from the engineering side. I don't have the data on total devices, wire runs(voltage drop over distance), etc. on these systems and some are quite complicated such as multiple buildings on one panel, etc. The systems are roughly ten years old and the original calcs were either lost or never provided.

I guess my two options are to put a new set of batteries in and run it down over the full 24 hour period to simulate actual conditions, or get someone to engineer it based on available info.
Mike
 
jrdsg,
Thank you as well, I will look at this option. I have dozens of buildings and any way to be quicker as well as more accurate are very welcome.

Plus, I'm actually learning something too!
Mike
 
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