Stupid balancing question...

Rednecktek

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So I've read the threads on top balancing your cells and the often multi-day process and specialized equipment that entails, and I've been looking at Daly BMS kits that say they come with active balancers, but I'm curious about balancing...

What's the point of spending the days and getting the special equipment to top balance cells if the BMS has active balancing? That sounds redundant like crawling around in the dirt to change my oil so I can drive to JiffyLube and get my oil changed.

Also, and I come from the FLA world on this, but when you connect up lead acid batteries in parallel they balance themselves, do LFP cells not do this?
 

DerpsyDoodler

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LFP stay at 3.2V for most of their usable capacity. one cell could be 3.2V @ 80% soc while another could be 3.2V @ 50% soc. With no voltage difference, no current will flow between them. They won't balance with each other. Getting them to a nearly full soc will allow current to flow, but with a 30% difference and charging in series they won't balance between themselves without something to facilitate that (active or passive balancing). Better to charge in parallel and let voltage difference be the decision maker on which absorbs charge. You dont have to spend weeks making them perfect, just get them close enough so that a small passive balancer (which most bms have) can bring them in line over time.
 

chrisski

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My BMSs will only balance when a cell is over 3.4 and the battery is charging. That is not very long, perhaps a few minutes. To avoid becoming the guy who receives all but one cell at the same capacity, but one was shipped much lower than others.
 

DerpsyDoodler

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My BMSs will only balance when a cell is over 3.4 and the battery is charging. That is not very long, perhaps a few minutes. To avoid becoming the guy who receives all but one cell at the same capacity, but one was shipped much lower than others.
I set mine to start balancing at 3.35. If you allow a long absorb stage, they will balance longer and each cycle will bring them more in line with each other.
 

740GLE

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Also some BMS ‘active balancers’ are very very slow to balance. You prob don’t want to keep one cell at 3.65 waiting for the BMS to bleed off excessive charge, it could take multiple hours if not days for other cells to all balance out.

The other this is Top balancing is only worth it if you are trying to every last Kwhr from the pack, if you’re good with getting 95-90% you should be fine just setting up the battery and monitoring for runner cells after a dozen or so charge cycles.
 

chrisski

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I really think if you don’t top balance, you ought to get an active balancer and not count on the one built in the BMS.

I pre charged my batteries by putting them in a pack with a BMS, disassembling, and then finish the top balance.

I admit top balancing is not fun whatsoever.
 

DerpsyDoodler

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I really think if you don’t top balance, you ought to get an active balancer and not count on the one built in the BMS.

I pre charged my batteries by putting them in a pack with a BMS, disassembling, and then finish the top balance.

I admit top balancing is not fun whatsoever.
it's worth doing, but the whole, charge to 3.65, let settle, charge to 3.65, let settle again, repetitive nonsense is unnecessary. Series is great to get them most of the way more quickly, but this is also just one more time youre torquing those soft aluminum threads. put them in parallel and let them go, unless you have welded studs. once they hit 3.65, call it done. let your bms balancer handle the rest over several cycles.
 

toms

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Get a BMS that can control current during its balancing phase and you will be fine.

The single biggest mistake people make with LiFePO4 is continuing to charge at their bulk charge current as the cells approach the upper voltage knee.

Not only does it contribute to short cell lifespan, it means a low balance current BMS may not keep up with cell drift.

A low balance current BMS is desirable especially if the balance resistors share an enclosure with the rest of the BMS.

I have seen too many active balancer failures to implement them full time in a system.
 

time2roll

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I agree with strapping them up in series ready to go and put them in service immediately. May have to limit charge voltage initially until the active balancer has time to work. In the mean time all is up and running protected by the BMS.

OK, if the delivered voltages are all over the place maybe more attention will be needed. Can be done manually while in series.
 

Substrate

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Also, and I come from the FLA world on this, but when you connect up lead acid batteries in parallel they balance themselves, do LFP cells not do this?
The problem is that most sweeping statements like the one about lead-acid self-balancing is that it lacks context, real-world variables that will cost you.

Lacking context in other areas of discussion, often with generalized statements can cost too, and often pit us against each other.

For example, for a balancing bleed resistor to be effective, it should be done when the cell voltage is well into the upper-knee of charge. That upper knee is generally 3.45 to 3.65v. Most shoot for the middle, around 3.5 to 3.55 to start the resistor doing it's thing.

Recent advice to set it much lower, like 3.35v is below the knee, and is what would be called "balance all the time", which has shown to actually UNbalance the bank, and was discarded as a viable method decades ago. Unless you consider de-rating your capacity by 50% a viable way to go. Or listen to an unknowledgeable vendor who might include such an option revealing their tech dept has no real background experience.

But I understand - sometimes the signal-to-noise ratio is just too much, and only the generalized sweeping statements without context are all we can stand. :)
 

Rednecktek

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But I understand - sometimes the signal-to-noise ratio is just too much, and only the generalized sweeping statements without context are all we can stand. :)
True, and I'll be the first to admit that I'm just a beginner at all this. The only real-world data I have to work with is my camp where my 4x WallyWorld batteries all read the same voltage with a meter when I get there and when they say they're all charged, so I figured that lead batteries just did that.

Also being a beginner, and not having the money and environment to play with LFP, I admit I'm not quite sure what the active, passive, and BMS do differently as far as balancing the cells. I figured that all 3 were supposed to be doing the same thing taking power from a high charged cell and pushing it to a lower charged cell. Basically balancing the cells the same way FLA's do, but not sure the exact process and I'm not an EE to grasp the higher concepts. :)
 

740GLE

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BMS is the safety control for the battery bank it’s primary goal is to prevent individual cell over voltage and under voltage, better ones will have multiple temp sensors to shut if charging to discharging if need be.

Passive balancers is bleed off small amounts of current from higher charged cells into heat a couple watts at a time

Active balancers shift charge from higher cells to lower cells efficiently avoiding heat burn off.

BMS can have either active or passive balancers, passive being the most common.

You can always add a capacitive active balancer to a BMS with a passive balancer. But it appears those active balancers need to be switched on only at high cell voltage >3.4v which requires some sort of monitoring system.
 

time2roll

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Recent advice to set it much lower, like 3.35v is below the knee, and is what would be called "balance all the time", which has shown to actually UNbalance the bank, and was discarded as a viable method decades ago. Unless you consider de-rating your capacity by 50% a viable way to go. Or listen to an unknowledgeable vendor who might include such an option revealing their tech dept has no real background experience.

But I understand - sometimes the signal-to-noise ratio is just too much, and only the generalized sweeping statements without context are all we can stand. :)
OK and that reduction to 3.35 needs to be used with a minimum delta to start balancing.
At 15 to 20 mV delta to start the balance nothing should actually happen until 3.45+ vpc unless they are way out.

Or if somehow the balanced cells are naturally at 30 mV delta in mid charge range then reset the delta appropriately. My B cells are all within a few mV at mid charge level.
 

ocular

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Your concept is correct , it is possible to top balance with an active balancer instead of paralleling the cells at 3.6v, but there are traps. You would have to have a constant voltage source to hold say 16s LiFePo4 at 55-56 volts for a long period. There are different types of active balancers with different specs. I don't know what the Daly specs are but I don't think they impress when it comes to integrated BMS/active balancers. There are some standalone smart active balancers that can be added to the existing BMS. Some of the cheaper capacitance type active balancers reduce the balance current significantly as the voltage difference decreases. Others continue the same balance current irrespective of delta V. But for a 300A set up it may take a long time using a 1-5A balance current. It is possible you could just hook things up to your solar and actively balance each day for ?1-2hrs when pack is fully charged (and active balancer turns off when cell V < 3.4V) using the protection of the BMS and eventually your goal will be achieved over months.
 
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