Balancing Matched VS. Mismatched Cells - How the BMS Does it and More!

Quite possibly the most comprehensive video on this subject yet!
This video is all about balancing matched VS. mismatched cells. It is more about how the BMS works rather than balancing methods; although, you could use this video as a balancing guide.
I will cover in detail how the BMS achieves balancing through active or passive balancing. We will also attempt to discover the method (if any) in which the BMS remembers the pack state for subsequent cycles. I also cover charging stages for LiFePO4 cells.
During the video, we will cover how to top balance cells using the raw, parallel pack method (no BMS), as well as series pack balancing using a BMS. The BMS balance will be time trial style, and we will see how fast the BMS can balance matched V.S mismatched cells.
See my website for more details:
www.natesdiysolar.com/project...
SFK 12V Battery Build #1:
• DIY LiFePO4 12V Batter...
* My Cell Distributor Recommendation *
If you mention my channel, it will help me out. Thanks! I have personally purchased 48 cells from this company, and it is one of the larger ones out there.
Lily Zhang (Docan Power)
Email: [e-mail]
Whatspp/Wechat: 0086 18269259127
Facebook: Lilyzhang456
Twitter: @Lilyzhang797
Cell buying tips:
- Do not put cells of largely differing capacity and / or manufacturer in the same pack.
- Confirm that the internal resistance is the same for all cells.
- Make sure the cells have welded studs.
- Ask the supplier if the cells are from the same batch.
- Check the datasheet for the cells to ensure the C rate are suitable for your application.
- For a 12V battery, you will want at least a 1C rate, continuous discharge.
Topics covered in this video:
0:00 - Intro
0:52 - Cell selection
1:48 - Matched set parallel pack quick charge
4:15 - How LiFePO4 batteries are charged
5:35 - Both sets pre-charged to 3.40v
7:00 - Matched VS. mismatched pre-charge chart
7:35 - Connecting the BMS
8:30 - About the plug-in charger
9:42 - The time trial test setup
11:50 - THE TIME TRIAL TEST
15:32 - How active balancing works
16:38 - How passive balancing works
17:35 - Cell buying tips
18:25 - Conclusion
BATTERY TESTER AND CHARGERS (affiliate links)
LiTime 14.6V 10A LiFePO4 Battery Smart Charger (used in this video)
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LiTime 14.6V 20A LiFePO4 Battery Smart Charger
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JK BMS JK_B2A8S20P
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BIG BLUE:
ZKE EBC-A40L 40A LiFePO4 5V 40A Fe Battery Cell Cyclic Tester
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Reputable Companies
Ready-to-run kits (affiliate link):
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For Solar Panels (affiliate link):
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For Battery Enclosures (affiliate link):
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Contact Information:
I am not available for personal solar system consult!
Email: [e-mail]
Website: www.natesdiysolar.com
FTC Disclosure Statement and Disclaimers:
This video is NOT sponsored. Some product links are affiliate links, which means if you buy something, We'll receive a small commission.
Videos on this channel are for EDUCATIONAL purposes only. Electricity is dangerous and can kill. Please use common sense.
Nate's DYI Solar is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com

[@natesdiysolar]

This was an insanely difficult video to make but I learned a ton and if you watch till the end
you will know everything there is to know about balancing. I also point out some general tips when buying cells. Enjoy!
My Cell Distributor Recommendation.
Lily Zhang (Docan Power)
Email: [e-mail]
Whatspp/Wechat: 0086 18269259127
Facebook: facebook.com/Lilyzhang456
Twitter: @Lilyzhang797
UPDATE 04-24-2023: Using the BMS, I charged up both packs then drew them both down to 95% then charged them back up again to ensure they would top off then drew them both back down to 95% again. I then put the BMS on the matched set and topped off again from 95% with no problems. Then I swapped the BMS to the mismatched pack and did the same thing, and it worked fine. So, the BMS probably just uses the balancer only to keep things in check once initially balanced. There does not seem to be any sort of memory in the BMS to remember the cell state once married to a particular set of cells.
Nate.

[@Code_hack_]

Great test and explanation. Looking forward to next video. Thanks for sharing!

[@natesdiysolar]

Thanks

[@tiborchren]

Very good tutorial for balancing the battery cells before building the battery pack. Well done, great video.

[@natesdiysolar]

Thanks. Glad you liked it.

[@RitinPali]

Thankyou 4 this.. more please

[@natesdiysolar]

Lol.. sure thing.

[@uhjyuff2095]

you should do a test by top balancing your "mismatched" pack and then specifically turning off the bms cell balancing function. Then do a full discharge cycle and charge cycle and see how the cells individual voltages change during the discharge and charge. Its a fun test and you will learn alot.

[@natesdiysolar]

That would be a great example of cell drift.. good idea!

[@jasonbroom7147]

That looked like it was a lot of work, so thank you for putting in the time to test this and show your results. The internal resistance is more important than the actual capacity. The BMS has no idea how much energy a given cell can store, it is only working with the amount of resistance the current it is feeding the cell encounters throughout the charge cycle. Charging LFP cells above 3.5 volts doesn't really make much sense, if you want the most life out of them. Between 3.5 and 3.65 volts you only see a small 1 or 2 percent increase in actual capacity...not enough to justify the potential damage done by charging the cells to that higher voltage.

[@natesdiysolar]

Yes, I agree. I personally charge up to 3.40V on my cells. This was just to show how those plug-in chargers work. It's not a charging recommendation.

[@offgridwanabe]

What I can draw from this video is only charge to 3.4 to avoid such conflict and take a full charge once a month or even longer.

[@natesdiysolar]

Yes, in general. It's not a bad idea to get maximum life out of your cells. But these cells are made to cycle 100%, so there is nothing wrong in doing that, but as the other guy said, the extra capacity is negligible.