Trying to make sense of the battery State of Charge voltages

Archerite

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Sep 22, 2021
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I have an experimental setup to try the different solar electrical components before I invest in a permanent installation. (see here for a bit more info on what I have)

To start with I bought two Kung Long 12v SLA 7.2Ah. (WP7.2-12) on amazon for almost no other reason than that they were cheap and had prime. Since my primary goal at the moment is getting experience with the various components and I do not "need" to power anything from solar 24/7 yet these will do just fine I think. They allow me to test both series or parallel connected battery bank setup. For now I keep only a single one connected to the Victron MPPT at a time since I don't really know how to properly connect the batteries in parallel. As in do the voltages and Soc need to match exactly?

Since at this time of the year the sun is not that much available and I only have about 30 watts of solar panels, charging from solar alone is not exactly fast yet. Looking at charges and prices I decided to go for a Victron Blue smart 12V/5A charger since I can configure it to charge at 2A maximum that the batteries can handle. Also have a smart shunt connected to one of the batteries to get a percentage and current statistics but that's not related to my actual question.

The common recomendation is to prevent discharging an SLA battery below 50-80% but I am having trouble to determine when and how this percentage is reached. I know voltages change during charging but my battery remained at 13.2v for about a day after charging with almost no load. Only the victron smartshunt with Bluetooth active was connected.
Then I used a USB charger and the "State of Charge" displayed by the smartshunt app showed the expected drop to 85% and consumed Ah made sense too. Since it was a slow charging USB device I left it in over night because I knew it would stop when full and not drain my battery all the way. When checking this morning it was back at 100% State of charge at a 12.83 voltage!!! That was a big suprise since I thought the smartshunt also calculated the aps while charging the 12v battery that go back into the battery. I did specify a "charged voltage" as 12.8 so it was not exactly wrong....but still it's confusing!

Looking at voltage and state of charge values online I found that 12.7+ is considered 100%...but mine started at 13.2v after charging. Is this exact value different for each battery? What would be the lowest "safe" voltage to go down too? I read the battery's datasheet but it raises more questions than giving me an idea on how to correctly use the battery.

Sorry if these are dumb questions. Still very new to the solar and batteries thing here :)

Thanks
 

chrisski

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Aug 14, 2020
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3,249
I have a Victron shunt and I don’t see what you do.

Current out takes the SOC down, and current in takes the SOC back up to 100%. Kind of like 10 amps out on a 100 ah fully charged battery is 90%, and 10 amps back in adjusted for the Puekert Effect coefficient algorithm in the setup puts it back at 100%.

What made my SOC jump up from 85% to 100% immediately was the tail current setting. THe factory setting was way to high and I had to bring it down quite a bit. I think your problem sounds like this, except you’d need a charging current for this tail current setting.
 

sunshine

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Apr 24, 2020
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681
Looking at voltage and state of charge values online I found that 12.7+ is considered 100%...but mine started at 13.2v after charging. Is this exact value different for each battery?
Each battery type is different. Your 13.2v does seem to be rather high for lead acid. Maybe someone with your type can be more precise.
The best way to measure state of charge is with a hydrometer. Slightly difficult to use with SLA, AGM, GEL so we resort to voltage for calibrating the amount of net charge going in over time. I can see a problem with the small amount going into your batteries may cause inaccurate readings. Maybe not take any amp/time readings as being 100% right. This error will reduce for larger capacities.
Chemical reaction in batteries is why they gain or lose voltage after large charge/discharge events and over time.

For the low voltage discharge setting the data sheet has a range of life cycles for certain depth of discharges and from this you can go down to whatever voltage at certain loads to give you the life expectancy you want. This is beyond me so I would use the other table of voltages for certain discharge rates 10.6v - 9.6v and assume 10.6v for long life and 9.6v for shorter life. Your choice if you need one.
 

Substrate

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Pretty typical data sheet for the conventional AGM. We can work with that provided the battery is new and fully charged, not some old abused trash pulled out of a ups or other system where it has already been worked over.

Instead of voltage, we're going to use the current of your application to determine a rough 50%, because with lead acid, a little thing called "Peukert-Effect" is in play. The more current you draw, the faster voltage drops.

Look at the "Nominal Capacity" chart they provide and the hour rates of discharge.

These indicate how long it takes at a specified current to fully (100%) discharge.

The simplest measure is to cut things in half. Example:

They specify that if your application draws 0.36A at the 20-hour rate, then you should only run your stuff for 10 hours if you want to cut that down to 50%.

Likewise, if your application is pulling near the 5-hour rate as seen in the chart at 1.224A, then you should stop at 2.5 hours.

You can get a better idea by looking at the "Discharge Time vs Discharge Current" chart they provide. The time in this chart is again indicating a FULL 100% discharge. But you only want 50%.

Based on your application's current, the ballpark way is to simply reduce your application draw time in half, or reduce your application current in half from these 100% discharge values.

Trying to determine SOC based on voltage with lead-acid is not recommended, because it cannot indicate how healthy the battery is. Ie, a highly sulfated battery may charge up quickly, and even retain what *looks* like a good voltage of say 12.9v, but as soon as you put a dinky little load on it, the voltage crashes fast, not supporting the application it was designed for.

Thats why those manufacturer charts are made as *predictions* based on the consumer receiving absolutely factory-fresh batteries, and fully recharging them properly after each cycle with no sulfation. :)
 
Last edited:

Archerite

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Joined
Sep 22, 2021
Messages
38
Wow, that's a lot of reaction in a few hours! :) I'll try to respond to all of you in one post here...

Anything above 12.8v is BONUS power
I am assuming you are joking, either way it sounds funny :ROFLMAO::ROFLMAO: and far too good to be true.

What made my SOC jump up from 85% to 100% immediately was the tail current setting. THe factory setting was way to high and I had to bring it down quite a bit. I think your problem sounds like this, except you’d need a charging current for this tail current setting.
I have been reading a little about the tail current but I am not fully understanding it. But if that might be the setting for the smartshunt to give a more accurate indication that would be great. I am not sure what you mean by "you'd need a charging current for this tail current setting". Probably because I don't fully understand what it is yet, thanks for pointing it out though. :)

Each battery type is different. Your 13.2v does seem to be rather high for lead acid. Maybe someone with your type can be more precise.
The best way to measure state of charge is with a hydrometer. Slightly difficult to use with SLA, AGM, GEL so we resort to voltage for calibrating the amount of net charge going in over time. I can see a problem with the small amount going into your batteries may cause inaccurate readings. Maybe not take any amp/time readings as being 100% right. This error will reduce for larger capacities.
Chemical reaction in batteries is why they gain or lose voltage after large charge/discharge events and over time.

For the low voltage discharge setting the data sheet has a range of life cycles for certain depth of discharges and from this you can go down to whatever voltage at certain loads to give you the life expectancy you want. This is beyond me so I would use the other table of voltages for certain discharge rates 10.6v - 9.6v and assume 10.6v for long life and 9.6v for shorter life. Your choice if you need one.
Yeah, from what I have been reading 13.2v seems high to me too. But that is the voltage shown on m smartshunt after about a day of rest after charging. The inaccuracy with the voltage for determining SOC is exactly the reason why I even bought the smartshunt, so I could see the actual Ah that have been used from the battery. Also ofcourse since I have fallen for the Victron vendor lock-in already, hahaha.

Going down to 10.6v or even lower feels stretching the limits right? That low of a voltage would be considered 0% SOC?? I don't even dare going below 12.0v so far or when the shunt shows I have been drawing too much current. Even though I have only been through maybe 3-4 charge cycles since I only got the batteries for about a week now.

Pretty typical data sheet for the conventional AGM. We can work with that provided the battery is new and fully charged, not some old abused trash pulled out of a ups or other system where it has already been worked over.
The batteries are not from an old UPS as far as I know. I ordered them from amazon for the simple reason they had prime delivery and were not to expensive to buy two at once. While I have not decided on a 12v or 24v system the idea was that I could experiment with two batteries in series or parallel. If they are AGM is doubtful as it's not mentioned anywhere in the datasheet or on the battery itself. But I don't know enough about it to argue with you on that ;)

Instead of voltage, we're going to use the current of your application to determine a rough 50%, because with lead acid, a little thing called "Peukert-Effect" is in play. The more current you draw, the faster voltage drops.

Look at the "Nominal Capacity" chart they provide and the hour rates of discharge.

These indicate how long it takes at a specified current to fully (100%) discharge.

The simplest measure is to cut things in half. Example:

They specify that if your application draws 0.36A at the 20-hour rate, then you should only run your stuff for 10 hours if you want to cut that down to 50%.

Likewise, if your application is pulling near the 5-hour rate as seen in the chart at 1.224A, then you should stop at 2.5 hours.

You can get a better idea by looking at the "Discharge Time vs Discharge Current" chart they provide. The time in this chart is again indicating a FULL 100% discharge. But you only want 50%.

Based on your application's current, the ballpark way is to simply reduce your application draw time in half, or reduce your application current in half from these 100% discharge values.

Trying to determine SOC based on voltage with lead-acid is not recommended, because it cannot indicate how healthy the battery is. Ie, a highly sulfated battery may charge up quickly, and even retain what *looks* like a good voltage of say 12.9v, but as soon as you put a dinky little load on it, the voltage crashes fast, not supporting the application it was designed for.

Thats why those manufacturer charts are made as *predictions* based on the consumer receiving absolutely factory-fresh batteries, and fully recharging them properly after each cycle with no sulfation. :)
While I have no idea how peukert works I did read about it a little. It's set at 1.25 in smartshunt but I left most settings at default because I have no clue what is correct. As the shunt is only a measuring device it won't hurt much if something is configured wrong there right? Only the SOC percentage would be wrong I guess.

I am assuming that you mean with "cutting things in half" is that the battery should not be discharged more than 50% right? That much I knew, hahaha. There is no real "application" yet in my setup as it's just some experiments and learning about how it works. Off course also trying to keep it safe with fuses but I also really wanted to try a big load like an inverter. I bought a pure sine wave 300W at 12v just to test a few things with it. I have no intention to actually draw 300W from it as that's 30A at 12 volt and my wiring is not capable of that at the moment.

There were a few things I had to solder so I thought lets hook up the inverter and give it a go! I kept an eye on the reported current draw and it spiked at 9-10A but when the temperature was reason on the solder station (it's small by the way) it dropped to 2-3A. So I am guessing the solder station uses 80-100watts at the 230v from the inverter right?
The shunt indicated that it could run for less than 15minutes when I first tried it. So I hooked up the charger for a couple of hours until it went into float. Voltage after disconnecting was arround 13.4v for more than 10 minutes...but I know that's kind of short. Tried it again with the solderstation and now I could run it for arround 1h15m and I have used it for like 30 minutes I think in total. Turning the inverter off when I was getting some heatshrink tube (what's that called again??) and preserving the battery charge a bit more. Never thought I would be more cautious about the energy I use during this project. :)

Anyway, after nearly burning my fingers soldering XT60 connectors to a power meter and maybe 15-20 minutes of runtime of the inverter the "consumed Ah" is only 1.5Ah. The SOC percentage is 65%......and the voltage: 12.86V!!! I really have no idea what to believe anymore now, hahaha. It might be that because of the relatively high current draw things got out of sync or something. I am happy that the inverter fan is not always on but kicks in every now and then. It does that full speed though so it's quite noisy but only for a couple of seconds. I would rather have it spin at a low RPM and cool the components inside constantly and spin up when needed. Guess I need to buy a more expensive one for that....but that's for another time.

I am really not good at short posts. Sorry😊

Thanks everyone for responding. I really appreciate it.
 

sunshine

Solar Addict
Joined
Apr 24, 2020
Messages
681
Going down to 10.6v or even lower feels stretching the limits right?
Yes. That is why, for that V, the number of life cycles is shorter.
For lead acid to extend its life, you prevent them dropping to this level by cutting the total daily draw or adding more battery capacity or even lowering the boost charge...anything to flatten the voltage range.
Everything you are learning applies to lithium as well with only a little variation.
 

chrisski

Solar Boondocker
Joined
Aug 14, 2020
Messages
3,249
I am not sure what you mean by "you'd need a charging current for this tail current setting". Probably because I don't fully understand what it is yet, thanks for pointing it out though.
That was poor grammar. Here is a screen shot of my Victron Shunt settings for my current RV Flooded Lead Acid Battery bank:
B9318465-FF06-487F-84C7-80A57D8C285C.jpeg
I forget what the factory settings was for tail currents. It was pretty high, like 8%. So with my flooded lead acid battery bank left build phase and entered Absorbtion phase the SOC would tick up slowly and the amps would drop. As soon as the charge current got less then 8% of the 458 ah battery bank or 36 amps (at the tail end of the charge so tailings current), the SOC would jump from 85% to 100%. I still had plenty of chargibig left when my SCC was still pushing 36 amps. I dropped to 2%, now when my battery is in absorption phase, when its chargibig at less than 9 amps, the SOC will jump to 100%. So not I go from 98% or 99% to 100%. I no longer jump 15 points; only 1 or 2.
 

Archerite

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Joined
Sep 22, 2021
Messages
38
Yes. That is why, for that V, the number of life cycles is shorter.
For lead acid to extend its life, you prevent them dropping to this level by cutting the total daily draw or adding more battery capacity or even lowering the boost charge...anything to flatten the voltage range.
Everything you are learning applies to lithium as well with only a little variation.
Good to know that this applies to lithium as well. But for the moment (read couple of years) any Lithium in useful capacity is way out of my budget! ;) I am glad that I got it right with the low 10.9v being the 0% mark. Having less cylces with these batteries would not be a huge issue actually, but it's better to learn the right way to use them now so I will not damage an expensive 200+ euro battery in the future! :)

That was poor grammar. Here is a screen shot of my Victron Shunt settings for my current RV Flooded Lead Acid Battery bank:
View attachment 66625
I forget what the factory settings was for tail currents. It was pretty high, like 8%. So with my flooded lead acid battery bank left build phase and entered Absorbtion phase the SOC would tick up slowly and the amps would drop. As soon as the charge current got less then 8% of the 458 ah battery bank or 36 amps (at the tail end of the charge so tailings current), the SOC would jump from 85% to 100%. I still had plenty of chargibig left when my SCC was still pushing 36 amps. I dropped to 2%, now when my battery is in absorption phase, when its chargibig at less than 9 amps, the SOC will jump to 100%. So not I go from 98% or 99% to 100%. I no longer jump 15 points; only 1 or 2.

I see, no worries! ;) It seems like we have almost the same settings.....but I do envy your capacity though :cool:
1632821730533.png1632822175685.png

Most of my settings are the defaults but I think maybe it's just the small capacity of the battery that's making it jumping around. The shunt is rated for 500A and I am pulling max 1.5-10A with my testing stuff here. I do notice you have a really high charged voltage. But you have flooded and I have sealed lead acid ofcourse and I did not lookup if those voltages are normal.

So the tail current is something that is used during charging??? I have a limit of 2A charging for this battery so maybe I need a really low tail current percentage or something? I do know when it made it's mystery jump to 100% there was no charger connected at all!
This one is on a bench with the shunt, fuses, inverter and a bunch of wires for testing it all out. Only been charging it with the BlueSmart and it takes about 3-4 hours to reach float state. The second battery is connected to my 28w foldable solar panel in the "full sun through some light clouds" at 17 watts! 😁 I know it's peanuts compared to what most of you probably have on your RV, camper or sheds but for testing it all out it's great!

Guess I still have a lot to learn. Thanks again. :)
 

chrisski

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Joined
Aug 14, 2020
Messages
3,249
So the tail current is something that is used during charging???
Tail current is only used to decide what the % is displayed in SOC. Does not effect charging at all.

When I had it set to 8%, 36 amps, the % of SOC would jump from 85% to 100%, but the battery would continue the absorption phase and slowly drop from 36 amps to almost 0 amps where the Float phase would kick in. For me, the batteries would continue to absorb for several hours.

For the voltage, that is what the spec sheet says.
 
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