Question About State Of Charge

[Mike90250]

Speaking of power budget, on days with close to the same temps (refrigerator loads) and if the watering schedule was the same, my load and harvest almost are the same. A couple days ago, my main water pump only ran a few minutes longer than the adjacent day, and my daily harvest and recharge was almost identical. This seldom happens and when putting stuff in the spreasdsheet, I saw the data was nearly the same. Close enough I had to double check. So it can be very repeatable.

[PNjunction]

Thing is, we *still* don't know what AL1952's actual load is!

So AL1952, how much current does all your gear pull, and for how much time?

What it sounds like to me based on post #1, you either have waaay too much load current from your devices and the batts, even if healthy are simply dropping like they do when Peukert effects raises it's head with the massive voltage drop.

Either that, or your old batts are just worn out / sulfated, and have much less capacity than you originally had.

Before buying any more stuff, can you install a current-meter, or even invest in a dc-clamp-meter, turn on your gear after a full charge, and take a reading of how many amps are being drawn at one of the battery terminal connections?

This will let us know if you are simply hammering your batteries right from the start with too much demand, even if they were in perfect health. You may also have a dangerous infrastructure problem developing from loose / poor wiring and interconnects.

THIS is where one starts - measuring your need, ie a "power budget". Then you can make educated purchases - otherwise, you are just throwing darts and wallets against the wall.

[Sunking]

Should I fuse my new system the same as the old?

Maybe, maybe not. Most likely you can throw them away as you likely had no use for them on your old system, nor likely needed for the new system. your old or new system. Over Current Protection Devices are only required if you have 3 or more parallel panel strings. Otherwise not needed or required. So if you only have 1 or 2 strings of panels, no fuse required or needed.

If needed, DO NOT ask anyone here what size to use as that is foolish. The manufacture provides you with that information as part of their specifications. If the manufactures gets it wrong, they pay for damages and have deep pockets. Take someone else's advice like a DIY, you get laughed at if the guess goes wrong and you end up paying.

[inetdog]

Some PV panels in the past, required positive ground. Some systems that provide electrolysis protection for remote underground pipelines, require positive ground.

But generally, negative ground is much more common.

The problem with some earlier design panels (otherwise reliable and high quality) was charge migration between the PV junction in the cells and the supporting substrate. A slow buildup of charge eventually cut the cell output nearly to zero. A few days connected with positive ground would restore the panels to normal operation. I am not aware of any current panels that have this interaction.

[AL1952]

While I am not familiar with the Renogy Rover it looks like an MPPT type CC. Maybe someone else can comment on its' quality.

I've decided on the 40a MPPT Renogy based on a lot of great comments and reviews, and on the size of my upgraded system. Should I fuse my new system the same as the old? With 600 watts of panels, 280 amp hour 12 volt battery bank, and a 900 watt inverter how large should my fuses be from the panels to the SCC. (currently using 30a) From the SCC to the batteries (currently using 30a), and from the batteries to the inverter (currently using 100a). Am I on target or do I need to change any of these?

[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

[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]

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

[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.

[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?

[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.

[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.

[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.

[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/

[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.