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Is there a need for bms?
#1
After many many many months of research and building battery packs, and messing around with bmss, I'm starting to think there isn't really. For a bms in terms of bancing of the pack burning off and wasting energy, I can see a need for sure for monitoring temp of each cell and if any becomes too hot then shut down the battery, and Maby monitoring current draw and having a max current draw.

But Balance of the pack is there a real Need? Each cell is charged, voltage goes up, then down as it is used, then recharged, balance Of the pack is just wasting energy?
Been using an active balance bms for a while and which works great. Fore sure setting voltage cut offs is great for longevity of the pack, but balancing really can't see a need for it it's just wasting energy.
Battery is li iron using 18650s, 14s, 0p, and growing cell so far 300, untesteded.
Bms is a diy open source by stuartpittaway.
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Cells 2000 to 2500ma.
Cells 1500 to 2000ma.
Cells 1000 to 1500ma.
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#2
Please read the FAQ about BMS.

It's not about just burning off excess, it's also about under/over voltage protection.
Also, how can you absolutely make sure that every pack is the exact same voltage 100% of the time? You can't. That's the BMS's job.

Let's assume 4s config with 16.8V fully charged (4.2V / cell). If you didn't have a BMS, there could be voltage drift.
4.2 * 4 = 16.8V
4.0 * 3 + 4.8 (imbalanced cell) = 16.8V

Without monitoring, you'd have no idea that the 1 cell is getting ahead of the others.
Or:
4.3 * 3 = 12.9 + 3.9V (lagging cell) = 16.8V

Without monitoring, you'd have no idea that the 1 cell is getting behind the others.

In both situations, you'd have a potential for catastrophe, or at the very least, cells whose life span is drastically reduced.

Sure, you could put those LED voltage meters on all your packs. But isn't that wasting energy burning in the form of light?

A proper BMS is designed to monitor and disconnect the charge/discharge if cells get too high or too low, and if it has a temp sensor, too hot.
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#3
You don't specify what size of pack you are talking about .....

For power-wall I have never used a BMS ... the vast number of cells involved mean each 3.7V block making up the wall will have around the same capacity so they will stay very close voltage wise ... I have a small voltmeter on each 3,7V block and the voltage of all is very close during use ... I did once notice that one pack was dropping behind ... when I touched all the cells in this pack I noticed one was slightly warm and so I removed it since it was self discharging and wasting energy ... If I had a bms connected this would never have been detected ...

Remember it is not necessary for packs to be 'ballanced' (have the same voltage) ... only that a pack does not go too high or too low ...

I did connect one of the voltmeters to just one 18650 to see what it's current consumption was , insignificant it lost about 0.01V per day.
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#4
Primary function of a bms is protection and notification.

Balancing is the last you add. Yes you need bms. No you dont need balancing but its nice to have and for some batteries it extends the life drastically.
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#5
(12-16-2019, 02:55 PM)daromer Wrote: Primary function of a bms is protection and notification.

Balancing is the last you add. Yes you need bms. No you dont need balancing but its nice to have and for some batteries it extends the life drastically.

My experience with second use cells is they do not fail catastrophically , but when dying self discharge at a rate depending on the voltage ... if you have a 3.7 V pack with one cell self discharging at 1A it will slowly drain that whole pack , the other packs will go high and the BMS will burn off that excess voltage ... the self discharging pack will cause massive loss and you will never know ...

I always use BMS for small mobile batteries .
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#6
You Will know IF you have a bms that reports status and even better the amount balanced. Like batrium do Smile

You are mixing balancing blindly with a bms that protects. Different things. Bms is needed om every pack that is charged with a dumb controller.

There is No argument Ever to skip out on a bms that protects. No matter what size it is.

Just because you can drive your car without seat belt once or twice its not a statement you Will live when you suddenly crashes.
Korishan and OffGridInTheCity like this post
The Ultimate DIY Solar and build place
YouTube / Forum system setup / My webpage  Diy Tech & Repairs

Current: 10kW Mpp Hybrid | 4kW PIP4048 | 2x PCM60x | 83kWh LiFePo4 | 10kWh 14s 18650 |  66*260W Poly
Upcoming: 14S 18650~30kWh | Automatic trip breakers, and alot more
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#7
(12-16-2019, 03:08 PM)ozz93666 Wrote: ...when dying self discharge at a rate depending on the voltage ... if you have a 3.7 V pack with one cell self discharging at 1A it will slowly drain that whole pack , the other packs will go high and the BMS will burn off that excess voltage ... the self discharging pack will cause massive loss and you will never know ...

Well, this is partially true. If you use a dumb BMS that does no reporting, then yes, you won't know. However, if you use one that has some sort of reporting mechanism then you will know that all but 1 pack is having excess energy burned. This would lead you to check out the pack.

Not having a BMS at all, you will "definitely 100%" never know of the dud cell.

As daromer says, Balancing and Monitoring are two different things. All BMS's monitor, but not all Balance. Also, not all Disconnect with over/under-voltage.

So when choosing a BMS, choose one that fits your needs. If you have brand new cells, maybe you don't need balancing. But over/under-voltage protection should be included though.
Proceed with caution. Knowledge is Power! Literally! Cool 
Knowledge is Power; Absolute Knowledge is Absolutely Shocking!
Certified 18650 Cell Reclamation Technician

Please come join in general chit-chat and randomness at Discord Chat (channels: general, 3d-printing, linux&coding, 18650, humor, ...)
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