Question About State Of Charge

[Ampster]

.....They are however 8 years old and about due for replacement. I don’t want to make the same mistake I did with these less than ideal batteries. I want something better that has less possibility of trouble. ............ My space is VERY limited. I can currently fit up to 8 smaller 35 Amp batteries, like my current ones, in the space I have, which is on a shelf 4 feet up from the floor. I now have 6 batteries. The space is not ventilated very well and somewhat close to an ignition source, which is why I chose sealed AGM batteries in the first place. I am also old and partially disabled so lifting and moving larger 50+ pound batteries isn’t possible for me. Can you suggest any batteries (6 or 12 volt) and how to set them up (series or parallel), that would give me the same (210 AH system) or better, up to the best capacity of my solar panels, that won’t break my bank account? I currently have 4, 100-watt panels on the roof of a shed next to my house, with just enough space and budget for 1 more. If you think I need it if you recommend a bigger battery storage capacity let me know. I’m open to any of your informed suggestions. .

I just committed to 28kWhs (560 Ahrs] of Lithium batteries delivered to my door for $116 per kWh. If you have the inclination for DIY pack assembly that might be an option. They will take up less space than any lead acid chemistry and at 11 lbs each are easier to shuffle about. There are also no hydrogen fumes to ignite. I am probably older than you and I have not tried to move a Lead Acid battery in years. Based on the size of your panels you could probably get by with less kWhs of lithium because they charge more efficiently and you can use more of their capacity.

[AL1952]

Ampster when you say I can use more of their capacity what does that mean? I can only use my AGM batteries down to 12.1 volts and I'm at 50% charge. Can I run the Lithium lower? How much lower? I'm not sure I understand this whole DOD/SOS thing. I'm trying to talk my wife into letting me use the stimulus check to buy 2, 100 AH lithium batteries. Does that sound about right for 4, 100 watt panels? I can go to 5 panels if I need to.

[Ampster]

The voltages are different for lithium but I will try to translate using kWhs. If you had 10 kWhs of Lithium you could use 80% or 8kWhs. Lithium is 90% efficient including inverter losses so it would take 8.8 kWhrs to recharge.

To get the same energy from Lead Acid at 50% you would need 16 kWhrs to get the same 8kWhs. But since charging Lead Acid is less efficient (70%) it would take 11.5 kWhrs to recharge.

[AL1952]

Interesting. So Ampster do these same values in this chart apply to lithium batteries? Could I run one down to 11.51 volts and still only be at 10% SOS? Would I be effectively getting double my run time from my current AGM batteries that I have to shut down at 12.10 volts (50%) and be able to recharge the lithium much faster? image_12216.jpg

[SunEagle]

Interesting. So Ampster do these same values in this chart apply to lithium batteries? Could I run one down to 11.51 volts and still only be at 10% SOS? Would I be effectively getting double my run time from my current AGM batteries that I have to shut down at 12.10 volts (50%) and be able to recharge the lithium much faster? image_12216.jpg

Li chemistry batteries have different cell voltages. SI could be wrong but I would say that chart is for Lead chemistry.

[AL1952]

Thanks for pointing that out to me Sun Eagle. I found this chart for lithium batteries that confirm what you said. Looks like I can run a lithium battery down to 12.0 volts and still not be totally discharged. That really would about double my run time. It actually makes the extra cost well worth it given the 2,000+ cycles of most Lithium batteries. My wife has agreed to let me get two 100 AH lithium Iron Phosphate batteries with our stimulus check. Thank you Uncle Sam! So which brand is best for this kind of battery? Battle Born, Renogy, or Life Blue seem to be the front runners. Any brand suggestions based on your years of experience? I want maximum cycles and reliability for my money.

[Ampster]

.......... So which brand is best for this kind of battery? Battle Born, Renogy, or Life Blue seem to be the front runners. Any brand suggestions based on your years of experience? I want maximum cycles and reliability for my money.

I have no experience with any of those. My overall experience with Lithium batteries suggest your wife will be happy. Wife Acceptance Factor is important.
There are some good reviews on this site for those products.

https://diysolarforum.com/

[Ampster]

Interesting. So Ampster do these same values in this chart apply to lithium batteries? Could I run one down to 11.51 volts and still only be at 10% SOS? Would I be effectively getting double my run time from my current AGM batteries that I have to shut down at 12.10 volts (50%) and be able to recharge the lithium much faster?

As @SunEagle said, those charts are for Lead Acid. Lithium has a much flatter discharge curve. Yes Lithium batteries can take greater charge rates but you have more flexibilty because you don't have to waste as much solar energy on the last Absorb stage to get them completly full like you have to do with Lead Acid. The brands BattleBorn and the others drop In replacements accept the usual Lead Acid charging parameters.

The second chart you posted looks correct. You can see how flat the curve is.

[chrisski]

Still on my first real system build. I spent a few hours looking at the Renogy Rovers, but opted to go with Victron MPPT instead. Basically came down to the website at https://www.renogy.com/products/char...e-controllers/ did not mention the charger was compatible with AGM batteries, just Gel, Sealed, flooded, and lithium. Maybe AGM and sealed are the same, not sure. Blue tooth was also not built in. I wanted to go with Renogy because I bough six renogy 100 watt panels, and I like to have matching equipment. When I looked, I found more data on Victron then Renogy controllers.

[Sunking]

I would not even look at any CC that have a positive ground.

Not trying to bust your chops SE, but if one had a chose between either negative or positive ground, I would pick Positive every time.

Way back during the American Civil War the Yankees had a secret weapon that won the North the war called the Telegraph. It was a single wire signalling device with a Battery. They connected Positive to the over head wire, and Negative to dirt. They noticed the Ground Electrode had problems with constant corrosion issues and required constant repair.

Then a few decades later Ma Bell was born. Telephone Systems use battery power, -48VDC. In the early days they bonded the Negative to Earth. Back in those days underground telephone cables had lead sheaths covering. That sheath had to be Bonded or Earthed for protection. Did not take long to learn that was a fatal mistake. The lead sheath covering corroded from Galvanic Corrosion which is when we learned about Cathodic Protection. The fix was stupid simple, bond the Positive and the problem reversed itself. Now the lead sheath cables grew from being plated by zinc or magnesium.

Today go to any airport or Disney park, and their Lightning Protection Systems and Ground Electrode Systems are protected by Cathodic Protection systems. Pretty simple to make. Take your facility Copper Ground Electrode System, and bond it to the Positive Terminal of a DC Current Generator. Then go drive a Anode Ground Rod made of Zinc or Magnesium as the sacrificial lamb. Off Shore Oil Platforms, Super Tankers, Ships, Submarines, High Rise Building all use Cathodic Protection to protect the structural integrity from corrosion and rust. I know this from experience, I spent my internship in Orlando building the Epcot Park. I was assigned to the two engineers who designed Disney Electrical System. Also spent 4 years in Silent Service on SSN-701

Now from an electrical POV, for CC, you want one that FLOATS like Midnite Solar. That allows you to bond either polarity, or smart money does not bond either polarity. Much safer and less prone to outages floating an electrical system. Put that in your pipe and smoke it.

[AL1952]

What wattage AC inverter would be the biggest I can use with 2, 6 volt, 280Ah batteries in series? (5 -100 watt mono panels and a 40a mppt SCC)

Would it be OK to use 48” #2 awg cables to go from the SCC to the batteries and also from the batteries to the inverter? I heard you have to keep them short but is there a minimum standard of some sort for these connections?

[SunEagle]

Not trying to bust your chops SE, but if one had a chose between either negative or positive ground, I would pick Positive every time.

Way back during the American Civil War the Yankees had a secret weapon that won the North the war called the Telegraph. It was a single wire signalling device with a Battery. They connected Positive to the over head wire, and Negative to dirt. They noticed the Ground Electrode had problems with constant corrosion issues and required constant repair.

Then a few decades later Ma Bell was born. Telephone Systems use battery power, -48VDC. In the early days they bonded the Negative to Earth. Back in those days underground telephone cables had lead sheaths covering. That sheath had to be Bonded or Earthed for protection. Did not take long to learn that was a fatal mistake. The lead sheath covering corroded from Galvanic Corrosion which is when we learned about Cathodic Protection. The fix was stupid simple, bond the Positive and the problem reversed itself. Now the lead sheath cables grew from being plated by zinc or magnesium.

Today go to any airport or Disney park, and their Lightning Protection Systems and Ground Electrode Systems are protected by Cathodic Protection systems. Pretty simple to make. Take your facility Copper Ground Electrode System, and bond it to the Positive Terminal of a DC Current Generator. Then go drive a Anode Ground Rod made of Zinc or Magnesium as the sacrificial lamb. Off Shore Oil Platforms, Super Tankers, Ships, Submarines, High Rise Building all use Cathodic Protection to protect the structural integrity from corrosion and rust. I know this from experience, I spent my internship in Orlando building the Epcot Park. I was assigned to the two engineers who designed Disney Electrical System. Also spent 4 years in Silent Service on SSN-701

Now from an electrical POV, for CC, you want one that FLOATS like Midnite Solar. That allows you to bond either polarity, or smart money does not bond either polarity. Much safer and less prone to outages floating an electrical system. Put that in your pipe and smoke it.

I stand corrected. Thanks for the history lesson.

[Sunking]

What wattage AC inverter would be the biggest I can use with 2, 6 volt, 280Ah batteries in series? (5 -100 watt mono panels and a 40a mppt SCC)

Would it be OK to use 48” #2 awg cables to go from the SCC to the batteries and also from the batteries to the inverter? I heard you have to keep them short but is there a minimum standard of some sort for these connections?

Are these batteries golf cart batteries by chance? If so you can easily draw a C/6 current or 280 AH / 6h = roughly 50 amps x 12 volts = 600 watts. Your system is properly sized with respect to equipment matching. 500 watts of panels into 12 volt battery requires a 40 amp MPPT controller, with a battery capacity of 240 to 450 AH.

There are two things that determine cable sizes you need to account for. To make it easy 2 AWG is overkill and safe for your application. Minimum cable size is determined by the Breaker or Fuse size. Heck with it, use this chart and use the 3% Voltage Loss Do that and you cannot go wrong. You don't t need to understand it.

Here is the LINK to full size

DC_wire_selection_chart.png

[Sunking]

Sun Eagle I strongly suspect the fixed negative ground found in many of the Controllers comes from the early days of Solar when everything was 12 volts and made to be compatible with automotive industry. I know you are not a Sparky by trade, but I think you understand some basic wiring principles used in AC electrical and NEC codes. So you know the neutral conductor can only be bonded once where service enters. Further you know why because if you bonded it downstream would allow current to flow on ground conductors right? The Automotive Industry is the only industry and technology that allows current to flow on ground. In fact they use ground as the Negative conductor, and do not use a Negative wire. Fine for Automotive, but completely unacceptable in any other technology or industry. Ground is forbidden to used as a circuit conductor. Extremely dangerous and generates a lot of noise issues.

So you can get an idea of the design quality of a Charge Controller and Inverters. Units that float the Inputs/outputs are modern design, and made to comply with electrical codes. If the Negative is hardwired to chassis, you know it is Automotive Quality not made to work in a house, plane or boat. No one cares if your car falls apart while driving due to corrosion. Everyone gets to go home.

Example if you took a Car Radio, and measured resistance between Chassis and Battery Negative Input, you would read a dead short. Do that same test on anything in your home measuring between neutral and chassis will read open circuit every time. If not throw it away or get it fixed.

There is nothing preventing any manufacture from making their equipment to Float or Bond either Polarity. That allows the designer to chose. In a vehicle bond the Negative because you have no other choice. In a home you can Float, or if you want to make over current protection cheap as possible bond the Positive.

For the Peanut Gallery; Raise your hand if you think this has something to do with Lightning protection!

Nope, has absolutely nothing to do with Lightning. Has everything to do with making fuses and breakers operate.

[Sunking]

Before I knew any better many years ago I bought some crappy 12 volt, 35amp AGM batteries to use with my 4, 100 watt solar panels. I have always been puzzled by the fact that when under full continuous load the system draws the batteries down from 12.7 resting volts to 10.3 volts (measured at the batteries) under load, over a period of 4 hours of continuous operation. The moment I remove all load the voltage goes up to 12.3 volts and stays there until I turn on the solar panels to recharge them. I was told that the state of charge should be measured at full rest, no load, no charging. If the state of charge is not supposed to go below 50%, or 12.1 volts for these batteries at rest, is it safe for me to continue operating at full load until the voltage drops to 12.1 at rest, even if the operating voltage under load drops as low as 10.0 volts or less?

Does not look like anyone answered your questions. What you are seeing is normal, and might indicate you have problems in your design and or workmanship. Possible batteries are about to give out. You have to figure it out.

All batteries have Internal Resistance no exceptions. If you know Ohms Law then you know Voltage = Current x Resistance. When you apply a load, or draw current from the battery. that current flows through the battery, wire, and connection resistance. So if you draw current you loose voltage. More current you draw, the the more voltage you loose. This tells you there is a limit to how much current you can draw from a battery before the voltage sags too low to be useful. Most equipment like Inverters have Low Voltage Shutdown set to around 11 volts on 12 volt toys.

In a properly designed system, you have to account for voltage losses. Those losses include battery voltage sag, wiring, and connectors. Your design goal is 5% maximum. 2% for battery sag, and 3% for wiring. The only head scratcher is how to determine how much current a battery can deliver before the voltage sags to 3%. Well quality manufactures tell you the internal resistance at various temps.Since that is not your battery a good ole rule of thumb works for Pb batteries. Both Max Charge and Discharge current is C/10. So if you have a 100 AH battery / 10 H = 10 Amps maximum. Wiring is very simple using charts like you see below.

Now go figure out what your problem is. 2% of 12 volts is only 0.25 volts and you are loosing 10X more than that. If I had to guess is your battery undersized , worn out from abuse , poorly installed with undersized wiring and poor connections.

DC_wire_selection_chartlg.jpg