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Battery monitor works...but what is it telling me?

465 Views 3 Replies 2 Participants Last post by  GoldRiver
I've finally managed to buy a couple of battery monitors, and get them hooked up. The system works well, and I posted info on it here. But to be honest, I'm not sure what it's telling me.

But I'm not sure where I am between the two extremes of a dramatic failure and a new battery. The in-service date for the car is December 2018, and it now has about 15,000 miles on it.

Here's my chart showing today's activity. I've imposed the Voltage chart on top of the SOC chart for ease of reading. I'm using the app-created graphs rather than Excel, since the Excel download is so difficult to work with. The car was not used during this day...all the craziness below was instigated by the car itself.

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Now here is the three-day chart generated by the app.
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In the chart above, the car was not used on 4/29, and was used for two five-minute trips on 4/30. It appears that during the evening on 4/30, the Aux battery started discharging, and then began that cycle of charging and discharging.

And in case it's of value, here's the five-day, app-generated chart

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After reading Ayepace's post on this topic, I expected to see charging activity when I used the car, as shown on the 27th and 28th. But not the activity on 4/30 and 5/1. I can see that my voltages are lower than some others I've seen, but that variance seems normal from the posts I've read.

This could be just a stray error... but now and then, I see the Aux battery reporting an SOC over 100%. Here's a couple of readings from Excel:

May 01/2023 05:4912.8588%24
May 01/2023 05:5112.85120%24
May 01/2023 05:5312.8588%24
May 01/2023 05:5512.8488%24

Any idea what's happening? What load is on the Aux battery (with the car parked and the doors unlocked) that would cause the discharge?

Any clues to the batteries' state of health?
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Here's an update to the readings above. As you can see in this chart, the situation stablilzed. The Start battery is now at 88%, and the Aux at 82%. No spikes in readings.
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But then I looked at the temperature, and saw the same spikes.
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Now, it looks to me like the Aux battery monitor is defective.

I don't know if an Aux battery at 82% is a problem when the car is idle.
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Hi GoldRiver,
I have posted in your other thread, it may be helpful.

I myself would ignore the SOC and Temperature graphs as confusing and not useful for you. Only compare the Voltage graphs they are actual true data. The small AGM battery should always show a higher voltage just after charging ceases. The AGM battery has almost no load when the car is locked, The Larger flooded battery runs all of the neccesary security, locking and monitoring data systems and will fall over time.

Cheers, Steve
Hey Steve,
You're right... the SOC does add confusion. The Aux battery voltage is reading consistently higher than the Start battery (12.74v and 12.61v respectively at the moment), even though the Aux battery is reporting a lower SOC, (the app knows it's an AGM, so it gets less SOC-credit per a given voltage). I'm assuming that the SOC is just a simple translation of the voltage anyway, based on battery type.

And your point from the other post is worth repeating here, that the monitor's negative terminal should go to a good ground rather than the battery's negative terminal on the Start battery. I've got a defective monitor to replace, so I'll make that change when I put the new one in.
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