Flitecell BMS question

EFOIL.builders Electronics (ESC, remote, batteries)
#1

We have a severely unbalanced battery. The BMS is not balancing the battery for some reason. Together with technical support, we performed the “Flitecell” update, but were unable to achieve the desired result. Does anyone have enough knowledge about the BMS installed in “Flitecell”? The following questions are of interest:

  1. What is the balancing algorithm? For example, does the BMS balance exclusively during charging or discharging?
  2. How is energy distributed during balancing? For example, does energy flow from the most charged cell to the most discharged one, or does the BMS discharge the most charged cell, releasing energy as heat?
  3. What is the balancing current? How much current can each cell draw during balancing?
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#2

I don’t know for certain, but I’d bet the answers were as follows:

1: balances during both charging and discharging.
2: uses higher cells to balance lower cells. Using heat would be a terrible idea.
3: that you would need to know which actual BMS it is.

#3

Almost all small BMS use resistive loads to balance high cells. Most only do a few mA so can take quite a while to balance if the pack is large, and the voltage difference high.

On a smart BMS you can see the temperatures of inside the pack, and another one of the BMS itself. This is usually always warmer than the center of the pack due to this resistive load.

#4

Yes, cheap BMS use passive balancing, however it’s highly inefficient technique.
For a company like Fliteboard who pride themselves on no major battery failures (like fires etc), I would assume they were using a much better BMS that can actually do active balancing…

#5

With such unbalanced cells on big pack BMS with around 100mA current will struggle forever. You may try with 5a active balancer. It will do it job in few hours…

#6

I also expected it to be a BMS with active balancing. But as far as I can tell from the results I’ve obtained, it is passive balancing. Moreover, it happens very strangely. Two days after we installed a new firmware on the battery, the cells gradually discharge. And what’s most surprising is that they discharge almost evenly. That is, the cell with higher voltage does not discharge faster than the cell with lower voltage, as one would expect from passive balancing. If you are interested, you can judge for yourself by looking at the two screenshots taken 30 hours apart.

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#7

I didn’t mean on BMS with active balancing but on active balancer with up to 5a balancing current. From my experience when you have such diference between series, some BMS see it as an error and they stop carging. In that case you can use active balancer or hobby balance charger to fill up the series. After that if you still have high unbalanced series probably you have some bad cell wich is pulling down one of series.

#8

Thats not behaving like active or passive balancing… It just looks like the battery lost a bit of charge overall. But from the screenshot, there’s an error flag. This is from the fliteboard manual:

“The battery management system (BMS) balances and optimises the battery during each charge cycle. Leave the charging magnet on Flitecell for 24 – 48 hours after between charges to allow the BMS to optimise Flitecell.”

I assume you’re doing that?

#9

Just noticed strange behaviour of BMS from screanshoot. On upper pic it’s 25% with 55.6V charged and on second it’s full charged with 54.9V… Also you have error flag on both pics. Somehow BMS in one moment see battery as 14s and in other as 13s… Thats why probably is stoped witb balancing.