Remote battery backup

I have a project that has multiple sealed outdoor sensors that run off their individual batteries spread out over a few acres of property. The sensors talk via radio. Everything runs great and I am happy with it. I would like a way for there to be a back up battery for each sensor that can kick in ONLY when the main battery (at each sensor) dies. This way my sensor will not be out and I can program it to send me a message that the main battery died.

I was playing around with relays but they use a huge amount of power and thus defeat the purpose. So, I guess it has to be some sort of solid state set up. I looked on the internet and did not see anything that I think might work.

Any help is much appreciated.

The sensors should monitor the battery voltage and inform you it is time to recharge/replace it.

I would also suggest that all you need to do is have the sensor read the battery voltage and report it.

Thanks for the feed back! I can use a voltage divider to measure it. That's doable. Some times I can't get to the sensors because of my work schedule, etc to change out the batterys, so i was hoping for a backup.

Perhaps you could re-design the sensors.

I just had a battery dies in a 'sensor' that I had on test.

This 'sensor' reported temperature, humidity, pressure and battery voltage every 15 mins. The 155mAhr battery lasted 14 months, so some AAs would last 20 years, so a leeway of maybe 2 years to get around to replacing them when they were say 90% gone.

srnet:
Perhaps you could re-design the sensors.

I just had a battery dies in a 'sensor' that I had on test.

This 'sensor' reported temperature, humidity, pressure and battery voltage every 15 mins. The 155mAhr battery lasted 14 months, so some AAs would last 20 years, so a leeway of maybe 2 years to get around to replacing them when they were say 90% gone.

Thanks for the info: I get about 6 months of a 6V big lantern battery. Another issue for me is if an animal pulls out the battery wire: so having a way of the system to tell me "wire pulled out, on back up battery" is necessary. I have can this happen in the past (I don't know why wires are so attractive to them).

What if the animal pulls wires of the backup?
What backup battery do you plan to use?
How much current your setup needs? (In other words what is reasonable current for the switchover circuit.)
Is it OK for power to be lost shortly during the switchover?

Smajdaf: I was hopeing to put a little 4 battery AA holder inside the weather proof box: just to run enough time for me to get here. The current is very small: Uno with 3 sensors lasts through the winter (so far). If I loose power it kinda screws p the arrays but I can live with it. The main thing for me is the battery does not have to last a long time. Even if I got about a few days - week out of the 4 AA batteries I would be happy. If an animal did pull out the main batter wires, the back up would give enough power to transmit the alert. This helps me because now I have a clearer picture of what kind of animal (bigger, more aggressive) around my hens.

Thank you.

I haven't really thought this through, but the circuit below might automatically switch over to the backup battery if the main battery is disconnected, or is discharged enough. You would want to pick a mosfet with the right Vgs threshold voltage so the main battery would have to discharge a good bit before the backup switches in, but with still enough juice to power the device so there's no interruption. And the switchover point might be kinda messy. It would go back and forth, and the two batteries would probably discharge together from that point.

If the backup battery is significantly lower voltage than the main, you would also need a Schottky diode in the battery line. Otherwise current would flow from the main battery through the body diode of the mosfet into the backup battery, which probably wouldn't be desireable.

Backup battery.png

Backup battery.png

Thank you!

Don't thank me yet. The circuit should work in case the main battery wires get pulled out, but if it's just that the main battery is low, there won't be a hard switchover. The backup will just begin to contribute current. Actually, I don't know how it would behave. And the circuit doesn't tell you what's happening. You would still have to monitor the main battery voltage.

They make ICs specifically for this purpose, but I don't have any experience with them.

This topic was automatically closed 120 days after the last reply. New replies are no longer allowed.