Hurting the alarm charging circuit integrity?

[redbaritone]

I described what I understood about using a step-down power supply <https://www.amazon.com/dp/B008BHAOQO> off of the alarm battery leads to power the Raspberry Pi with a AD2PI board, to a friend who's an electrical engineer. His was response was:
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I have had time to sleep on your proposal and I have heavy reservations of the approach taken. Using voltage from the backup battery steals from what was designed to float charge the battery. Without full design information, we could be jeopardizing the integrity of the charging circuit and backup protection of your panel controller. I highly recommend against this as we don't have the design details.

The only way I could suggest using a Pi as an add-on would be a separate power supply with it's own backup that is correctly rated.
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Anyone do the research on this? Thanks.

[kevin]

We have done pretty extensive testing in our lab with this - I will see if I can get the engineer to chime in.

[redbaritone]

Thanks for that. I'm eager to hear you're engineer's response.

[kevin]

He's out until Monday at this point. But our default bundle comes with a wall based power supply that you could attach to a small APC battery backup unit or something if it's that big of a worry for you. We've had ours running on the battery charge circuit for a few years at this point...

[redbaritone]

That's good to know, but it's still possible that it could wear out the charging circuit over time. I imagine Honeywell made it more robust than it had to be, but I don't want to take that chance. I'd have to install a new power outlet in my hall closet if I were to use another adapter and UPS. That may be my only option, because my wife already hates the way the alarm's power adapter looks in our front foyer.

I'd still like to hear what your engineer has to say about it. I doubt I could call Honeywell and get a straight answer on the internals of their system. Perhaps your engineer would have more influence.

Thanks for the great feedback!

[scbrews]

I'm in the same camp as your buddy.
I read in one of the Vista docs that the charge current was in the "trickle" category (something like 100-150 ma @12v ) and as far as I recall, the Pi can draw 500ma or more.
You'd be better off (IMO) to determine exactly what the Pi is drawing, and connect it (the voltage regulator) to the Aux power output instead ...*as long as you're within the rated current limit*

My $.02
-Steve

[mathewss]

It has been a while since I did this research but here is what I have for some facts to help.

The Ademco Vista panels indicate they have 650ma is the max charging current.
The AC power supply is 16.5VAC or 25VA and that translates to ~1500ma
The Rasberry pi uses 250ma at idle but can reach 700 if overloaded on each CPU.
The Pi is mostly idle when running our image.
The Pi can be limited in its CPU power usage by setting arm_freq_min=100 and force_turbo=0 in the boot config file.

So based upon these numbers it is possible to maintain battery charge and power the Raspberry Pi with some conditions.
The total alarm load needs to be below 600ma leaving the remaining power for the alarm panel itself and battery charging.

One design issue currently is that regulator we are offering is a low voltage dropout regulator that is capable of going below the safe voltage level on the battery. This will result in damage to the battery if the system looses power for too long.

In the next generation of Ad2Pi board I hope to finish this summer I am looking at including this DC-DC circuit as an addon option with battery protection.

Best
Sean Mathews
Director of R&D
AlarmDecoder/NuTech.com

[redbaritone]

Thanks, Sean! I really appreciate the feedback.