12V battery voltage monitor for a backup battery (e.g. Sump pump, Reef Tanks ... )?

I have a full-size 12 volt car battery that I use for my Reef Tank battery backup system, to keep the water flowing. It worked fine in testing, but failed during a massive storm/power outage last weekend. It turned out the battery had gone down to 10.66 volts somehow. Therefore, I'm trying to find a z-wave or other smart battery voltage monitor to keep this from happening again.

In my research so far, I haven't found anything, except for the device @Cobra mentioned in his RV Automation thread ... Motorhome (RV) Automation (A small side project). The device mentioned was this "Shelly Uni" ... Shelly Uni | Shelly Shop Europe. At first glance it looks fairly involved to install it.

So ... has anyone heard of a 12 volt battery monitor that would work with Hubitat?


Yes ... but.

I know @iharyadi has a Zigbee board that will measure DC voltage. It may need a few external components.
The @haas Arduino shield will measure DC voltage, again Zigbee and will need a few resistors to measure DC.

An automotive battery has a internal leakage of approx 50ma. This means you have to "charge" the battery with at least 50ma to keep it from discharging. How long was the battery idle before you discovered the 10.6 voltage? Is there any other drain on the battery?

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Normally the backup battery is charged by a "Deltran Battery Tender", which are typically very dependable and the gold standard in classic cars and motorcycles. I'm not sure how long the charger was off-line, but I'm guessing I bumped the charger or something else in charging system. It probably was a few days, since it's a big SLA battery.

So, it looks like I'll need to get my soldering iron out :slightly_smiling_face:

I can't see such a battery dropping to 10.6 volts in a few days. Minimum in a car (with the inherent drain from the vehicle electronics) is a month (i.e. at the airport).

Perhaps there is more draining the battery.

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I could have been a few weeks, since I rarely check the voltage and whether the charger was working. You're right, though, something else could be draining it. Could it be the Inverter I have connected to it? I've heard an Inverter may draw power even though it's turned off. I probably should disconnect it, since I wouldn't use it during a "normal" power outage.

Yes an inverter will draw current even with no load. Think of a engine idling. You either have to switch off the inverter or increase the maintenance current to account for the inverter loss.

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I have the Shelli Uni device and it works great! And most importantly, it helped me resolve my battery under-voltage issue!

I found it relatively easy to install. You just need to know which cables to connect where, and essentially you just plug 2 wires to the positive (red and white) and 2 to the negative (green and black).

Hopefully, your battery will survive the ordeal - any chance it might just be way past its prime?

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Its been my experience using AGM batteries only last about 3-5 years installed in the typical UPS battery backup. I've been burned several times when there is a power failure the UPS fails to supply the necessary load or only has enough reserve for a few seconds. The UPS I have is supposed to check the condition of the batteries every so often, and even that is not a guarantee the batteries are good. Measuring voltage when the UPS is in standby show 13+ volts but disconnecting them and then measuring each battery I usually find a bad one somewhere in the bank.

I stopped using those store bought UPS's in favor of a stand alone power inverter along with a smart charge control and two independant battery banks using deep cycle marine batteries. Just before switching in a battery bank I do load test, similar to those used for testing car batteries, before switching the bank into service. Each month I repeat the process. That way I find the defective battery before putting it into service.

Of coarse this is overkill for just a sump pump but if you have things you cannot afford to loose power its a good setup to have, plus using a regular power inverter you can get much more runtime than any consumer grade UPS.

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Have you ever considered a Lithium Battery Pack ?

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If you do go with a li-ion battery, I'd go with a LiFePo4 battery pack. They're much safer for home use. Pretty easy to find high capacity cells as well.

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I use a number of shelly uni to monitor the voltage of both my cab battery and my leisure batteries.
It’s only a case of connecting a few wires and I use the battery that I’m monitoring to power the uni as the current draw is minimal

Although I put it all on a monitoring dashboard, you could easily just use a driver to send a pushover alert if the power drops too much

The uni have proven to be really reliable and they are relatively cheap

Have you thought of a small solar panel to keep the battery ‘topped up’?

I would be happy to go through the wiring with you in a PM or email if you decide to go with the Uni


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Thank you ... that looks encouraging.

With regard to the battery, it's a few years old, but it's always been maintained with the Deltran Battery Tender. To my knowledge, this was the only time it's been depleted. Right now it's holding the charge, but I should do a load test to make sure it's ok.

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So, it sounds like might need a new battery and remove the inverter. I don't really need it, since my Reef Tank pumps are all DC. I just had the inverter to run the lights and heater, if I needed to. First, though, I'm going to test it with a cool automotive battery tester I got last year. It's funny, I never thought of using it on this battery.

... Yes, I've considered it, but didn't think I would need it. Although, now it's sounding like a great idea! In fact, I have a ton of reclaimed 18650 Lithium Ion batteries that I was going to create a battery pack with, So this might a good application ... or just buy one.

Thanks for pointing that out. I'll see what's out there. I really need to calculate the Amp/hours required and go from there.

That sounds awesome. Do I still have to set it up through and Arduino?
Also, a solar panel probably wouldn't work well in Michigan, but be a fun project.
Thanks for the PM offer ... We see how this project goes.

I know little to nothing about battery packs but I was under the impression that paralleling battery's required some level of matching.

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They need to be close but don't have to be exact. Your lower and higher cells will be your limits. The more off they are the more load individual cells will take. You'll want to match capacity though.

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To match capacity would one have to perform some monitored discharge? Do you know a reference to how the capacities are matched? i.e. pure AHr or voltage vs some level of discharge.

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You need to get a BMS / PCB board that mount to the battery, that manages the charge and discharge of each 18650 battery in the pack.

Here's one ... Amazon.com: 14.8V 4S 30A 18650 Lithium Battery BMS PCB Integrated Circuits Protection Board: Electronics

I'd go with a bms with all the standard protections, overvolt, undervolt, overcharge, heat, short etc.

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No arduino required.
You just connect the wires and join your wifi network
With the hubitat driver you can easily get the voltage reading into hubitat

As for solar...
I live in the north-east of uk :slight_smile: