Cell low voltage cutoff blocks charging ....sometimes??

jbatx

I make stuff with things
Joined
Jun 8, 2021
Messages
178
Location
Austin, TX
I'm seeing what I think is inconsistent behavior from my Overkill Solar 48v 16s bms. However, it's likely working as it should and the problem is me and something I've done. Anyhow, when one cell hits the low voltage cut off, the bms appears to completely disconnect from the charge controller preventing charging. However, it doesn't do this every time. When it does, it causes the controller to see 0v on the battery and completely stop pushing current to it. The only solution I've found is to restart the bms by unplugging it and plugging it back in. I think it's likely that I've made a mistake somewhere in a SW or HW config.. Just don't know where. Please help.

Yesterday in high sun I lost power due to lv cut off and it kept charging. Last night I lost power again due to lv cut off and this morning I had to restart the bms.

System details:
1. 2p16s using batteryhookup lifepo4 cylindrical modules. 32 module delivering 200ah

2. Single 48v/16s overkill bms.

3. Renogy rover 100amp charge controller

4. App version 3.1.1026

5. Cells need to be top balanced badly. I am only able to charge the pack to about 54v before one cell causes hv cutoff. I am only able to discharge the pack to about 48v before lv cut off. This is a problem I will be dealing with shortly.

6. Four 90v 750w pv strings online. Five total - but one has an issue and it switched off.


IMG_20210901_123208.jpg
 

shavermcspud

Solar Addict
Joined
Mar 12, 2020
Messages
432
Low voltage recovery not set on BMS to correct cell value maybe? And you have half answered your question by stating that you haven't balanced the pack so you're hitting low cell voltage cut off rather than total pack voltage cut off it would seem.
 

jbatx

I make stuff with things
Joined
Jun 8, 2021
Messages
178
Location
Austin, TX
Low voltage recovery not set on BMS to correct cell value maybe? And you have half answered your question by stating that you haven't balanced the pack so you're hitting low cell voltage cut off rather than total pack voltage cut off it would seem.
I'll confirm what the low voltage recovery is set at. Though, that doesn't seem like it would cause the inconsistent recovery behavior.
I'm working on the balance issue. I had balanced them prior to building the pack - however there was a bad module in the bunch that appears to have caused an issue with it's neighbor. ....that's the theory anyhow. Batteryhookup warrantied it. Anyway, I know why it's hitting lv so frequently. That just doesn't seem like a cause for the inconsistency - it's a coincidence. Would you agree?
 

MisterSandals

Participation Medalist
Joined
Nov 5, 2019
Messages
6,257
Location
Silicon Valley
1. 2p16s using batteryhookup lifepo4 cylindrical modules.
5. Cells need to be top balanced badly.
I found that if you match your 2P pairs better it will help your overall balance.
Start by removing and separating your weakest 2P pair and your strongest 2P pair. Find the strongest of the 4 cells and pair it with the weakest of the 4 cells. I often did this from a discharged state near LVD. When all 4 are disconnected from each other, the lowest voltage was the weakest (lowest capacity), the highest was the strongest.

I've done this with many iterations and I am convinced it helps quite a bit.

Hopefully this makes sense.
 

jbatx

I make stuff with things
Joined
Jun 8, 2021
Messages
178
Location
Austin, TX
I found that if you match your 2P pairs better it will help your overall balance.
Start by removing and separating your weakest 2P pair and your strongest 2P pair. Find the strongest of the 4 cells and pair it with the weakest of the 4 cells. I often did this from a discharged state near LVD. When all 4 are disconnected from each other, the lowest voltage was the weakest (lowest capacity), the highest was the strongest.

I've done this with many iterations and I am convinced it helps quite a bit.

Hopefully this makes sense.
Solid plan. I was thinking similarly sort of. The plan is the split them, rebalance them and reconstruct two 16s banks instead of the one. I just ordered a second bms.
 

MisterSandals

Participation Medalist
Joined
Nov 5, 2019
Messages
6,257
Location
Silicon Valley
Make sure you label them (i recommend using letters A,B,C...) and take good notes of voltages and cell position (which is a number). Good luck.

I'm not sure if its better to put the best 16 in one 16S battery or what. I found that pairing strongest with weakest in a 2P was a good way of helping out the weakest because they can't hide or cut the mustard on their own.

Keep us posted on what you learn!
 

jbatx

I make stuff with things
Joined
Jun 8, 2021
Messages
178
Location
Austin, TX
Make sure you label them (i recommend using letters A,B,C...) and take good notes of voltages and cell position (which is a number). Good luck.

I'm not sure if its better to put the best 16 in one 16S battery or what. I found that pairing strongest with weakest in a 2P was a good way of helping out the weakest because they can't hide or cut the mustard on their own.

Keep us posted on what you learn!
The two benefits that I want from splitting them in the two banks are..
1. Redundancy in the event of an issue with one bank. I don't loose power if I need to fix something, etc, etc
2. Troubleshooting a wonky module is simpler since the bms data is specific to one module (made of many cylinders) instead of a pair made from 2x the cylinders.

...primarily, I want the redundancy.

[UPDATE since the original post]
...This might be pretty cringy to some folks. This was just an experiment because I'm in a pinch to get more capacity from this bank right now.

I sort of sandbagged the one module pair that liked to hit the cell over voltage early. This pair is/was limiting the bank to 53.4-53.7 volts max. I said 54 earlier for simplicity sake. All day yesterday, I left three 12v 26w old school automotive bulbs (the sandbags) connected in parallel to that pair while the bank was charging. I also moved the minimum balance voltage setting to 3.5 from 3.2 - this was to prevent the bms from trying to balance while the sandbags were connected. I reasoned that the sandbags would interfere with balancing and would just clutter up the experiment in the first place. Anyhow, by 4pm, the v diff on the bank was .007 - .010 and the bank was at 54.1. After removing the sandbags and monitoring the charging for another 45 minutes, the v diff fluctuated up and down .008-.011. I don't expect this to be a long term fix... but, it got me more capacity for now.
 

MisterSandals

Participation Medalist
Joined
Nov 5, 2019
Messages
6,257
Location
Silicon Valley
...primarily, I want the redundancy.
The reality of having 2P pairs is that you also have redundancy in a finer grained form.
Troubleshooting a wonky module is simpler since the bms data is specific to one module (made of many cylinders) instead of a pair made from 2x the cylinders.
Absolutely! Some of my initial cell capacity findings came from having the cells in 4S so that i could see individual cells. I have 13 cells (1 extra) and the goal was to make a good 2P4S battery for the RV and a 4S emergency and greenhouse battery made with the worst 4 cells (excluding the absolute worst 1 cell).

Great work so far! Thanks for keeping us posted.
 

HRTKD

Boondocker
Joined
Apr 24, 2020
Messages
7,722
Location
Somewhere South of Denver
Figure out why your BMS had to cutoff discharge. Your load, presumably an inverter, should be programmed to shut off well before the BMS has to. By doing this, it still allows discharge to other devices, like the solar charge controller.
 

MisterSandals

Participation Medalist
Joined
Nov 5, 2019
Messages
6,257
Location
Silicon Valley
Your load, presumably an inverter, should be programmed to shut off well before the BMS has to.
That would be ideal. I've never owned an inverter that could be programmed! And I think the LVD that I recall is from 10V to 11V. Both far lower than i hope to ever see from my precious batteries.
What would you recommend for BMS settings for this situation?
 

HRTKD

Boondocker
Joined
Apr 24, 2020
Messages
7,722
Location
Somewhere South of Denver
That would be ideal. I've never owned an inverter that could be programmed! And I think the LVD that I recall is from 10V to 11V. Both far lower than i hope to ever see from my precious batteries.
What would you recommend for BMS settings for this situation?

Well, my inverter can't be programmed either. But it's only a puny 1000 watt inverter that doesn't have much to do in my RV. My next inverter will be programmable.

My thought is that if the battery is hitting the low voltage disconnect and assuming nothing is wrong (wiring, settings, etc) then the system is not designed correctly. I would start with the theory that there is not enough battery for the loads, or there is simply too much load for the battery.
 

jbatx

I make stuff with things
Joined
Jun 8, 2021
Messages
178
Location
Austin, TX
The reality of having 2P pairs is that you also have redundancy in a finer grained form.

Absolutely! Some of my initial cell capacity findings came from having the cells in 4S so that i could see individual cells. I have 13 cells (1 extra) and the goal was to make a good 2P4S battery for the RV and a 4S emergency and greenhouse battery made with the worst 4 cells (excluding the absolute worst 1 cell).

Great work so far! Thanks for keeping us posted.

Not arguing, just stating my own qualifications in order to meet the needs of this system. That is, redundancy.

This is redundant:

1. I must be able to service the thing with out taking the system off-line. "hot-swappable" batteries, in this case.
2. If the thing stops functioning for whatever reason, the system does not go offline. The reasons not with standing, these are just the qualifications. Use case - If any cell in bank A hits lvc for any reason, cutting off discharge from A, bank B continues to discharge and the system stays online. Fire, explosion, flood, aliens, theft, are outside the scope here... those are not things I can plan and architect around in this particular case.

Too much load can be adjusted for by adding additional banks to the hot swappable layout - which is just busbars with additional studs and an additional breaker.

Caveat... This is my first big-ish solar build. I'm not an ee. Just an old hacker with obsessive tendencies. :)
 

jbatx

I make stuff with things
Joined
Jun 8, 2021
Messages
178
Location
Austin, TX
Well, my inverter can't be programmed either. But it's only a puny 1000 watt inverter that doesn't have much to do in my RV. My next inverter will be programmable.

My thought is that if the battery is hitting the low voltage disconnect and assuming nothing is wrong (wiring, settings, etc) then the system is not designed correctly. I would start with the theory that there is not enough battery for the loads, or there is simply too much load for the battery.
Mine has a low v cut-off. At this moment, I don't recall what it is. It's not user programmable.

Regarding your comment:
By doing this, it still allows discharge to other devices, like the solar charge controller.
I don't understand what the ramifications of that would be. The charge controller stops charging when the bms does the issue that this thread started with. That is, sometimes it completely blocks charging when it triggers the low voltage cut-off. ...sometimes it doesn't

We know with certainty that modules in a pair (pair #4 to be specific) were well out of balance with the other modules. This may have been remedied, however temporarily, with the sandbagging hack that I wrote about above. We shall see. Regardless, when I split the modules into two banks, I will rebalance them all.... not looking forward to that saga - I need a better power supply!

I have a moderate amount of load on the system. Peak usage is about 2200w while running the MrCool 12K on AC mode and an induction cook top and charging my laptop. But, that's a peak just for a 15-20 minutes while cooking. With the Mr Cool on dry mode, it pulls only about 200w... which is amazing :cool:. I'm in central TX and at relatively high elevation for the area - a lot of sun. ...but, I digress.

As for the hardware design, the part that I'm concerned about is the series connection for the + mppt output to the inverter and then to the + bus and then to the + battery terminal. I probably should have connected all to the bus.
 

MisterSandals

Participation Medalist
Joined
Nov 5, 2019
Messages
6,257
Location
Silicon Valley
the part that I'm concerned about is the series connection for the + mppt output to the inverter and then to the + bus and then to the + battery terminal. I probably should have connected all to the bus.
Yea, that doesn't look right. All to the bus is better.
I really like to keep charging wires/fuses separate from load wires/fuses mostly for simplicity and usually a better fit for the fuses (who charges at the same amperage as their loads?)

So i go from SCC to breaker to battery.
Also, battery to fuse to loads (bus bars).
This helps me conceptually and operationally if i have to work on or test (voltage/amps) any part, i can isolate smaller areas.
 

HRTKD

Boondocker
Joined
Apr 24, 2020
Messages
7,722
Location
Somewhere South of Denver
I don't understand what the ramifications of that would be. The charge controller stops charging when the bms does the issue that this thread started with. That is, sometimes it completely blocks charging when it triggers the low voltage cut-off. ...sometimes it doesn't

We know with certainty that modules in a pair (pair #4 to be specific) were well out of balance with the other modules. This may have been remedied, however temporarily, with the sandbagging hack that I wrote about above. We shall see. Regardless, when I split the modules into two banks, I will rebalance them all.... not looking forward to that saga - I need a better power supply!

When cells are no longer in balance (i.e. you have a runner) then all bets are off. Chargers and programmable loads know nothing about the cell voltages unless they have an inside source to the BMS. The battery could be at 12.6 while one cell could be below 3.0v.

Under normal operations (well balanced cells), a programmable load may be set to stop operating when the battery voltage is 12.0v (throwing out a round number). The BMS may have a low voltage disconnect of 11.9v. This way, the BMS/battery will still be active, with the big load shutdown.

In theory, a non-smart charge source will keep things humming along should things go bad enough that the BMS shuts down discharge. It's going to pump out some current if it sees a low voltage. The Overkill Solar/JBD BMS is supposed to keep the charge "port" open even if the discharge port is closed. That would allow the charge from the non-smart charger to bring the battery back up.
 

jbatx

I make stuff with things
Joined
Jun 8, 2021
Messages
178
Location
Austin, TX
When cells are no longer in balance (i.e. you have a runner) then all bets are off. Chargers and programmable loads know nothing about the cell voltages unless they have an inside source to the BMS. The battery could be at 12.6 while one cell could be below 3.0v.

Under normal operations (well balanced cells), a programmable load may be set to stop operating when the battery voltage is 12.0v (throwing out a round number). The BMS may have a low voltage disconnect of 11.9v. This way, the BMS/battery will still be active, with the big load shutdown.

In theory, a non-smart charge source will keep things humming along should things go bad enough that the BMS shuts down discharge. It's going to pump out some current if it sees a low voltage. The Overkill Solar/JBD BMS is supposed to keep the charge "port" open even if the discharge port is closed. That would allow the charge from the non-smart charger to bring the battery back up.

gotcha - we're on the same page. Do you think that the unbalanced cell is causing the BMS to behave like it is? That is, cutting charge as well when it executes the lvc?
 

HRTKD

Boondocker
Joined
Apr 24, 2020
Messages
7,722
Location
Somewhere South of Denver
gotcha - we're on the same page. Do you think that the unbalanced cell is causing the BMS to behave like it is? That is, cutting charge as well when it executes the lvc?

No. What I think is happening is that your charge device isn't getting any juice from the battery, so it doesn't know what to do, or is simply not on.

Somewhere on the forum, someone posted results of a high voltage disconnect test on the brand of BMS we're using. Charging was blocked, but discharge was still very much alive. I would expect the same behavior on low voltage disconnect.

However, I do think the low cell voltage is the root of your problem.
 

jbatx

I make stuff with things
Joined
Jun 8, 2021
Messages
178
Location
Austin, TX
I'll do the following
  1. Rewire scc, inverter, battery to all use the bus instead of the goofy series connection, as I mentioned above.
  2. Once I receive the new bms
    1. rebalance all cells
    2. build two banks
    3. connect both banks on the buses on their own posts
    4. test/monitor
 

HRTKD

Boondocker
Joined
Apr 24, 2020
Messages
7,722
Location
Somewhere South of Denver
I'll do the following
  1. Rewire scc, inverter, battery to all use the bus instead of the goofy series connection, as I mentioned above.
  2. Once I receive the new bms
    1. rebalance all cells
    2. build two banks
    3. connect both banks on the buses on their own posts
    4. test/monitor

That is similar to my implementation. For #3, my two batteries connect at the shunt on the negative side and the fuse on the positive side. If I end up with more than two batteries, I will put in a new bus bar where all three batteries negative cables can connect to and then that bus bar will connect to the shunt. I'll something similar on the positive side.
 

jbatx

I make stuff with things
Joined
Jun 8, 2021
Messages
178
Location
Austin, TX
Which shunt are you
That is similar to my implementation. For #3, my two batteries connect at the shunt on the negative side and the fuse on the positive side. If I end up with more than two batteries, I will put in a new bus bar where all three batteries negative cables can connect to and then that bus bar will connect to the shunt. I'll something similar on the positive side.
Which shunt are you using? I don't have one installed yet
 
Top