Thread Rating:
  • 0 Vote(s) - 0 Average
  • 1
  • 2
  • 3
  • 4
  • 5
BMS balance voltage and cell maximum charge voltage
#1
Hello,

I am looking at cells with a maximum charge voltage of 4.15V. My charger maximum output voltage means this is not exceeded as i can only achieve 4V per cell average.

However, i appreciate theres a good chance some cells will deviate hence the need for balancing/protection.

I dont have budget just now for fancy BMS like Batrium, but a 14S 60A "scooter" BMS like ebay item 323926226256 would provide essential safety.

The issue i have is most of these types of BMS balance at higher than 4.15V, eg the one linked is 4.18V balance and over voltage protection at 4.25V.

2 questions...

1. Is it generally required protection and balancing voltages should be less than the cell listed maximum charge voltage?

2. What BMS options are there which may permit adjustment of these settings in the sub £100 price range.

Cells are EIG eplb-c020 and i am looking at 14S5P for 48V 100Ah configuration in a low power (20-40A) home energy store.
Reply
#2
Yes, BMS protection is generally required, and voltage should be clamped to whatever the cell manufacturer stipulates. They will have set aside some safety buffer, but going above the rated voltage will drastically lower the lifespan of the cell, and the overall health and capacity of the battery. A common quoted rough value is: "0.1V above will halve the lifepan, 0.1V below will double the lifepan" (lifepan=charge/discharge cycle count).
Usually, the cell with the lowest capacity will reach full voltage the earliest. So that weakest cell will stay in the unhealthy high voltage stage for the longest, making it even weaker. It's a death spiral.

You could use this BMS. Balancing and cutoff voltages can be configured from a smartphone via bluetooth. It also shows the current charge state, individual cell voltages, etc. I currently have 5x 60A units in operation, and I'm very happy with them.
https://www.aliexpress.com/item/32879451402.html

Some (all?) pouch cells need to be compressed to prevent gases forming inside. You might want to check that.
Modular PowerShelf using 3D printed packs.  60kWh and growing.
https://secondlifestorage.com/showthread.php?tid=6458
Reply
#3
(03-11-2020, 09:42 PM)Stuartsjgs most of these types of BMS balance at higher than 4.15V, eg the one linked is 4.18V balance and over voltage protection at 4.25V. Wrote: 1.  Is it generally required protection and balancing voltages should be less than the cell listed maximum charge voltage?
2.  What BMS options are there which may permit adjustment of these settings in the sub £100 price range.
Agree with @ajw22 and I would add some comments.

Its not the BMS 'balance to be less than cell max' that is the key.    The 'key' is the charging of the overall battery bank and the absorb/float voltage settings of the charger will set the max battery voltage. Divide this max v by number in series and you get 'max cell voltage' the charger will push the battery toward.  The BMS is not ment to 'set a top' - but rather protect against cell(s) in the battery going too hi or too low for safety as the charger does it's thing. So as long as a BMS does 4.2v cut-off (and 3.0 or 2.7 low cutt-off) then that's all you need - and you should focus on the max charger settings rather than BMS settings.

There are several cheaper 14s BMSs  (search 14s lithium-ion ebay or alibaba) that can provide the hi and low voltage protection and can even be settable thru android app etc - but again, the charger is the primary place to focus on max (absorb/float) battery voltage (which give max cell voltage) in cooperation with this.
Reply
#4
Thanks for both excellent replies.

I had hoped the manufacturer would document a maximum charge voltage and perhaps an absolute maximum, but im thinking to just stay well under the maximum.

I am used to electronics where a normal, maximum and absolute maximum is often defined. I.e. normal being what you should aim for in design, maximum be what your safety margins dont exceed and absolute maximum being the point of destruction.

Thanks for the pointer on the bluetooth BMS too, perfect to see its configurable.

My plan would be to aim for my 4.0v normal charge, balance at 4.1v and shut off above 4.15V perhaps.

I dont need to squeeze every Ah from the pack and i would rather under charge to gain lifetime and safety margins.

Many thanks Smile
Reply


Forum Jump:


Users browsing this thread: 1 Guest(s)