How to monitor off grid battery bank state of charge

[BrianZ]

Hi everyone,

I just put together a small off-grid solar setup. The system consists of:
- (2) 100 watt solar panels
- (2) 235AH 6 volt Crown RE batteries
- Renogy Tracer 4210 charge controller with LCD display (20ft of 10 gauge wire going from the panels to the charge controller, 8ft of 4 guage wire from charge controller to the batteries)
- Aims 6,000 watt pure sine inverter

This is my first dive into anything solar related so forgive me for my lack of knowledge. My issue I'm having is on the LCD screen, it says my battery voltage is at 12.7 volts. The minute I switch on any load on the inverter (16 watt LED light bulb for instance), the LCD display shows my voltage dropping immediately down to around 12.3 volts. When I then turn the load off, the volts within about 60 seconds go back to 12.7. My question is whether or not this is normal? I don't understand how 0.4 volts could be used almost instantaneously when under such a light load.

If that is typical, then how am I ever supposed to know when I run the risk of running the batteries down below 50%? From what I read online the batteries are at a 50% state of charge when the volts read 12.06. When I put a heavier load (small water distiller) the volts almost immediately drop to 11.9. So almost immediately after plugging in my small water distiller which draws about 550 watts my batteries go from full down to below 50%? That doesn't make any sense to me, but when the load is unplugged the volts quickly climb back up even when it's overcast outside. This leads me to believe that the volts did not go down as far as the LCD is telling me since there is no way it could recharge that quick with overcast skies.

The charge controller LED's are all lit green, so it looks like everything is working correctly. The batteries are both brand new so I don't think anything is wrong with those.

How can you get an accurate state of charge on the batteries when it changes so dramatically with any sort of load on it? Or are my batteries not working properly?

Sorry if this is a "rookie" question. Any help would be appreciated.

[Mike90250]

You don't have enough solar PV to charge those batteries properly, at least one more panel would help.
Also, the battery is not fully charged, that will take a couple days with the panels you have, and no loads to get them charged up to where they should be.

[Raul]

Another think is that battery bank can support a 300w inverter only . Anything higher than that it's just killing it and will see massive voltage drop because the load is way higher than the battery can cope with.

[BrianZ]

You don't have enough solar PV to charge those batteries properly, at least one more panel would help.
Also, the battery is not fully charged, that will take a couple days with the panels you have, and no loads to get them charged up to where they should be.

Interesting, I started out small and was going to increase the number of panels and batteries anyway. I guess I just assumed wrong that the batteries had a full charge after buying them new. Thanks for the quick response.

[BrianZ]

Another think is that battery bank can support a 300w inverter only . Anything higher than that it's just killing it and will see massive voltage drop because the load is way higher than the battery can cope with.

Thanks for the quick response. Strange, I figured it didn't matter what size inverter you have as long as you weren't using a large load on it. So somehow the inverter pulls more power than the load on it just because it's rated for 6,000 watts? By using your calculation of (2) 12v 235amp hour batteries only are supported by a 300w inverter, does that mean for this system to run efficiently I would need to have 40 batteries ((6000watts / 300w) X 2))?

[Sunking]

You don't have enough solar PV to charge those batteries properly, at least one more panel would help.

Nor does he have near enough battery to to power a 6000 watt 12 volt Inverter. He just made another bomb.gif

Brian nothing you have is made to work with the other. What you failed to do is make any sort of plan to do what you want. You just went out and mindlessly elected NoBama because it sounded cool. Look what it got you.

I will show you how far off you are by working your system both ways. ! based on what a 200 wat solar panel can support, and another is minimum requiremeent using a 6000 wat 12 volt inverter that should be illegal.

A 200 watt panel takes a minimum 15 Amp MPPT Controller, to charge a 12 volt 120 to 140 AH battery to support a 200 to 300 watt Inverter.

To Support a 6000 watt Inverter at 12 volts (Bomb) requires a 12 volt 4000 AH battery weighing 3000 pounds costing $10,000. To support that battery requires a minimum 5000 watt Panel with 6 x 80 amp MPPT controllers. The cable required between the 3000 pound battery and 6000 watt coffin of an Inverter would need to be the size of your leg costing $25/ft .

Lastly 6000 watt Inverter at 12 volts is just plain ole ignorance and border line STUPID. You are asking for one hell of a fire. Solar Industry considers anything over 100 amps dangerous which in my professional oppinion is wat to high because DIY have no clue how to properly terminate cables. 100 amps @ 12 volts is a maximum 1000 watt Inverter. A 12 volt 1000 watt inverter is very inefficient and dangerous.

The largest Charge Controller you can buy is 80 amps. Assuming it is a MPPT type you have the following power input limitations vs battery voltage:

1000 watts @ 12 Volt Battery
2000 Watts @ 24 Volt Battery
4000 watts @ 48 Volt Battery

But here is all you need to know as a general rule. Inverter Power shall not be greater than Panel Wattage.

[Raul]

You don't have 2 235a 12v , you have 2 6v 235 and you wired them for 12v so you still got 235a now at 12v . This can support a max 23ax12v=276w load.
A 6000w inverter should only be powered by a 48v bank or higher. A12v system can safely and efficiently use only a 1000w inverter .

[Mike90250]

Just the act of turning on my 300w suresine inverter, it draws about 0.02 amps

Turning on a 6,000 w inverter will likely consume 3 or 4 amps, just to power up the control circuits, so, going way oversize on an inverter will increase even your idle consumption. And if you have a 200w load on it, and it's only 50% efficient at that load, you are consuming 300w. The suresune with 200w load is about 90% efficient, so only about 220w is consumed.

The Bomb part comes in if you have a 3,000 w load, @ 12V, that would be 250amps, which will quickly over heat the cheap tiny wires in the cheap inverters. (plus losses)

[BrianZ]

Nor does he have near enough battery to to power a 6000 watt 12 volt Inverter. He just made another [ATTACH=CONFIG]8093[/ATTACH]

Brian nothing you have is made to work with the other. What you failed to do is make any sort of plan to do what you want. You just went out and mindlessly elected NoBama because it sounded cool. Look what it got you.

I will show you how far off you are by working your system both ways. ! based on what a 200 wat solar panel can support, and another is minimum requiremeent using a 6000 wat 12 volt inverter that should be illegal.

A 200 watt panel takes a minimum 15 Amp MPPT Controller, to charge a 12 volt 120 to 140 AH battery to support a 200 to 300 watt Inverter.

To Support a 6000 watt Inverter at 12 volts (Bomb) requires a 12 volt 4000 AH battery weighing 3000 pounds costing $10,000. To support that battery requires a minimum 5000 watt Panel with 6 x 80 amp MPPT controllers. The cable required between the 3000 pound battery and 6000 watt coffin of an Inverter would need to be the size of your leg costing $25/ft .

Lastly 6000 watt Inverter at 12 volts is just plain ole ignorance and border line STUPID. You are asking for one hell of a fire. Solar Industry considers anything over 100 amps dangerous which in my professional oppinion is wat to high because DIY have no clue how to properly terminate cables. 100 amps @ 12 volts is a maximum 1000 watt Inverter. A 12 volt 1000 watt inverter is very inefficient and dangerous.

The largest Charge Controller you can buy is 80 amps. Assuming it is a MPPT type you have the following power input limitations vs battery voltage:

1000 watts @ 12 Volt Battery
2000 Watts @ 24 Volt Battery
4000 watts @ 48 Volt Battery

But here is all you need to know as a general rule. Inverter Power shall not be greater than Panel Wattage.

Thanks for the response. I actually have a 3k watt inverter, the surge watts are 6k. My mistake. I actually have read a highly rated solar book and watched about 100 videos on solar. Funny nobody ever mentioned that your panels need to match your inverter. The only comments made on the inverter was that it has to be rated high enough to handle whatever type of loads you have on it. I went large because that was the only piece of the system that could not just be added onto like batteries / solar panels. Seems to be a critical piece of information that people neglect to talk about.

LOL, by the way I didn't vote for Obama. I actually worked under Peter Schiff at Euro Pacific Capital and believe in the teachings of people like Ludwig Von Mises and Lew Rockwell. So I'm about as free market as you get I do believe in learning by getting your hands dirty, and making mistakes is the best way to learn. I have quite a bit of experience with A/C electrical as many years ago I worked as an apprentice for an electrical contractor. I'm new to DC though.

[ButchDeal]

Thanks for the response. I actually have a 3k watt inverter, the surge watts are 6k. My mistake. I actually have read a highly rated solar book and watched about 100 videos on solar. Funny nobody ever mentioned that your panels need to match your inverter. The only comments made on the inverter was that it has to be rated high enough to handle whatever type of loads you have on it. I went large because that was the only piece of the system that could not just be added onto like batteries / solar panels. Seems to be a critical piece of information that people neglect to talk about.

Most of the online videos are garbage. You don't need to match the inverter to the PV, you need to match the inverter to the load, then the battery to the inverter/load, then the PV to the batter/load. You have a completely unmatched system. In fact a 3kw 12V inverter is a bit of a mess. Hard to supply enough power at 12V for that. A 3kw inverter should be 24v or even better 48v.
At 3kw 12v you would need some heavy copper from battery to inverter to keep from dropping and some nice breakers.

[Sunking]

Thanks for the response. I actually have a 3k watt inverter, the surge watts are 6k.

Does not change a thing, still 1000% more than what you battery can handle. Instead of you sitting on a 1000 bomb, it is a 500 pound bomb. Either way when it blows up you are vaporized and only a picture of you for the funeral.


Kind of like that looser John Daily the pro golfer. He says he quit drinking. Instead of drinking a quart of whiskey every day, he drinks a case of beer everyday.

[BrianZ]

Most of the online videos are garbage. You don't need to match the inverter to the PV, you need to match the inverter to the load, then the battery to the inverter/load, then the PV to the batter/load. You have a completely unmatched system. In fact a 3kw 12V inverter is a bit of a mess. Hard to supply enough power at 12V for that. A 3kw inverter should be 24v or even better 48v.
At 3kw 12v you would need some heavy copper from battery to inverter to keep from dropping and some nice breakers.

Thanks for the info. You mentioned that my system is completely unmatched. Can you provide more clarity on that? I understand the 3kw inverter is more powerful than I need with the battery bank I currently have. I was going to increase the battery bank to (8) 6 volt batteries very soon. I also wasn't going to put much of a load on the inverter until getting more batteries. My charge controller is rated for a max of 500watts and 40 amps. I was going to add another 3 100watt solar panels very soon as well. Can you tell me where else I'm unmatched besides having an inverter that if running a large load is way too big form my battery bank? By the way, I am using 4 guage wire from the batteries to the inverter, I also added an in-line 200amp fuse between the batteries an inverter on the positive leg for safety. Thanks!

[Sunking]

Funny nobody ever mentioned that your panels need to match your inverter. The only comments made on the inverter was that it has to be rated high enough to handle whatever type of loads you have on it. I went large because that was the only piece of the system that could not just be added onto like batteries / solar panels. Seems to be a critical piece of information that people neglect to talk about.

Why would anyone talk about it? Info is out there if you look to professionals. It is not the job of manufactures, schools, and suppliers to educate you. You can buy ganja, booze, tobacco, guns, or whatever legally. You can buy all sort of products and no way to use it safely or in some cases legally.

To use your analogy what kind of vehicle do you need to haul say a motor cycle around? Most people will say a PU Truck. You bought a 18-wheel tractor-trailer. Don't feel bad, you are not alone. You made the same mistake 99% of the folks who come here, they failed to plan, and that is a plan to fail every time.

[ButchDeal]

Thanks for the info. You mentioned that my system is completely unmatched. Can you provide more clarity on that? I understand the 3kw inverter is more powerful than I need with the battery bank I currently have. I was going to increase the battery bank to (8) 6 volt batteries very soon. I also wasn't going to put much of a load on the inverter until getting more batteries. My charge controller is rated for a max of 500watts and 40 amps. I was going to add another 3 100watt solar panels very soon as well. Can you tell me where else I'm unmatched besides having an inverter that if running a large load is way too big form my battery bank? By the way, I am using 4 guage wire from the batteries to the inverter, I also added an in-line 200amp fuse between the batteries an inverter on the positive leg for safety. Thanks!

You really need to review some of the documents on here. Look at how much copper is needed for a 3kw at 12v system. your best bet really is to sell the 12v inverter and get a 48v one, get a matching 48v MPPT CC. If you keep going with the 12v 3kw system, you will just be tossing more good money after bad design.
bTW 4 gauge wire is WAY too small. I run 0 gauge ( size of my thumb) on a 3kw 48v system which is minimal. At 12v would need copper bars to handle the load.
You might check your CC as welling, might be rated for max 500w at 24v but only 250w at 12v ( that is output voltage, meaning battery voltage) A common situation.

[Sunking]

Thanks for the info. You mentioned that my system is completely unmatched. Can you provide more clarity on that?

It has already been explained to you. Let's try another approach.

Batteries have a maximum discharge rate they can sustain without significant voltage loss and heating. For Deep Cycle FLA batteries C/8 is that maximum limit. Some Hygrids and AGM's can go as hih as C/4 and pure lead AGM's can go as high as C/1 or 1C.

C = Amp Hour Capacity of a battery rated at its 20 hour discharge rate. In your case 225 AH. So if discharged at the 8 hour rate the maximum current that battery can sustain without significant voltage drop is 225 AH / 8 Hours = 28 to 29 amps. So how much power is that at 12 volts? 3rd grade math. Power = Voltage x Amps = 12 volts x 29 amps = 348 watts. If you draw 350 watts from your 12 volt 225 AH battery battery assuming it is fully charged up to 12.6 volts you have to limit voltage loss to 3% or less. .03 x 12 volts is only .36 volts. Which means you voltage drops instantly to roughly 12.2 volts.

It also tells you your battery Ri aka inte

rnal resistance is .36 volts / 29 amps = .0124 Ohms. So now lets see what happens when you try to draw 300 amps that is required to run your 3000 watt inverter. 300 amps x .0124 Ohms = 3.75 volts, or your battery voltage drops from 12.6 volts to 8.8 volts. You would never see that because your Inverter quit working at 11 volts and shut down. Your battery is fully charged but will not work. If by some miracle you found an Inverter that could go to that low of a voltage using 300 amps woul dheat that battery up to the boiling point is a few short minutes spewing battery acid every where. But that would be the last of your worries. Unless the cables are the size of your arm and properly terminated, the copper would melt and start a fire with a big BOOM of an explosion. So now you are on fire covered in battery acid.

Now let's talk charge side. The minimum requirement is C/12 to produce enough gassing to prevent stratification and dissolve soft lead sulfate crystals. Maximum charge rate is the same C/8 current as on the discharge side. So for a 225 AH battery you need a minimum 225/12 = 19 amps and a maximum 225/8 = 28 amps. Let's just say minimum is 20 amps and max is 30 amps. If you assume 12 volt battery and using a MPT controller means the minimum panel wattage required for a 12 volt 225 AH battery is 20 amps x 12 volts = 240 watts. Max is 30 amps x 360 watts. Perfect is C/10 or 22.5 amps x 12 volts = 270 watts.

So if you tie it all together you can instantly tell without even thinking if a system components are matched up. When someone comes here and says they have a 200 watt panel, 12 volt 225 AH battery with a 3000 watt Inverter, we are ROFL. Now you know the reason as you are not even in the Ball Park of a workable system. Heck not even on the right planet and came from a Make Believe World of fantasy. Happens here on a daily basis.

Not personal, just plain ole ignorance.

All you had to do was read the Stickies like:

How Many Batteries

Battery vs Inverter

Sizing Off-Grid Battery System