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Top vs bottom balancing?
#21
@Headrc: Yeah, that is kinda what I trying to get at. I wasn't including the charging curve portion (forgot about that portion). This makes sense as each of the packs in series would have the same curve. However, this is if you fully charge your packs to 4.2V. Remember, LiIon (Cobolt/Manganese) need to have the CC reduced at 4.1V to 0 by the time it gets to 4.2V. So the charge curve needs to be the same across the board. With an EV, they are trying to pull out as much capacity as possible out of the batteries. This is a little different than powerwall storage as this setup usually has a (semi-)constant supply of power to recharge (solar, wind, hydro or grid).

@brwainer: Bottom balancing is only part of the full equation. You can't do a bottom balance without top balancing as well. But, you can top balance without bottom balancing.
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#22
I think maybe I stated the top balance wrong ....meaning if you bottom balance then charge to the top ...they will be in balance...as stated above.  I guess I was terming this as top balancing ...thinking they were one and the same at that point.  But it still makes sense to me to do the bottom balancing ...and whatever you may call it ...top balancing or just top charging.  After bottom balancing ...is there really any difference?
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#23
Top balancing is pretty much done all the time when you use a bms. it maintains like voltages across all packs. It just depends on what your application is. If you need "full" capacity at your disposal, then top and bottom balancing should be done. If you just need temp storage then top balancing would be all that's required as the bms will disconnect if 1 pack or another drops too low
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#24
If you bottom balance them and then charge to the top they will be in balance regarding the stored energy but not regarding their voltage. That's the whole point of bottom balancing, you balance on discharge while top balancing is balancing on charge and brings all cells to the same voltage.

Bottom balancing: Discharge all cells to the end of discharge voltage, charge until first cell hits end of charge voltage or just charge time-based, cell voltages drift at the top and are the same at the bottom

Top balancing: Charge all cells to the end of charge voltage, discharge until first cell hits end of discharge voltage, cell voltage drift at the bottom and are the same at the top

These methods are working 180 degrees to each other, you can't do one AND the other, you have to decide what you want.
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#25
OK ...sounds like Darkraven disagrees on this topic.  You essentially can't do both?  If that is true I believe what I am getting here is if you want the most capacity for an E vehicle of some kind, you bottom balance.  If your goal is temporary storage with a constant recharge like a Powerwall then top balancing is the approach you take.
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#26
My understanding is the same as DarkRaven’s - he stated it better than I did.
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#27
(03-04-2018, 06:03 PM)Korishan Wrote: Top balancing is pretty much done all the time when you use a bms.
True, but only if the BMS you've picked has balancing.
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#28
(03-04-2018, 07:30 PM)thanar Wrote:
(03-04-2018, 06:03 PM)Korishan Wrote: Top balancing is pretty much done all the time when you use a bms.
True, but only if the BMS you've picked has balancing.

Well, yeah, this is correct. I personally wouldn't buy a bms that doesn't have balancing. Otherwise, I think it's a waste of money and time (shipping and installation). But, that's me Smile
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#29
You can only do either or. NOT both as stated. No matter what you need to beware of and controll/monitor individual cells since if you bottom balance you will some day get them out of balance in the top so much that you get cells above 4.2V. Same is with top balance.. One day you will have 1 pack hitting bottom Smile


Active balancing will on other hand balance both on top and bottom. Thats a continuous balancing that works good on banks that are pretty good from start
But cost more! And in the end if you do your work there is no issue.
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#30
I knew someone with some experience would speak up sooner or later! Big Grin
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