Most efficient top-balancing of lithium cells?

learntorv

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I've got 34 304ah cells coming from Amy Zheng with Docan for 2 x 16S cell packs and 2 spares. My bench charger came today (the one linked to from the document on top balancing and linked to from his video).

What's going to be my most efficient method of top balancing them? Do each 48v pack together/separately? Do them in batches of 4 so they charge in a reasonable amount of time?

I did have one friend suggest hooking up the BMS + my 48v inverter to bottom balance them first, then use it to top-balance them since it's significantly higher amperage. But I'm hesitant to vary from Will's advice.

Thoughts?
 
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Do you have a 48 volt charger?
Make 2x 16s battery each with its own bms and charge them at system voltage until the high cell triggers disconnect.
Then top off each cell individually.
You don't even have to break the battery down to top off the individual cells.
 
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learntorv

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Do you have a 48 volt charger?
Make 4x 16s battery each with its own bms and charge them at system voltage until the high cell triggers disconnect.
Then top off each cell individually.
You don't even have to break the battery down to top off the individual cells.

I'm ordering a pair of Victron Multiplus II inverter/chargers. Probably will be a day or two before I pull the trigger on them, though.
 

pellicle

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What's going to be my most efficient method of top balancing them? Do each 48v pack together/separately? Do them in batches of 4 so they charge in a reasonable amount of time?
my view is this:
  1. assemble the pack with its BMS (btw, is it a 2 wire or 3 wire bms)?
  2. allow a lengthy saturation time and view that top balance will not cease until the BMS no longer accepts current (amps) I always use a unit like the 150A in-line with the charge source to the BMS input. Note down all values of Wh for each iteration.
  3. unplug the charge and let it settle for a little while and then check the V or each of the cells, if any cell is down it most likely means that the BMS has given up on that (limitations of some BMS's) OR that the cell is not up to the standards (which will become clear in subsequent iterations).
  4. record the voltage of each cell and spreadsheet them (I always number my cells)
  5. if required (meaning they weren't all ideal) do topping charge on the cells which are down until they are nearly full (meaning current has fallen to milliamps and voltage still remains below 3.6v) (this does not require the pack to be disassembled, just attach charger to each cell). This step will require you to monitor carefully so as to not damage the cells if you do not have a dedicated LiFePO4 capable charger, I use an iMAX B6AC for this task. Do not use the 150A with the iMAX
  6. repeat from 2 and monitor the V of cells and at BMS termination of the charge, repeat from 3 as needed
NOTE: some additional information
the 150A unit I used inline
1647467115239.png

keep in mind the charge curve of LiFePO4
1647467240225.png
you'll observe that the current going in will drop significantly past the 90% SoC point

be careful on purchase of the 6BAC .. there are plenty of low priced counterfeit versions on eBay. Buy from a good store.
 

toms

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As above, except use your BMS high voltage disconnect voltage as your “balance” voltage rather than 3.65V
 

pellicle

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As above, except use your BMS high voltage disconnect voltage as your “balance” voltage rather than 3.65V
Hi
not sure if this was a reply directed to my post (ignore if it isn't) I would always prefer to let the BMS manage the pack, however in the case of individually charging one cell in the pack how can you use the BMS to "high voltage protect" the cell.
 

toms

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Hi
not sure if this was a reply directed to my post (ignore if it isn't) I would always prefer to let the BMS manage the pack, however in the case of individually charging one cell in the pack how can you use the BMS to "high voltage protect" the cell.

You can connect an individual cell charger through the HVD relay of your BMS. That’s not what i meant though.

What i meant was if your BMS high voltage disconnect is 3.5V, then top balance your cells at 3.5V (not 3.65V)
 

pellicle

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What i meant was if your BMS high voltage disconnect is 3.5V, then top balance your cells at 3.5V (not 3.65V)
ahh
well I also wonder about this and have asked a question over here:

I'd be pleased if you would answer that either here or there, as your advice of 3.5 is not consistent with what I know. (I could be wrong)
 
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toms

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I agree with sunshine eggo’s explanation in that thread.
 

learntorv

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Thanks everyone. I need to get my inverter/chargers bought and then figure out how to hook them up to plug into the stick&bricks house temporarily so that I can do a fast charge most of the way up and then top balance from there.
 

Just John

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Thanks everyone. I need to get my inverter/chargers bought and then figure out how to hook them up to plug into the stick&bricks house temporarily so that I can do a fast charge most of the way up and then top balance from there.
Yes, the top balance resource that was linked in this thread explains how to do it. Depending on the state of charge when you get the cells, it can be up to 16 times faster for a 48v system.
 
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