Cell Drift Under Load

[Steve_S]

Just to add to the conundrum, different chemistries will drift differently, as will different capacity cells. The larger the capacity the larger potential to drift.... Some BMS' allows you to set the Cell Voltage Difference threshold which can be used to allow for a wider drift while under load to prevent disconnects. Momentary Under/Over voltages should not be a huge issue but if there is a persistent problem, then obviously the Cell(s) will need to be addressed.

Imagine watching a pack built with 1000AH cells, you'd go nuts I think. Hmmm, as if any of us could afford 1000AH cells at $1,100 USD a pop.
Boy would be nice to have 1000 AH packs but 38kgs/83.7 lbs a cell, I wouldn't want to lift them... gosh imagine the shipping costs.... Ahhh to win a Lotto !

Something to note in general. A BMS is not a device which can correct a defective cell, they are quite limited as to what their functions are.

Some links of related interest.

BU-504: How to Verify Sufficient Battery Capacity https://batteryuniversity.com/learn/article/bu_504_how_to_verify_sufficient_battery_capacity
BU-908: Battery Management System (BMS)

BU-908: Battery Management System (BMS) – Battery University

Learn about old and new methods and how innovations may change old habits

batteryuniversity.com

BU-1101: Glossary

BU-1101: Glossary – Battery University

Learn about battery technology, battery maintenance and battery life at Battery University

batteryuniversity.com

What causes Lithium-ion to die? https://batteryuniversity.com/learn/article/what_causes_lithium_ion_to_die

 

[Sojourner1]

Looking at this image of my battery display & the inverter dislpay. Here are 2 different SOC 72% (top) & 37% (bottom). If you see the min/ max voltage #s those will flux without having to switch to the cell screen. This is with a larger load than what you might ever have.

 

[Steve_S]

@Sojourner1 is that two separate battery packs ?
If you take a DVOM/DMM and check the voltage (no loads) at the battery pack BATT terminals and then at the SCC & Inverter do the numbers match up or is there a slight deviation ? If there is a difference, have you corrected for that on the SCC & Inverter ?
Depending on tour gear, there may be offset values to correct for that, my Midnite SCC & Samlex Inverter have that capability. I would think that Magnum would as well although might be a different term for it.

 

[Ampster]

Something to note in general. A BMS is not a device which can correct a defective cell, they are quite limited as to what their functions are.

And I would add that a BMS would take forever to balance a 1000 Amp cell or group of parallel cells of that capacity. At that size pack a BMS is a reporting device that can be programmed to signal when a cell goes over or under a limit.

 

[Sojourner1]

One battery pack. There is a deviation between the 2 displays I never worried about it over the years and if it works why fix it.

The Magnum inverter is set to disconnect at 12.0v (first line of protection), over the years that image showing that inverting voltage is the lowest it's ever been (also would be the largest load I'll ever have). During the winter in the morning I might get down to 26-35% SOC even making coffee (82a) or using the microwave (145a) it will be inverting 12.4-12.6v. These are the "normal" low inverting voltages, that's why I don't worry about.

 

[Steve_S]

And I would add that a BMS would take forever to balance a 1000 Amp cell or group of parallel cells of that capacity. At that size pack a BMS is a reporting device that can be programmed to signal when a cell goes over or under a limit.

Actually, I would say that when any pack above 250AH is a likely candidate for an Active Balancer as opposed to a passive balancer especially with wandering cells.

White Paper - Dissipative vs. nondissipative balancing (a.k.a.: Passive vs. Active balancing)

Li-Ion BMS - White Paper - Dissipative vs. nondissipative balancing (a.k.a.: Passive vs. Active balancing)

 

[Ampster]

That book is ten years old but makes more sense today than when I read it eight years ago. Then I thought he was just pushing his product. Today I believe that is the concept used by Tesla. It reminds me of why heat pumps are more efficient because they move heat rather than create it.

 

[Mrdanielmh]

Thanks for the information, I think the fluctuations are normail, however an active balancer interests me. Is this something you js @Steve_S?

 

[Steve_S]

Thanks for the information, I think the fluctuations are normail, however an active balancer interests me. Is this something you js @Steve_S?

I cannot say YAY or NAY that's your call if you want to try it. I can't recommend one but I've seen cheapo's (to be leery of, most certainly) and some crazy expensive ones which you'd have to be Bill Gates to buy. That's something you'll have to search on and find out about.