# Cell Balance Board ?



## evmetro (Apr 9, 2012)

If the batteries are already top balanced, .5 amp should be plenty to maintain the balance. If they are way out of balance, it would indeed take awhile to get them balanced at .5 amps.


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## Sunking (Aug 10, 2009)

OK so let me see if I understand you correctly. My charger is say 25 amps and batteries are far out of balance.

Charge at 25 amps until the first Balance Board turns ON. Then limit charge current to .5 amps until all remaining Balance Boards turn ON?

Is that correct?


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## dcb (Dec 5, 2009)

it sounds kinda hokey, the cells are charged when they reach 1/10c under constant voltage. half an amp sounds like overcharging (without a fair bit of fiddling with shunt voltage).


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## Hollie Maea (Dec 9, 2009)

Your best bet is to top balance manually at first to get the cells all close to balance. Once you do that, the cells will unbalance so slowly that the shunt will be plenty fast to keep them balanced even when you are charging at full speed. When a cell is shunting, it would be charging at an effective rate of 24.5 while the others charge at 25 amps. That's enough difference to compensate for a tiny imbalance. As long as you don't have a cell with an internal short, it shouldn't get far enough out of balance to ever get beyond the shunt's ability. If your pack can't regularly maintain close enough balance so that the shunt can keep up with it, you have bigger problems than just choice of BMS.

In reality, most of the shunting will occur after the charge hits the elbow at the top of the SOC curve, since that's when an unbalance in SOC manifests itself as a significant voltage differential. At that point, your charger will be in CV mode anyway, slowing the charge way down so that the .5V becomes way more significant in comparison to the charge rate.


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## dcb (Dec 5, 2009)

I tend to think bottom balancing would be more important, especially with a brain dead bms on lithium.


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## Hollie Maea (Dec 9, 2009)

dcb said:


> I tend to think bottom balancing would be more important, especially with a brain dead bms on lithium.


Top balancing works better with BMS. Bottom balancing works better without.

Shunt BMS can only pull down a cell, so balancing works way better at the top.


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## dcb (Dec 5, 2009)

This isn't really a BMS though. The only feedback seems to be if a cell is bleeding or not. The reason for bottom balancing is because it is far worse for 1 cell to be out of balance without a "real" bms reporting individual low voltages while discharging. Top balancing is about the worst thing you can do without a good bms keeping an eye on individual cell voltages (and the shunt approach is seriously hokey at that).

Just because a shunt "bms" can sort-of top balance, while overcharging, doesn't make it a good idea.


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## Sunking (Aug 10, 2009)

dcb said:


> This isn't really a BMS though.


Well yes and no.

What I have come to terms with there are two kinds of BMS

Active and Passive.

The Cell Balance Board vampires (bleeders/shunt) are Passive dumb uni-taskers. Cheap and effective at preventing over charging cells and do not communicate with other cells or the charger.


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## dougingraham (Jul 26, 2011)

Generally speaking the BMS is the greatest source of imbalance. In other words, if you don't have one then you don't need one. It seems like engineers and scientists have the most problem with this idea because they tend to be data junkies and without some kind of monitoring system you just don't have the data you need to be sure that you don't need the data. The problem is that bad data collection proves that you needed the data collection in the first place.

I am going on 3 years now without any BMS. I added cells to my pack (33 cells up to 52 cells) and re bottom balanced at that time (about 2 years in). The original balance was still better than what the typical shunt balancer does to your pack on a daily basis.

As to the cell bleeders, they depend on the fact that the pack is already mostly balanced and all they have to do is fix the imbalance they imparted on the pack since the last charge cycle.

I did some testing of my 100AH prismatic cells while I was building the car. It was the fact that during the year I was building my car that the batteries didn't drift in any measurable way that I concluded these cells don't self discharge. And the next three years of actual use have confirmed this.


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## glyndwr1998 (Apr 27, 2013)

I agree with Doug,

I have a 76 cells in series set up in my prius plug in conversion kit.
I manually balanced the cells at the install and use a cell logger to monitor the cells, I do if rom time to time add or discharge from some cells to do minor rebalance , only because I didn't balance perfectly at install, my cells haven't drifted a great deal after a over a year of use, no bms just basic cell monitoring with cheap equipment.
It seems some of these bms systems cause more damage than what they wee intended to protect.
Plus Costa small fortune in the meantime purchase and install.

Anthony.


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## dcb (Dec 5, 2009)

I was really impressed with the lee-heart batt-bridge, to at least give you some indication when it is time to break out the clip on resistor and voltmeter for a few pennies.

https://www.youtube.com/watch?v=a-Ke_k0Dcoo

But for all the hardware on the shunt "bms"s it seems like you are 9/10ths of the way to getting individual voltage readings. And I don't think gobs of data is required, just what is the minimum cell voltage and what is the max.


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