Software person here, so the battery side is new to me. We have a gateway in an unmanned pump hut that reads 4 meters over RS-485 and sends to Hermetry over LTE. Mains drops a few times a month, usually for a few minutes, and we lose data every time and get a burst of offline alarms.
My plan was to build a small UPS: 12 V 7 Ah sealed lead-acid, a charger, and a diode-OR to the 12 V rail. Questions:
What charging circuit is sane for a lead-acid that’s on float 99 % of the time? I found LM317 constant-voltage designs and some bare «12 V battery charger» boards online.
A colleague says if we move to a licensed gateway with Hermetry Edge we don’t need a UPS at all because it buffers. Is that right?
For the LM317 design I think you set it to 13.8 V and add a current limit resistor, we did that in a lab once
Your colleague is half right. Buffering on licensed gateways covers the link going down: they keep readings locally and send them later with their original timestamps, so the history fills in. How much they keep depends on the model, worth asking about the specific one before relying on it. Licensed gateways and Hermetry Edge
But a buffer doesn’t help when the power is off. The gateway is off, and so are your meters’ RS-485 interfaces if they’re on the same supply. There’s nothing to read. So in your hut the UPS is what keeps data flowing, and the buffering is what covers LTE dropouts.
For the charger, I’d not build one. A DIN-rail DC UPS module (24 V or 12 V, takes the battery directly, does float charge and switchover, has a «mains fail» relay output) is about the price of the parts and the time you’d spend. If you do build, Rahul’s numbers are right: about 13.6-13.8 V float at 20 °C, current limited to around 0.1C (0.7 A for 7 Ah), and temperature compensation if the hut gets hot.
Wire the «mains fail» contact to a gateway input and send it as mains_ok. Then an alarm on mains_ok == 0 tells you about the outage itself, and you can make the offline alarm of the meters wait longer than the UPS runtime minus a margin. If you use the Remote sites ready solution, its UPS profile already has keys like ups_a_status and ups_a_charge you could fill.
That cleared up my confusion, thanks Emily. Ordered a 12 V DC UPS module with a relay output and wired it to a spare input. Next mains drop was Tuesday, 11 minutes: we got one «mains failed» alarm, no offline alarms, and no gaps in the meter charts. We’ll keep the LTE buffering question for when we replace the gateway.
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