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What is going on? Battery Issue?

dnshowell1

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May 8, 2021
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At my off grid cabin I have a fairly small system. I have four 100 watt panels, an MPPT controller, and three identical deep cell AGM batteries (96 Ah) hooked up in parallel. The panels are roof mounted facing south and get great exposure from about 8:30 AM through to about 3:30 PM. On a sunny day the controller is on float from about 11:00 AM.

1657031180860.jpeg

Attached to the batteries I have a 12 volt Unique fridge that draws about 3 -3.5 amps/hour when it is running. In addition I have a few LED 12 volt lights for night time and a 1500 watt modified sine inverter that I connect only when I need it.

On a sunny day, the system is on float most of the day - even when the fridge is running (voltage stays steady at 13.8 volts and the AMPs in from the panels drops to whatever the draw on the system is). There is a ramp up period in the morning as the batteries recharge but the system is pretty steady until about 5:00 PM. Then the number drop pretty significantly. The batteries drop from 100% (13.8 amps) to the mid/low 70s as soon as the sun gets low. If the fridge goes on (which it does about very two hours in hot weather) the percentage drops to about 68% (12.4 amps). The shows consistently on the controller and on the inverter when I have it hooked up. By the time it is full dark the batteries are showing about 64%. When I get up in the morning before the sun is up the batteries are at about 58% to 54%.

I would have thought that with 288 amp hours (three batteries linked in parallel) I would have more reserve power. Is it possible my batteries are a problem? As mentioned, the three batteries are the same, but two are 2 years old and one is only a year old. In the winter I keep them at home in my basement and trickle charge them from time to time.

Question 1 - does it sound like my batteries are the problem?
Question 2 - can you spot any other issues that could be causing the sudden drop?
Question 3 - my assumption is that when your system is on float, you are not using all the power that is available from the panels, so it would logical to expand the battery capacity - does that make sense?

Any help is appreciated - Dave
 
1) No.
2) Yes.
3) No.

The % value is voltage based and is completely worthless. The only time it has any meaning is when the batteries have been completely at rest (no charge or discharge of any kind) for 10+ hours (kinda accurate at 2+ hours).

The only way to accurately determine state of charge is to count the net current flow to the battery.

Will recommends some:

 
Thanks for the quick reply sunshine-eggo. So, if I understand you, the numbers I am getting from the controller are not useful in determining how well charged (or discharged) my batteries are. So what is the point of the fancy read out?
Can you explain more about the net current flow to the battery and how that determines the state of charge? Keeping in mind this is "Beginner's Corner". So for example if I got the AiLi Battery monitor it would show a simple diagram of a battery with the charge indicator - is that based on the net current flow?

Thanks again

 
The display is to mislead the public that it's useful. Many devices do this.

Battery monitors count current. They compute the number of Ah going in and out of the battery and compare the net to the programmed amount. In your case, you would program it for 288Ah. If you used 28.8Ah net outgoing, it would report (288-28.8)/288 = 90% SoC.

The monitors are also programmed with the "full" criteria, e.g., your batteries are fully charged once they reach 14.4V and 3A of current simultaneously. This would trigger the monitor to indicate 100%.

The AiLi has several different display options, but a simple % readout would give you true state of charge.
 
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Isn't AGM batteries (96 Ah) has usable capacity of 48Ah?

Yep, but that doesn't mean you program them for half. Many have cycle life ratings down to 20%, too.

What happens if you're forced to go lower than 50% due to an emergency, problem, or an accident? Do you want your SoC to become completely inaccurate below 50% when you would likely want it even more accurate?
 
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