SOK 48v: draining extremely quickly

[dothedr3w]

Hey everyone! I was hoping I may be able to share the problem I'm experiencing with you folks, and see if you experts have any insight. For reference, here is my current setup:

- SolArk 15k Inverter/Charger
- 5x SOK 48v 100Ah batteries
- grid tied
- no solar (yet)

Basically I live in an area where power outages are super common. My batteries aren't lasting worth a damn, and my draw is not high. I only charge the batteries to 90%, and the BMS is set to shut down them down at 20%, so I'm not taxing the batteries. 3 outages in a row I've only gotten about 6 hours of battery backup.
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I've attached a collage of screenshots from the solark app. P-Grid displays a steady 0W because it's a power outage. Notice the SOC when compared against the time stamps.

05:47-05:52 displays a 20% drop in SOC....very bad.

05:52-06:02 displays a 1% drop in SOC...more normal, still worse than expected.

06:02-08:37 displays a 42% drop in SOC....very bad. Extra bad because the BMS is programmed to shut down at 20%, and the load during this period is no more than 200W at any time.
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Anyways here's some quick math. Let's round this up so we're at max draw for those 6 hours of standby time we're getting:

1500W x 6hours = 9kWh

I've got 5 batteries connected right now:
5 x 5120Wh = 25.6kWh

I only run the batteries between 20%-90%, so I'll subtract 30% of potential from the theoretical max capacity:
25.6kWh x 0.7 = 17,920Wh

So even with my very lenient math, the batteries are running at 50%....best case scenario. And that doesn't explain the extremely sharp/almost immediate drop from 63% to 43% SOC.
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I'm absolutely stumped. I think I've got dud batteries. I bought 6, and only 5 are in the system right now as #6 is getting a BMS replacement under warranty. Batteries are kept at room temperature

 

[BentleyJ]

With the limited information provided (no individual battery or cell data) its just a guess.

It sounds like the batteries are not getting fully charged and are probably at different SoCs. Are there any warnings or alarms on any of the batteries such as low voltage shut down?

Are the settings on the inverter correct? Without Solar, the batteries have to rely on Grid Charging. First thing I would do is set the inverter to charge to 100% and enable float at 54.4V. Let them float for several hours and then review the BMS data for each battery and individual cell voltages in each battery.

 

[Q-Dog]

How do you determine your batteries are 90% charged?

 

[dothedr3w]

How do you determine your batteries are 90% charged?

The batteries are connected in a 'closed loop' with the Inverter/Charger; all the battery BMS's can communicate with one another, and to the SolArk. I set the 90% value in the SolArk.

 

[dothedr3w]

With the limited information provided (no individual battery or cell data) its just a guess.

It sounds like the batteries are not getting fully charged and are probably at different SoCs. Are there any warnings or alarms on any of the batteries such as low voltage shut down?

Are the settings on the inverter correct? Without Solar, the batteries have to rely on Grid Charging. First thing I would do is set the inverter to charge to 100% and enable float at 54.4V. Let them float for several hours and then review the BMS data for each battery and individual cell voltages in each battery.

No warning or alarms on the batteries...battery #6 was showing an alarm, but it's been out of the equation for about 6 months while it's being serviced.

After changing the float to 54.4, and charge to 100%, and record the individual cell voltages for each battery, what should I do with that information?

 

[BentleyJ]

Let the batteries charge until the inverter switches to float for a few hours then take pics of (or record) the individual cell voltages from each battery. Post them here so we can see what's going on.

EDIT: The batteries are not fully charged until you see the current drop off to below 2 amps. Called tail current.

 

[hwy17]

It's gonna be the 90/20 boundaries somehow.

Probably the 90 not balancing the batteries.

 

[dothedr3w]

Let the batteries charge until the inverter switches to float for a few hours then take pics of (or record) the individual cell voltages from each battery. Post them here so we can see what's going on.

EDIT: The batteries are not fully charged until you see the current drop off to below 2 amps. Called tail current.

Awesome will do. I'm out of town, so it won't be until late this evening, or early tomorrow morning before I can post it. But thank you!

 

[Lt.Dan]

If you never charge over 90%, then over time, the BMS is going to loose where 100% is actually at, and get worse the more time passes. So it might THINK its at 90%, but in reality its at 72.5%, 50%, or even 30%. It has no idea. Its guessing because you never give it the chance to calibrate it.

Stop worrying about babying them, and use them. Charge to 100% (SOK recommends 57.6v I think), and take them down to 5-10% (48-50v). Calendar aging is going to kill the batteries faster than cycling will.

Like mentioned above as well, charging to 90% will never allow balancing, even further exacerbating your problem.

 

[dothedr3w]

If you never charge over 90%, then over time, the BMS is going to loose where 100% is actually at, and get worse the more time passes. So it might THINK its at 90%, but in reality its at 72.5%, 50%, or even 30%. It has no idea. Its guessing because you never give it the chance to calibrate it.

Stop worrying about babying them, and use them. Charge to 100% (SOK recommends 57.6v I think), and take them down to 5-10% (48-50v). Calendar aging is going to kill the batteries faster than cycling will.

Like mentioned above as well, charging to 90% will never allow balancing, even further exacerbating your problem.

Ok...10% - 100% and 54.4v float it is. Any other parameters you fellas suggest? Ie. At what percentage should I allow the batteries to drain to before the inverter begins charging again.

 

[ABJ]

I have 3x SOK batteries and they are bad at reporting an accurate SOC if you don't fully charge to 100%.
Over the winter months I charge fully from the grid every couple of weeks.

 

[dothedr3w]

I have 3x SOK batteries and they are bad at reporting an accurate SOC if you don't fully charge to 100%.
Over the winter months I charge fully from the grid every couple of weeks.

Do you allow them to regularly charge to 100% by PV, assuming there's enough sun?

 

[ABJ]

Do you allow them to regularly charge to 100% by PV, assuming there's enough sun?

In the summer yes, but during the winter in UK we need the grid.
Before I started charge to full in winter I'd see over 25% difference between the packs.

 

[Tomthumb62]

Ok...10% - 100% and 54.4v float it is. Any other parameters you fellas suggest? Ie. At what percentage should I allow the batteries to drain to before the inverter begins charging again.

Having read a LOT on the 90/20 vs 100/10 (or 100/5 or 100/0) debate, here are my take aways:

In theory, not constantly charging to 100% or discharging to 0% will extend the life of your battery.

However, there can be costs to that method: 1)Imbalanced cells (unless you have an active balancer setup, not sure if SOK does or not) can become a problem. 2) The SoC meter getting out of true (such as what you're likely experiencing here).

In theory, using the battery "hard" by charging to 100% to 0% will enable you to get the most value from your battery, because the cells will degrade faster due to calendar aging than your "hard" use of it.

However, there can also be a cost to this method: Imbalanced cells and too high of a charge voltage (ie 3.65v, 58.4V for a 48V system) can cause the BMS to stop charging before the charger can get to float. The BMS's cell over-voltage protection can only trigger so many times before it no longer works and you need to replace the BMS. On the SOK, this isn't too difficult and adds a small $ cost to replace it. Easy enough for the DIY'r but if you're not DIY, you'll need to ship it off for repair at $$$ to do so.

If you want a "set it and forget it" method, then choose one of the theories above that makes sense to you and your use pattern and forget about it.

What I have decided is a middle ground of the two theories:

1) Most of the time, I charge conservatively. I charge to 3.5V/cell (14.0V for my 12V system, 56V for a 48V system), as I have found through testing that this is a consistent voltage that my battery can charge to without triggering the BMS cell over-volt protection and thus it reaches the float stage. Next summer (my system is of seasonal use), I will see if I can edge the balance of the battery (mine is prebuilt, only passive balancing) up to 14.2 or maybe even 14.4V (56.8 or 57.6V for a 48V system) without triggering the BMS protection.

2) On occasion I charge/discharge "hard". About 1x/mo during summer, I do a full charge-discharge cycle to re-calibrate the BMS's SoC meter. I do this by charging completely to 14.6V, which the maximum recommended for LiFePO4 (3.65V/cell or 58.4V for a 48V system) and then completely discharge to 0%. This will certainly trigger (at least on my battery) cell-over-volt and cell-under-volt protection on the BMS before all the cells reach 3.65V, but that's fine. No big deal and no harm since the BMS likely can offer this protection hundreds of times, which will outlive the calendar age of the battery. The mfr even recommends charging to 14.4 or 14.6V.

This 2-way method does require some more effort on my part, but it's not that big of a deal. Easy to do with the Victron charger and charge controller I have, which I can save different charging profiles for. Plus, it's fun as a way to learn more about LiFePO4 batteries, charging in general, etc. If I hated this sort of thing, I would just set it and forget it.

In your case, your batteries are not used daily. They're for backup purposes and so they will degrade due to calendar aging faster than anything else. I would probably fully cycle them once a month or every two months, to keep the SoC meter better calibrated. The rest of the time, I'd probably charge to 56-57V or so. 56V is about 99% SOC (your battery's SOC meter may read anywhere from 95-100% at 56V), which is "good enough" in my book. That final 1% is what stresses the battery the most and then again, there is debate about how much that stress really matters or not.

Finally, something that confused me as a beginner: The charging voltage is different than the battery's resting voltage. Say you charge your battery to 56V. If you remove all loads from the battery and wait at least 30 minutes (up to 24 or more hours for more accuracy), you will see the voltage quickly fall to 53.2 to 54V. This is the "resting voltage" and is what is completely normal for a fully-charged LiFePO4 battery. So anytime you see people mention voltage like 56, 57 or 58V, they're referring to the charging voltage, not the resting one.

 

[dothedr3w]

I should add that our battery serve more of a purpose than just backup power. We've installed our system in a fully electric tiny house, so the batteries are used daily atleast in some capacity.

We're hooked up to 50A service, so if for example we fire up the hot-water-on-demand, we easily exceed what the service can provide, so the batteries consistently jump in to 'help out'.

 

[Lt.Dan]

Ok...10% - 100% and 54.4v float it is. Any other parameters you fellas suggest? Ie. At what percentage should I allow the batteries to drain to before the inverter begins charging again.

There is common debate for float as well. I personally do not float. So I set my Bulk/Absorb/Float all to the same voltage. The argument is if you Absorb to 56-57v (roughly 3.5v), then immediately drop to 54.4v (3.4v), then you are still not allowing much (if any) balancing to happen. I like to bring my cells up to 3.5v, then hold it there, and give the balancers enough time to balance properly, where there is no confusion if a cell is actually at 100% or not. This argument is moot though, if the BMS will only balance while charging, which quite a few BMS's do.

 

[dothedr3w]

Ok. I've attached 3 photos. I'll go thru each of them by title for clarity:

Limiter Param - Limiter
- this is where I've defined that I want the batteries to charge all the way to 100%

Batt Setup - Charge
- Start % is now set to '0' so it won't attempt to begin recharging until the batteries have been fully drained
- 45A is the charging rate
- Float changed to 54.4v as advised
- Absorption/equalization left at default settings
- other parameters not pertinent

Batt Setup - Discharge
- Shutdown set up 46v/0% so the BMS actually allows it to fully drain
- Low Batt also set to 0%, I believe this is just an alarm
- Restart set to 52v/35% so the battery doesn't become available for use again until it hits that mark. From 0-35% it must charge only.

I'm all ears for what you guys think the next step should be, or for how I should adjust these parameters. And thanks everyone already for all the help

 

[dothedr3w]

Ok so after about 20 hours off the batteries being allowed to charge to 100% and actually balance themselves, here are the Pack Voltages:

Batt #1: 55.83v
Batt #2: 53.62v
Batt #3: 55.80v
Batt #4: 55.84v
Batt #5: 53.65v

Are the batteries toast? Especially #2 and #5?

 

[42OhmsPA]

Ok so after about 20 hours off the batteries being allowed to charge to 100% and actually balance themselves, here are the Pack Voltages:

Batt #1: 55.83v
Batt #2: 53.62v
Batt #3: 55.80v
Batt #4: 55.84v
Batt #5: 53.65v

Are the batteries toast? Especially #2 and #5?

Can you see individual cell voltages?
Have you checked all connections?
Are you measuring the voltages with a meter?

 

[marionw]

Do you have a copy of "SOKTools.exe"? This is the software you can use to drill down into the details of each battery, including individual cell voltages, whether a cell is Balancing (BL or BAL), Over Voltage (OVP), or Under Voltage (UVP) as well as any alarm conditions in the battery. In addition you can monitor the Charging and Discharging "MOSFETS", Charge Current and SOC

You can also save the data to a local database and export it to a spreadsheet.

You will need an RS232 to USB communications cable which connects to the RS232 port on the "Master/Host" battery.

To monitor all batteries select "FF" for "Pack" in the "Serial Port" box before opening the serial port

SOK Tools Link

RS323-USB Cable