How to monitor off grid battery bank state of charge

[BrianZ]

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

Thanks for the info and links. So it seems like to me that what I could do is add another (2) 12v 100 watt panels, and never put a load on the inverter that is over 1,000 watts or just get a new 1,000 watt inverter all together. The paperwork for my 3kw inverter says that it has a no load current of 0.9 amps. I understand that if I max out the inverter that I am risking pulling too much from the battery bank and also my 4 gauge wire I'm using from the batteries to the inverter is too small. But if I use the 3kw inverter like a 1kw inverter common sense tells me that would work safely. I don't see the circuitry in the 3kw inverter being that much higher than a 1kw inverter, in fact my Aims manual says the no load current on their 1500 watt inverter is 0.64 amps.

[Mike90250]

..... 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!

I'm going to stand way back and watch the sparks. 200A fuse is NOT any sort of safety for 4 ga wire.

4 ga wire is generally not safe beyond 75A.

https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

[lkruper]

Thanks for the info and links. So it seems like to me that what I could do is add another (2) 12v 100 watt panels, and never put a load on the inverter that is over 1,000 watts or just get a new 1,000 watt inverter all together. The paperwork for my 3kw inverter says that it has a no load current of 0.9 amps. I understand that if I max out the inverter that I am risking pulling too much from the battery bank and also my 4 gauge wire I'm using from the batteries to the inverter is too small. But if I use the 3kw inverter like a 1kw inverter common sense tells me that would work safely. I don't see the circuitry in the 3kw inverter being that much higher than a 1kw inverter, in fact my Aims manual says the no load current on their 1500 watt inverter is 0.64 amps.

It sounds reasonable that you could limit your loads to 1000w as long as you can prevent anyone else from plugging in something that exceeds it when you are not around. However, if there is a failure somewhere in your AC load, that inverter will quite happily surge double its rated capacity and maintain its 3kw power even if it means fire.

[ButchDeal]

I'm going to stand way back and watch the sparks. 200A fuse is NOT any sort of safety for 4 ga wire.

4 ga wire is generally not safe beyond 75A.
https://lugsdirect.com/WireCurrentAm...ble-301-16.htm

Agreed. He needs to change the fuse, add batteries, change the CC, add modules just to get it to a reasonable safe 1kw system. Would be safer, more efficient and possibly cheaper, to go 24v or 48v since so much needs to be changed anyway. But at minimum change that fuse.

[ButchDeal]

It sounds reasonable that you could limit your loads to 1000w as long as you can prevent anyone else from plugging in something that exceeds it when you are not around. However, if there is a failure somewhere in your AC load, that inverter will quite happily surge double its rated capacity and maintain its 3kw power even if it means fire.

As it is, it needs to be limited to 300w , it needs work to get it to 1kw

[ButchDeal]

Thanks for the info and links. So it seems like to me that what I could do is add another (2) 12v 100 watt panels, and never put a load on the inverter that is over 1,000 watts or just get a new 1,000 watt inverter all together. The paperwork for my 3kw inverter says that it has a no load current of 0.9 amps. I understand that if I max out the inverter that I am risking pulling too much from the battery bank and also my 4 gauge wire I'm using from the batteries to the inverter is too small. But if I use the 3kw inverter like a 1kw inverter common sense tells me that would work safely. I don't see the circuitry in the 3kw inverter being that much higher than a 1kw inverter, in fact my Aims manual says the no load current on their 1500 watt inverter is 0.64 amps.

Note 12v modules are more costly and as I mentioned your charge controller likely is at its limit anyway. You would need an MPPT cc to add more modules. Thus the big costs start for getting 12v systems that large.

[lkruper]

As it is, it needs to be limited to 300w , it needs work to get it to 1kw

What if an automobile was designed to be safe up to 70 miles an hour but its engine would catch fire over 70? Would anyone feel safe if they rationalized that since 70 is the maximum speed limit where they drive, they will never exceed it and be safe?

There are lots of reasons why good intentions to keep a load below 300w won't work in every scenario, especially when components can fail in unexpected ways.

[ButchDeal]

What if an automobile was designed to be safe up to 70 miles an hour but its engine would catch fire over 70? Would anyone feel safe if they rationalized that since 70 is the maximum speed limit where they drive, they will never exceed it and be safe?

There are lots of reasons why good intentions to keep a load below 300w won't work in every scenario, especially when components can fail in unexpected ways.

I was not recommending that course of action but rather pointing out the OPs plan was still way off

[BrianZ]

As it is, it needs to be limited to 300w , it needs work to get it to 1kw

Thanks everyone. I do have a MPPT controller so I'll get a couple more panels and batteries, then swap out the 200amp fuse with a 75amp. I based my system off the design by Tin Hat Ranch. Here is a link to their schematic: http://tinhatranch.com/wp-content/up...chematicR1.pdf

I purchased the same items (except I'm 2 panels and 2 batteries short at the moment), but I went with a larger inverter. That is the only difference. I thought if I mimicked their system I would be ok.

[lkruper]

I was not recommending that course of action but rather pointing out the OPs plan was still way off

Yes, I understood. I was just taking the opportunity to add the illustration that just came to me.

[ButchDeal]

Thanks everyone. I do have a MPPT controller so I'll get a couple more panels and batteries, then swap out the 200amp fuse with a 75amp. I based my system off the design by Tin Hat Ranch. Here is a link to their schematic: http://tinhatranch.com/wp-content/up...chematicR1.pdf

I purchased the same items (except I'm 2 panels and 2 batteries short at the moment), but I went with a larger inverter. That is the only difference. I thought if I mimicked their system I would be ok.

Great so you have a cc capable of 24v and doubled the modules. I would recommend you turn it off, read up a bunch before doing anything more, and consider a course of action around going to 24v as it involves much of what you already plan with just the addition changing the inverter and wiring batteries differently. Has added bonus of being able to add more modules.

[BrianZ]

Great so you have a cc capable of 24v and doubled the modules. I would recommend you turn it off, read up a bunch before doing anything more, and consider a course of action around going to 24v as it involves much of what you already plan with just the addition changing the inverter and wiring batteries differently. Has added bonus of being able to add more modules.

Yep, I think that is what I'm going to do. Thanks.