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Tesla Battery Module Voltage Imbalance/Drift in 48v systems

SolarBro

Solar Budgeteer
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My system for reference. https://diysolarforum.com/threads/10-kw-diy-install-mpp-solar-lv5048-2x-tesla-modules.10285/

So i'm having an issue, over the past 2 months by tesla modules have been shifting in voltage between each other. @Will Prowse

In week one - Module 1 - 24.1v / Module 2.0 - 24.1v.

2 months later - Module 1 - 23.9v / Module 2 - 24.2v - A .3v difference!

Im trying to understand why there is this shifting voltage difference between the modules. I've been having a hunch about it.

1. Do my positive and negative leads from the battery's need to be the EXACT same length?

If so, Do i compensate for the positive lead having to go through my breakers and fuse, thus adding length to the positive run? Is this an issue? should i make my negative lead longer then?

1a. I have 2 gauge windy nation wire (200 ampacity) going to my LV5048 inverter but i have 4/0 copper cobra-flex (400 ampacity) inbetween the modules. Is this an issue?

2. Is my LV5048 doing un-even charging? Any reason why it would do this?

3. Should I switch the battery position so that i reverse how the negative and positive currently run through the packs? Maybe there is an imbalance in one pack over another?

4. Is one pack weaker than the other? and is there anything i could do to compensate for this?

I'm trying to figure this out, meantime i will pull the battery's and put them in parallel to balance them again for now.
 
1. No. you're in series. it doesn't matter if one is 2X the length of the other. The electrons don't know the difference - only the total path.

1a. No.

2. No. Not possible. Series connection means ALL components in the series experience the EXACT same current. Period. No exceptions.

3. No, but you're very imaginative. See #2

4. Yep. You simply have two different packs with two different capacities and/or internal resistances, OR, you've had a few cells pop their fused wires (which would equate to different capacities.

Welcome to the reality of un-managed Lithium batteries. I hope you're checking individual cell voltages.

Assuming this isn't traceable to a gross individual cell imbalance...

Options:
  1. Get a 12S balancing BMS
  2. Get 12 of these and attach to each cell group: https://www.electriccarpartscompany.com/3v-1s-lithium-lighted-battery-balancers - they will actively shift up to 4A of current between individual cell groups and all CELLS will eventually be within 0.01V. I've personally used these when I had a 7S NMC 24V bank.
  3. Periodically break your pack down and balance charge them to 24.3V while monitoring individual cell groups.
 
1. No. you're in series. it doesn't matter if one is 2X the length of the other. The electrons don't know the difference - only the total path.

1a. No.

2. No. Not possible. Series connection means ALL components in the series experience the EXACT same current. Period. No exceptions.

3. No, but you're very imaginative. See #2

4. Yep. You simply have two different packs with two different capacities and/or internal resistances, OR, you've had a few cells pop their fused wires (which would equate to different capacities.

Welcome to the reality of un-managed Lithium batteries. I hope you're checking individual cell voltages.

Assuming this isn't traceable to a gross individual cell imbalance...

Options:
  1. Get a 12S balancing BMS
  2. Get 12 of these and attach to each cell group: https://www.electriccarpartscompany.com/3v-1s-lithium-lighted-battery-balancers - they will actively shift up to 4A of current between individual cell groups and all CELLS will eventually be within 0.01V. I've personally used these when I had a 7S NMC 24V bank.
  3. Periodically break your pack down and balance charge them to 24.3V while monitoring individual cell groups.

Sounds good. I'll do a passive balance between the packs for now but I think plan no.2 is easier for me to understand and implement.

I'll get 12 of those 1s units. I'm not super literate on BMS's and I think wiring each balancer individually will help me learn about BMS.

Did you have a pic or photo or diagram on how you used that on your 7s pack? It would be greatly appreciated.
 
On the product page, there is an example:

1599883949363.png

I have added green lines to indicate each cell is wired to the next in series.

The challenge will be in how you affix them to the cell groups. I don't know how they're laid out or if there are convenient places to attach them. You might need to cut the ring terminals off and solder them somewhere.

Note that this is in no way a BMS. These are just active balancers. They will attempt to keep all connected cells to the same voltage within 0.01V by shifting up to 4 amps from high to low cells. Note that 4A is pretty rare. When I did experiments with grossly imbalanced cells, I never saw more than 1.3A or so. Even so, that beats the snot out of the passive balancers that only bleed down high cells at 80mA or so.

You might also consider a couple of these, one for each module:


They have an extremely low current passive balancing mechanism, but their ability to report all cell voltages would be very handy.

To be clear, I vigorously recommend a meaningful BMS over these options; however, education, diligence, active balancing and monitoring may suffice.
 
Allright, i got a 12s active balancer. I'll update once i start installing it soon.
 
Was out of town at Honey Badger Ranch. Too much to do to get on the computer. I know nothing about that unit, but it looks like it will do the trick if it meets spec; however, it does not appear to have a "protect" function where the battery is disconnected if a cell goes out of range.
 
Was out of town at Honey Badger Ranch. Too much to do to get on the computer. I know nothing about that unit, but it looks like it will do the trick if it meets spec; however, it does not appear to have a "protect" function where the battery is disconnected if a cell goes out of range.

Thanks for the help. I think the balancer is ok because im never getting close to 100% State of charge. If a cell goes out of range it should be far still under charge. Atleast i think it will be ok. Ill still be keeping an eye on the modules
 
Watch it like a hawk. It's a substantial improvement over what you had. I would also recommend some sort of scheduled log/evaluation, e.g., every day/week/month/whatever record high (at "float")/low (right before charging starts in the morning) voltages. Watch for trends.

2A can keep pace with a lot of imbalance, but it's still limited by the very small voltage difference between the cells, and the cells' response. The moment you start to move current into a cell, its voltage raises further reducing the voltage difference and reducing the current. 2A is substantially better than the 80mA of typical balancers, but it's likely to be in the 200-500mA range far more often than 2A.
 
@snoobler i got the balancer installed. Set to 12s and i have an error of the resistence being too high. It does balance a single module in 6s mode. It has an issue once i try 12s.

Should i use 2x 6s balancers per battery instead? I was also thinking would it be possible to use a 2s balancer between the batterys to send power between them if they are off? IE: one at 24.1v and the other at 23.9v?

1001.jpg
 
It's odd that that 24S are represented by 01-24 in the voltage section and 00-24 in the resistance section.

Suspect:
Wiring
Settings
Bad unit

Have you contacted the seller?
 
It appears to work just fine in 6s mode. It looks like there is one cell pack that is slightly higher than all the rest. Will post back after testing for a week. Idk if i should go with 2x 6s balancers.

Thanks @snoobler
19-173759.jpg
 
Last edited:
@snoobler

The balance was successful. I will be running 2x balancers and i think this will work just fine. I feel alot safer with the modules now.

638.jpg
 
I have a 48 volt setup with my Tesla modules using 6 modules. 2s3p. 4 packs are nicely balanced, my 2 middle ones are drifting. I top balanced all of them 3 months ago. Now modules 3 and 4 are .4 apart. Can I see a picture of your active balancer setup?
 
I have a 48 volt setup with my Tesla modules using 6 modules. 2s3p. 4 packs are nicely balanced, my 2 middle ones are drifting. I top balanced all of them 3 months ago. Now modules 3 and 4 are .4 apart. Can I see a picture of your active balancer setup?
Are you using any type of BMS like the EVTV one ect?

Jen
 
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