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BMS Choices
#11
I plan on building my own based off the TI BQ76940 www.ti.com/product/bq76940

But it will be awhile before I start working on it. I'm still learning all the stuff that I need to know before hand
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#12
As i mention my electronics knowledge/skills are not that good... but if i go this right that will read each cell voltage and send that information via i2C? Looks interesting... add an ESP32 and you have the monitoring hardware...
Looking forward for your BMS
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#13
It'll be nice to come up with something! The one thing I noticed with the chinese 1S balancer is that it seems to take the high voltage cells and dump into the low voltage cells. I'm not sure how they are doing this, possibly with a supercap as in intermediate storage. But that's a better design in a way than the batrium where I think it's just doing a load resistor dump. My idea initially was to keep the cost low by utulizing only a few balancers and attaching to the high and low cells through arduino somehow.
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#14
I didn't think about using a super-cap. That's an interesting idea. I will have to keep that on note. But we'll see how things progress as I put it all together. Again, it's a few months away before I get into it. I gotta start tinkering with other arduino stuff to get more familiar with it.
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#15
not2bme they use biderectional dc/dc on an isolated bus. Where all cells send their energy. The one with lower will take and the one with higher will give.


And then i looked at another alternative the other day:
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#16
(11-19-2017, 12:13 PM)daromer Wrote: not2bme they use biderectional dc/dc on an isolated bus. Where all cells send their energy. The one with lower will take and the one with higher will give.


Wow good review on the 1s balancer. That got me to look more into active balancers instead of a whole bms. It's more of a dumbed down version that just does the basics, and that active balancing (taking cells from high to low) is a better alternative than passive balancing (like a resistive load). This may also be better for smaller packs so there's less power loss since passive balancing is just dumping the load. Also active means very little heat generated since it's transferring the load (basically charging another cell, well i hope that's the case since it's claiming over 90% efficiency at 1A).

I found two that looks ideal. High active current transfer > 2amps.

8s balancer (scalable to 24s) - Capable of 4 amps active transfer, with serial connection for maybe future arduino BMS?

16s balancer - Has display, and limited bms to control over voltage (through small limited passive load) and a regular active balancer at around 2-3 amps

I only have a 7s so I may try out the 8s balancer, but both may be ideal starter balancers for under $100.
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#17
not2bme generally tha lost energy due to balancing is not a problem. On such systems you try to only balance in the top when they are full and you have excess or you are charging fast.
Doing so the lost energy is minimal and does not affect the overall functionality.

Meanwhile the active is on always in most cases "loosing" energy by transfering it back and forth.

For me Active balancing is for systems where you have battery banks with banks that are far apart in terms of capacity. Lets say you got 20Ah 20Ah 10Ah. Then an active will help you out so you potentially can use 15Ah instead of 10Ah in total. This a passive cant.
For a battery bank that have same amount of capacity in the packs you need to balance very very little per week and that should not be an issue. If it is the packs have a greater issue!

Note that active balancing tend to balance back and forth. So when you drain them they will balance to the cell with lower resistance. And when they are charged back up again they balance it back again. This is not really needed. Unfortunately on all the cheap ones out there this is what will happen and in long run you loose more on the active system


So my point is that you dont save energy on the cheap active balancers because they balance without a thought behind it. They tend to waste more energy than gained. And if you get one make sure you get one that can show how much energy have been transfered back and forth and you see what I talk about Big Grin
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#18
In other words active balancing can be inefficient iif done incorrectly. so if there was some combination with some arduino voltage monitoring and some 8 relay boards to turn on when the deviation between the cells are withing certain set limits so active balancing isn't active 100% of the time then it will be a cost effective solution. Or is this still a foolish idea?
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#19
not necessarily foolish, just interesting Wink
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#20
Phew.. I was thinking that maybe it's just futile to even think about anything other than Batrium
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