“Normal” Cell Delta at Charge Voltage

[Dynoman]

I got a 25% off coupon from Harbor Freight this week, so I went there and bought a set of low profile sockets (needed 10mm for the nuts on the batteries) and a composite handled insulated ratchet (so I don't risk shorting any battery terminals).
I found out cell #2 is on the right hand side, second from the rear, so I was able to just barely get the wrench on those two nuts and torque them down just a bit. Less than 2" clearance above the batteries to the shelf above, so it was tight, but I succeeded.
It turns out that must have been the problem. Now my cell delta voltage is among the lowest of the three banks. Interestingly, the cells wire resistance on that cell is still .122 while the rest are .117, but the delta came way down by over an order of magnitude.

I'm not sure why the default active cell balancing voltage was set to 3.00V from the factory. I will do some research on the effects of raising this value. I may walk it up .1V at a time and observe over days what effect it has. If it lengthens battery life, I'm all for it.

Thanks to all who've offered their suggestions here.

Glad to hear the problem was solved and only a loose/bad connection.

Using a FLIR camera to check wiring connections found problems that were unknown until the thermal camera was used on my system.

https://www.amazon.com/gp/product/B07DM8DG1B

I agree with @Steve_S. At 27.5 Volts (around 3.43 volts per cell) is when active balancing occurs on my 2 battery banks. It is usually suggested to start active balancing 3.4 volts per cell or greater.

 

[Brett V]

I must be too nit-picky. I have four 16s batteries and if one of them goes above .006 with the voltage around 3.5, I start to worry.

 

[PotusGobbleBottom]

nit

You and me both. I have seen .030 delta at 3.5 and I was like wtf happened!!! OMG

 

[Brett V]

You and me both. I have seen .030 delta at 3.5 and I was like wtf happened!!! OMG

.030?!

 

[Steve_S]

Cell deviations always appear after 3.400Vpc and with a deviation of 0.030 @ 3.500 is still in chuckle worthy territory as it's insignificant.... now 0.100 out is cause for an eyebrow raise & closer look. The main reason a lot of folks have issues with battery packs & balancing is 95% of the time because they charge BEYOND the working voltage range.

 

[MWeiss]

I did some research and some of the consensus is to go no higher than 3.2V for the start of active balance because under high charging current, a "soft" cell can run away. I've decided to try stepping it up in .1V increments. I've set 3.1V today and will watch for a week and then if all looks well, bump it to 3.2V.

 

[PotusGobbleBottom]

I've set 3.1V

Oh my!

 

[42OhmsPA]

I did some research and some of the consensus is to go no higher than 3.2V for the start of active balance because under high charging current, a "soft" cell can run away. I've decided to try stepping it up in .1V increments. I've set 3.1V today and will watch for a week and then if all looks well, bump it to 3.2V.

I think you meant to say 3.4V.

Here's what my packs look like with JBD passive balancing set to start ~3.4V.
Note - pack 2, the newest, was not top balanced...

 

[MWeiss]

The video presenter said "no higher than 3.2V" for the cell balance threshold. I see no reason not to go higher, as my cell voltage is at 3.5V right now, being nearly fully charged, but I want to walk this up in .1V increments and watch it carefully.

 

[42OhmsPA]

Good luck.

 

[PotusGobbleBottom]

video presenter

Kindly name this presenter. Thanks

 

[OffGridInTheCity]

I operate an 18650 3.0v -> 4.2v chemistry - which has a littler wider voltage range the LifePo4 but you might find my comments helpful.

I have large powerwall with 126 x 100p packs of various cell types - e.g. analogous to 126 x 260ah LifePo4 cells. My max difference ranges from 30mv-70mv (tight balance) up to 50mv-90mv after 9 months of no balance. Since I have different cell types (different manufacturers, slightly different charge/discharge curves) it's impossible for me to be perfectly balanced all thru the charge/discharge curve.

A couple of principles for you to think about...

Since I operate in the middle DOD (3.5v -> 4.0v) a 90mv max difference means worst case I could have a few cells at 4.09v hi and 3.41v low - neither is near the max of 4.2v hi or 3.0v low - so there's no operational risk. For example, I'm running 80mv max difference right this second....


Given the above, it's not the daily difference that's so important, it's how fast does it changes over the cycles/months of operation with no balance. I find that healthy cells don't need much balance. I do touch-up balances every 9 months or so when the drift reaches 50-100mv max difference and bring it back to 30-70mv.

To conclude - if you're cells are close (58mv max difference in you're example) and do not affect you're desired hi / low operating voltage range and they stay that way day after day - then I wouldn't worry too much and just monitor to make sure you're BMS keeps them within what you need.

 

[MWeiss]

Kindly name this presenter. Thanks

This video:

 

[MWeiss]

To conclude - if you're cells are close (58mv max difference in you're example) and do not affect you're desired hi / low operating voltage range and they stay that way day after day - then I wouldn't worry too much and just monitor to make sure you're BMS keeps them within what you need.

In relatively static conditions (low charge or discharge rate) my Cell delta is around 0.015V. It's during aggressive charging in full sun late morning when the batteries are nearing full charge that the balance now goes out as much as 0.15V or an order of magnitude more delta. I think that's normal though. All three banks behave in similar fashion.