EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: e100 on September 10, 2022, 01:27:41 pm
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I would like to measure the voltages of some 12v lead acid batteries used in an alarm system without having errors due to the voltage drop in the ground wire powering the alarm panel 20 meters away.
I was thinking of putting a micro next to the battery and conveying the voltage measurements via a digital opto isolated link.
Is that a sensible idea? What do commercial bits of kit use for this kind of problem.
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Run remote sense wiring, fused at 100mA in both the positive and negative wires, right next to the battery terminals for safety, then make a differential measurement at the panel end. If using a digital display it must *NOT* be powered from the sense wires, and must either have fully separate power ground and signal negative, or if they are commoned, be powered from a fully floating isolated supply (e.g. its own transformer secondary or an isolated DC-DC converter).
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I would use a Fluke meter with Fluke Connect and use an Iphone to monitor the voltage. The phone can also log the voltage. Or I just use the Fluke meter to log the voltage for a day or so and view it to see if at any time the voltage drop. That's a comercial solution I already have.
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Use a micro like an ESP32.
This will help: https://www.espruino.com/INA226 (https://www.espruino.com/INA226)
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Use a micro like an ESP32.
This will help: https://www.espruino.com/INA226 (https://www.espruino.com/INA226)
I already have an INA219 which is similar, but not identical to the INA226.
For a remote voltage measurement without ground effects, is it OK to repurpose the differential measurement feature intended for current so it looks like this
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This seems to work. I'm using the INA219 differential inputs to measure the voltage drop across a resistor in a 1mA current loop and then calculating the full voltage across the battery.
[attachimg=2]
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Some questions...
Is the 1mA current loop always active? Is it used for anything else?
Is the alarm system powered only by the battery or is the battery just a backup power source?
What's the capacity of the battery?
What microcontroller do you have in the panel?
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Some questions...
Is the 1mA current loop always active? Is it used for anything else?
Is the alarm system powered only by the battery or is the battery just a backup power source?
What's the capacity of the battery?
What microcontroller do you have in the panel?
The panel and network gear consume 20W so the efficiency of the monitoring circuit isn't a concern.
The micro will eventually be an ESP32 with wired ethernet to the internet gateway.