Battery bank drop in voltage after dark and lose capacity too quick

[Johnny16]

My system info: (1 month old new system)
(8) 270watt solar panels
100 amp Epever Charge Controller
(8) Duracell GC2 6V 230AH lead acid deep cycle batteries .
3000 watt inverter with 24v input.
All cables are 0, to 2 guage copper strand cables.

Batteries hooked to 2 strings of (4) 6v Duracel GC2 230AH batteries in serial to 24V.

Solar Panels hooked 2 panels to a string in series to get 80v. Total 4 strings going to combiner box before connecting solar charge controller.

From 10:30am to 5:00pm, i am getting 1,600watts from panels. During the day, my power usage is less than 500w to 700w at any giving time so charge controller always have power to charge battery bank and usually thinks my battery bank is full around 2pm (27.5 to 28.25 volts) and goes into float charge mode until night time.

Once night time comes around 8:45pm, my batteries voltage dropped from 27v to 25.5v which is supposed to be 100% charge. Using my single phase 120v well pump (450 watt using a killawatt meter) for 10 minutes and stopping it, battery voltage drop to 25v which is around 80% capacity according to most charts.

How can i drop from 100% charge at 25.5v to 80% charge at 25v using 450watt for 10 minutes. My battery bank is 460AH at 24V, which is around 11,000 watt hour.

Are my batteries defective ? Is there a flaw with using battery voltage to determine battery capacity ? The battery indicator on my Epever is useless because it will be 3 bar after dark from 100% full at 5 bars. I have a battery monitor with a shunt from amazon for around $50 but it lose track of my battery capacity once charger starts charging. The voltage is displayed correctly. Just not capacity.

System voltage is correctly measured and displayed from inverter, charge controller and my battery monitor.

With my normal enery consumptions over night, i end up with around 24.2 to 24.5 volts in the morning which is around 50%. According to my battery monitor, system used 70 to 80 amps overnight. If so, that is less than 25% of my capacity, yet based on my voltage, i am close to 50%.

 

[snoobler]

Voltage based SoC is only accurate when the batteries have sat without load or charge for 24 hours.

Any time you apply a load, the voltage will drop. Any time you apply a charge, voltage will raise.

When you read the voltage in the morning, there is likely a load being applied, so voltage is a bad indicator of SoC, but your voltage might be a little low.

Is your battery monitor indicating 70-80Ah overnight properly setup and calibrated? I have a $50 monitor from Amazon, and I have to reset it to 100% daily when the bank is charged to reliably have it report accurate %.

27.5-28.25V is not full. For a typical FLA, bulk/absorp is 14.4-14.7V/12V or 28.8 to 29.4V in your case. Float will occur around 26.4V AFTER the batteries will no longer take about 9A at the bulk/absorp voltage.

For your particular batteries, when you hit the bulk voltage at 46A, the battery is 80% charged.

I recommend you review the charge parameters for your batter and ensure your charge controller is properly setup.

 

[Johnny16]

Thanks for the reply,

I will have to customize the charge parameters since it is currently set to the Flood Acid Battery preset on my Charge controller. I am afraid to overcharge my batteries using custom settings.

I do have constant power draw on inverter with my Refrigerator, and small electronic totaling 40w to 100w so there will be slight voltage drop.

After dark, i zero out my battery monitor, disconnect my solar array, to monitor my Amp Hour power draw before charging again the following morning. Average 70 to 80AH power usage before next charge. When charging, I zero out my battery monitor and keep track of the AmpHour charging battery bank. Usually send more than 100+AmpHour back to battery before night.

I verified that battery bank is receiving the stated AH charge current from my battery monitor and charge controller by using a clamp on Amp meter on the main battery cables.

 

[snoobler]

In addition to the known amounts, your inverter likely has a 15-20W draw just to be an inverter. When you consider that's for a full 24 hours, it's a notable share of the energy - about 1/4 the energy of a standard residential refrigerator.

Worth familiarizing yourself with this:

https://www.eastpennmanufacturing.com/wp-content/uploads/Golf-Car_EV-Battery-Care-Maintenance-1932.pdf

Since you're concerned about SoC, you should get a hydrometer and check a cell. Specific gravity is the best way to determine SoC.

You should know what the charge controller voltage are even if you're using the pre-programmed option and the termination criteria.

Hard to find specific charge data for those batteries, but best I can find is 29.2V for bulk/absorp and 26.4 for float, though that document I linked indicates 13.8V for charge during storage. Some motive batteries (golf cart/forklift) have higher float voltages, but you should regularly check fluid levels to confirm as float higher than 13.2V (for 12V) can cause fluid loss.

You should limit your charge current to 46A. If the charge controller has an absorption time limit, use 5 hours. If the charge controller has an absorption termination current, use 9A. At that limit (9A @ 29.2V), the battery is well and truly fully charged.

You can use a hydrometer to confirm full charge. Since you're learning with poorly documented batteries, as charge approaches full, you should be checking the batteries for heat and specific gravity. I believe the values I've specified are safe, but YMMV.

I am 100% certain you have never fully charged your batteries based on the voltages referenced.

Note also that these are fairly cheap batteries with little cycle data available - meaning they likely won't last you very long. Maybe 1-3 years.

 

[tictag]

Firstly, I do not believe anything is wrong with your system. That's the good news. You are simply relying on a parameter that is inherently inaccurate, that is, terminal voltage as an analogy for state of charge. Everything @snoobler has said is excellent advice, the only thing I would add is that your current battery monitor is clearly inadequate for your needs:

1. You need one that counts electrons in and out so that you have a proper state of charge based on coulomb count rather than voltage.
2. With high power loads on a lead-acid battery bank you must account for the puekert effect.

I would recommend:

• Follow @snoobler's advice regarding charge voltage and current i.e. in lieu of battery specs: 28.8V - 29.4V (2.40V - 2.45V per cell). If you get a lot of gassing at the higher voltage drop it slowly towards the lower voltage in the range. A lot would look like a kettle boiling. Normal would look like a few bubbles per second. The battery should NOT get hot, just luke-warm. Anything below 2.3V per cell (27.6V) and you're going to cause damage to your battery long term (sulphation).
• Get yourself a proper battery monitor that incorporates peukert, I can personally recommend the Victron BMV range.

Victron's battery monitor do not base the indicated SoC on voltage, instead it resets to 100% based on time above float and charge current dropping to, usually, C/10 Amps e.g. 4.6A, then counts charge in and out to give you a reliable SoC.

My guess is that your pump is drawing more capacity from your battery than you think (see puekert effect) but the resulting voltage drop is indicating a lower SoC than reality.

 

[DASH]

In addition to the known amounts, your inverter likely has a 15-20W draw just to be an inverter. When you consider that's for a full 24 hours, it's a notable share of the energy - about 1/4 the energy of a standard residential refrigerator.

Worth familiarizing yourself with this:


Since you're concerned about SoC, you should get a hydrometer and check a cell. Specific gravity is the best way to determine SoC.

Highly recommend a hydrometer and check each cell. As snoobler pointed out, there are places on the web that can give You specific instructions on how to use it to trouble shoot and maintain your batteries. Unlike volt/amp meters it can also detect how each cell is performing and if You have a bad one.

The Duracel GC2 Batteries are Golf Cart Batteries and not intended to be used as Battery Storage for Solar Systems, so they wouldn't have the life, performance, or forgiveness that the true lead acid deep cycle storage batteries have (but then the cost of these batteries are significantly higher then the Golf Cart Batteries). You didn't mention how long You have had the batteries. I would definitely do some research on the best way of maintaining these batteries to get the longest life You can out of them.