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Top vs bottom balancing?
#11
(02-08-2018, 02:22 PM)jdeadman Wrote: ...and I watch them during the discharge...

Sounds like a lot of watching... Wink
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#12
(02-08-2018, 04:29 AM)sil5er Wrote:
Quote:Cells in parallel is like having two (or more) tanks of water that have their bottoms all interconnected. No matter how much water you take out of one tank, the others will flow and balance out. No matter how much you dump into one cell/tank, it will flow out into the other cells/tanks in parallel. They cannot be in any other way.


um ... no, not quite.

in my experience this very rationale expectation is only true when charge/discharge rate is relatively low.  think < 0.1C ... once the current gets up to 0.5C and certainly any more, then the cell's relatively different internal resistances start messing with everything.  IE effective capacity changes, differently for each pack, and therefore battery goes out of balance.

The cells that are in a parallel pack will feed each other at a very low resistance. We aren't talking about connecting a 4.2V cell with a 3.8V cell or of greater distance. The difference is of milliVolts of one another, if any at all. So, as you stated, "this very rationale expectation is only true when charge/discharge rate is relatively low" is exactly what I was referring to and what is going on here.


The only way this won't be true is if there are so few cells in parallel, and a high load is applied to them, that they can't balance themselves out rapidly. However, once the load stops, they will balance themselves out with each other, even at a higher resistance, and even at higher voltage difference (which, voltage can't be different due to Laws involved)
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#13
(02-08-2018, 07:52 PM)Korishan Wrote:
(02-08-2018, 04:29 AM)sil5er Wrote: [...]

in my experience this very rationale expectation is only true when charge/discharge rate is relatively low.  think < 0.1C ... once the current gets up to 0.5C and certainly any more, then the cell's relatively different internal resistances start messing with everything.  IE effective capacity changes, differently for each pack, and therefore battery goes out of balance.

The cells that are in a parallel pack will feed each other at a very low resistance. We aren't talking about connecting a 4.2V cell with a 3.8V cell or of greater distance. The difference is of milliVolts of one another, if any at all. So, as you stated, "this very rationale expectation is only true when charge/discharge rate is relatively low" is exactly what I was referring to and what is going on here.

The only way this won't be true is if there are so few cells in parallel, and a high load is applied to them, that they can't balance themselves out rapidly. However, once the load stops, they will balance themselves out with each other, even at a higher resistance, and even at higher voltage difference (which, voltage can't be different due to Laws involved)

true dat.

but in the context of the OP's questions, I think its still fair to point out problems with balancing when high ( >0.2C?) dis/charge rates are involved.  To use your analogy, its like some of the cups get holes in and start leaking water.  which then creates a further problem when load/charge is removed, and pack internally 'balances' ... further compounding the issue of a pack with different capacity from its neighbours, thus throwing the battery out of balance. 

yes, this is very definitely more of a problem on smaller batteries, which are easier to load up.

if it was only one pack, IE a 1s 'battery' then this would all be academic.  balancing by definition is only in play when there are multiple packs in series.  the aim is to have a battery that stays balanced from top to bottom of its charge, and back again.
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#14
I studied the whole bottom balancing concept on some of the Ecar/Ebike videos on Youtube this weekend.  It looks like the guys involved with that aspect of battery packs are very concerned about bottom balancing first then bringing the whole pack up and top balancing ...then maintaining an on-going monitoring of the pack for long term while in actual use.  The procedure I got was to discharge the pack with a load to a safe bottom balance before top balancing and this was done at a low discharge rate that could be monitored.  They then top balance.  But again it is done as a whole pack ...meaning cells in parallel are looked at as just one cell and are all brought to a safe discharge voltage that matches other cells in parallel.  What I wonder is if this is a good procedure ..why not discharge each cell to a specific safe voltage of say 60% capacity....before building a pack.  Then once the pack is built you should have every cell be the same voltage and now you top balance for actual use.  It seems to me then every cell would be balanced both in parallel and in series.  Thoughts anyone?
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#15
(03-04-2018, 03:09 PM)Headrc Wrote: But again it is done as a whole pack ...meaning cells in parallel are looked at as just one cell and are all brought to a safe discharge voltage that matches other cells in parallel.  What I wonder is if this is a good procedure ..why not discharge each cell to a specific safe voltage of say 60% capacity....before building a pack.  Then once the pack is built you should have every cell be the same voltage and now you top balance for actual use.  It seems to me then every cell would be balanced both in parallel and in series.  Thoughts anyone?

Time. It takes less time to discharge the whole pack than individual cells. Plus, it's a lot easier to manage the load. It's more difficult to keep a constant 300mA over 1 cell, than it is 3A over 10 cells. Each cell still sees 300mA, but it's easier with the 3A load vs the 300mA load.
Also, every cell in parallel will 100% be at the same voltage, regardless. It not possible for them to not be the same. Some smart guys math/theory/principle.

The reason why the eVehicle and eBike world do more of the bottom and top balancing is because they need to know exactly if they have 5mins left, or 15mins left, or 30mins left on a pack voltage. With lithiums, the SoC curve is so flat for so long, it's hard to tell based on voltage and overall capacity alone.

Plus, doing a bottom balance with a load on the whole pack, you can quickly see if a pack is having an issue as it'll drop load voltage quicker/farther than a fuller voltage. Example, a weak cell could drop from 4.2V to 3.9V under 2A load. But at 3.4V it could drop to 2.8V under the same load.
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#16
The thing with packs getting out of balance is due to packs' internal resistance being different. If the difference is too high, the packs with higher internal resistance burn some energy internally as heat; the higher the current, the more they'll burn and they will keep getting more and more out of sync as high current is being applied.

If -on the other hand- you manage to get all the packs on the same internal resistance, even if the capacities are different, you could keep them in sync through many cycles, by carefully calculating the top or bottom battery voltage, depending on whether you bottom or top-balanced respectively.

However, with packs containing lots of cells (say 50 or more), internal resistance averages out and it really cannot differ that much between the battery's packs, so big packs matched in capacity rarely get out of sync.
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#17
(03-04-2018, 03:09 PM)Headrc Wrote: Thoughts anyone?

Well, it doesn't make much sense to do both. Top balancing the battery after bottom balancing it leaves you with a top balanced battery and the bottom balancing it was rather pointless.
Top and bottom balancing follow two different concepts that don't work together.

Top balancing means the cells will drift on the discharge and will balance on charge while bottom balancing is the opposite.
A bottom balanced battery can be charged with any CCCV charger without balancing if you calculate in advance how long you need to charge it so you don't put too much energy in or have a automated cutoff once the first cells reaches the set end of charge voltage. The cells will have different end of charge voltages when you do that but all of them have the same energy stored and will have equal voltages when discharged.
A top balanced battery works the other way round, the cells will drift on discharge and you have to stop discharging at the point where the first cell hits the end of discharge voltage. All remaining cells will have some charge left at this point which can't be used. When fully charged all cells will have the same voltage, but not the same amount of energy stored.
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#18
Yeah, the greater the number of cells in parallel, the easier to balance, sort of. As long as you don't have a leaker in the mix, then it should be fine.

Also, with the greater the number in parallel, the harder to imbalance them as the amps gets spread out over the cells. So, even if a cell(s) have high resistance, 200A across 100 cells is 2A/cell, but across 300 cells is only 660mA / cell. So that makes a huge difference too. If you plan on large amp draws quite often, go with larger packs (or multiple series in parallel). In this case, almost, quantity is better over than quality (in reference to IR)
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#19
Ok ...this a; makes sense to me.  Except for what Darkraven just stated.  As I understand it from the Ecar community, the bottom balancing is to make sure your top balancing is equal voltage at the top of the charge curve.  That argument is made here on this video at around 5:30. https://www.youtube.com/watch?v=rQC_9TwyjTk
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#20
My understanding was that if you bottom balance, then when you get to the “top” it should already be in balance. You don’t do both bottom and top balancing (i.e. after discharging all to the same voltage at the bottom you don’t charge all to the same voltage at the top). A “perfect” bottom balancing should result in a “pretty good” top balance. If you actively top balance after that, then you won’t know exactly how much capacity the system has, unless you are using a monitoring system that includes a shunt or other current sensor that computes capacity.
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