Active/Passive balancers, the best debate in batteries. Balancing in Absorption?

[hwy17]

My favorite part of a technical hobby is the persistent controversial and dogmatic topics of debate. The nature of a good persistent topic will center around a ubiquitous performance frustration, and on one side will be a practical alleviating measure, opposed by a purist faction who argue that the alleviating treatment is ignoring or covering up the root cause.

In pools, we have "filtration is primary" vs. "filtration is ancillary to the chemical sanitation". 1% of the USA's entire electric consumption is caught up in that debate.

In dirt bikes, we have "lower the mix ratio for trail riding" vs. "never touch the mix, all rich conditions can be fixed in the jetting".

I only land on one side of these debates, with the contrarians. The reason I do that is probably for enjoyment. The practical faction will offer a good enough solution that you can apply and move on from but the purist faction will give you a forever problem, a bottomless pit of refinement. I know that jetting is the only true way to tune a bike, and yet my bike still smokes and the lean mixers don't. But that's the point, because every time I get my bike out I get to work with this problem again and dig into half step needle swaps and various brass until I find the perfect one for today's weather, which is spoiled again by noon.

Anyway, waxing on the zen and art of motorcycle maintenance aside, back to the title:

Do you guys really try to balance in absorption? Doesn't this give a very narrow window of like an hour or less in a daily solar charge cycle to get the balancing done? Maybe with some carryover as they come off the absorption into float but if you balance at 3.42 and float at 54 you're going to lose your balancing pressure right?

 

[sunshine_eggo]

Do you guys really try to balance in absorption? Doesn't this give a very narrow window of like an hour or less in a daily solar charge cycle to get the balancing done? Maybe with some carryover as they come off the absorption into float but if you balance at 3.42 and float at 54 you're going to lose your balancing pressure right?

That's the general consensus.

Healthy, well matched and top balanced cells shouldn't need more balance time than a few hours a month.

 

[Lt.Dan]

My $0.02 is to not float. I charge to 56.8v, then hold it there to balance.

I personally have found balancing while still absorbing (in the 54.4v-56.8v range), is sometimes counteractive to what you want. Some cells in that range might be higher voltage, which charging, and the balancer is trying to bring it down, but once current tapers off, that cell is actually a lower voltage, meaning that the balancer was just exacerbating the problem. So I try to get it up to 56.8v as fast as possible, to prevent unnecessary discharging of a cell that doesn't require it.

My Sol-Ark is programmed to ensure the battery is fully charged around 2pm, so it has 2 hours to balance anything before it starts to sell back at 4pm. This keeps my batteries within 10mV (at 56.8v) without an active balancer, and sh*tty mismatched grade B cells.

 

[sunshine_eggo]

My $0.02 is to not float. I charge to 56.8v, then hold it there to balance.

This is the literal definition of balancing in absorption.

I personally have found balancing while still absorbing (in the 54.4v-56.8v range),

You are implying that absorption occurs in a voltage range. this is not the case.

Absorption is defined as the constant voltage phase of charging. A voltage range is not constant voltage.

 

[rhino]

My only question would be what is the behavior of the cells closer to their end-of-life.. i.e. after 6000 cycles will they behave exactly the same or will the balancing method start to matter then?

 

[sunshine_eggo]

My only question would be what is the behavior of the cells closer to their end-of-life.. i.e. after 6000 cycles will they behave exactly the same or will the balancing method start to matter then?

Aging will change:

1. capacity
2. resistance
3. self-discharge rate

If all cells "age" perfectly equally, the effects will be minimal. If they don't, they will diverge.

 

[Lt.Dan]

This is the literal definition of balancing in absorption.

Yup, not arguing with you there.

You are implying that absorption occurs in a voltage range. this is not the case.

I should have said while still "charging" instead of absorbing. I was implying that I would prefer the balancing not occur while the battery is still charging from the 54.4v to 56.8v, and the balancing only occur once the 56.8v is met or current tapers off to a low amount. Sorry if I was confusing. Sometimes I have a hard time putting thoughts into words lol.

 

[rhino]

Aging will change:

1. capacity
2. resistance
3. self-discharge rate

If all cells "age" perfectly equally, the effects will be minimal. If they don't, they will diverge.

Could a passive balancer then have trouble keeping them balanced? I realize this is purely hypothetical since it would take quite a long time to get that many cycles in a solar installation but just curious.

 

[sunshine_eggo]

I should have said while still "charging" instead of absorbing. I was implying that I would prefer the balancing not occur while the battery is still charging from the 54.4v to 56.8v, and the balancing only occur once the 56.8v is met or current tapers off to a low amount. Sorry if I was confusing. Sometimes I have a hard time putting thoughts into words lol.

If I were smarter, I would have realized you meant this:

I personally have found balancing BEFORE absorbing (in the 54.4v-56.8v range), is sometimes counteractive to what you want.

Could a passive balancer then have trouble keeping them balanced? I realize this is purely hypothetical since it would take quite a long time to get that many cycles in a solar installation but just curious.

Given that some low quality or poorly matched "NEW" cells struggle to maintain balance with only passive balancers, yes.

I have found the single most influential aspect to maintain balance is the rate of self discharge. I've personally seen imbalances of as little as 0.1% SoC to trigger over-voltage protection at the cell level. I have seen "new" cells that vary dramatically in their self-discharge rate, but they still fall well within the typical 1%/month losses in the datasheets.

I wouldn't be surprised if "Grade B" cells start manifesting balance issues sooner rather than later.

 

[hwy17]

My $0.02 is to not float. I charge to 56.8v, then hold it there to balance.

This is the literal definition of balancing in absorption.

Well is it balancing in absorption, or floating at 56.8? Some say it's not float if it's absorption voltage, I say it is float if you're holding it at near zero amps.

Just a terminology rabbit hole though. We all understand the functional process described. Potato potahto what to call it.

 

[Lt.Dan]

Well is it balancing in absorption, or floating at 56.8? Some say it's not float if it's absorption voltage, I say it is float if you're holding it at near zero amps.

Just a terminology rabbit hole though. We all understand the functional process described. Potato potahto what to call it.

Yeah it all means something different to everybody. I would consider floating as lowering the charge voltage after the Constant Current phase is complete. In this case, I am never lowering the charge voltage.

 

[Steve_S]

Snickers, just to toss a rock in the pond.
1) I charge Bulk/Absorb to 3.450 Volts per cell Then Float @ 3.430
2) JKBMS Active Balancing starts at 3.420
When Bulk transitions to Absorb (Midnite SCC's) Active balancing starts up by then. Some cells may be out by 0.050 but by the time we reach float they are all within 0.015 and typically < 0.010 an hour after reaching float.

There is an Assortmemt of packs in this parallel mix.
1 - Bulk Cell pack (from XUBA yrs ago, worst of the bunch)
1 - B-Grade (with reports) (Luyuan - fair, 1 weakling cell)
1 - A-Grade (with reports) (Luyuan - Good) rarely farther than 0.020 variance.
1b - B-Grade (with reports) Luyuan - fair, 1 weaker cell but not bad)
2b X2 - A Grade 280KV3 (with reports) Luyuan)
*b packs in process of being assembled & tested now.
The 175AH & 100AH Packs have been shifted to other duties now.

 

[wpns]

Is there a good writeup on the differences between active and passive balancing?

 

[hwy17]

Is there a good writeup on the differences between active and passive balancing?

It can be described briefly.

1. Passive balancing burns off energy from a cell with a tiny heater. Active balancing moves energy from the high cell to a low cell.
2. Passive balancers are usually small, 200mA, while active balancers are larger, 2A is common.

#2 isn't an inherent difference, but it is the more important practical difference. Right now if people want 2A of balancing current, active is the way to get that. There could be big fast passive balancers, but they aren't currently common our BMS options. Probably because once you're doing 2A you'd rather save that energy than waste it.

Nuvation has a good video on it. But their focus on grid scale ESS is a different viewpoint than small DIY, and they discuss some exotic non-voltage balancing methods that we don't currently consider using in DIY.

Webinar: Battery Management System Impacts on Energy Storage - Nuvation Energy

Battery management systems impact the utility, effectiveness, and life of an energy storage system. Passive balancing is less expensive than active balancing, but weaker cells or modules may need to be replaced near the end of the ESS’s lifespan. Active balancing transfers energy from stronger...

nuvationenergy.com

 

[hwy17]

I personally have found balancing while still absorbing (in the 54.4v-56.8v range), is sometimes counteractive to what you want. Some cells in that range might be higher voltage, which charging, and the balancer is trying to bring it down, but once current tapers off, that cell is actually a lower voltage, meaning that the balancer was just exacerbating the problem.

This is my feeling too. Balancing moving cells seems like a juggling act and you'll see one cell getting balancing coming up and then later on it'll turn out to be a low cell.

If I balance and float at the same voltage (or what you're calling the prolonged end of the absorb) then I know the cell is high, once a cell is above it's 1/16th of the float voltage then it's completely certain that it needs the balancing.

 

[Lt.Dan]

This is my feeling too. Balancing moving cells seems like a juggling act and you'll see one cell getting balancing coming up and then later on it'll turn out to be a low cell.

If I balance and float at the same voltage (or what you're calling the prolonged end of the absorb) then I know the cell is high, once a cell is above it's 1/16th of the float voltage then it's completely certain that it needs the balancing.

Yes, I have raised my balance start voltage to 3.45v/cell for this reason. prevents premature balancing

 

[wpns]

It can be described briefly.

1. Passive balancing burns off energy from a cell with a tiny heater. Active balancing moves energy from the high cell to a low cell.
2. Passive balancers are usually small, 200mA, while active balancers are larger, 2A is common.

#2 isn't an inherent difference, but it is the more important practical difference. Right now if people want 2A of balancing current, active is the way to get that. There could be big fast passive balancers, but they aren't currently common our BMS options. Probably because once you're doing 2A you'd rather save that energy than waste it.

Nuvation has a good video on it. But their focus on grid scale ESS is a different viewpoint than small DIY, and they discuss some exotic non-voltage balancing methods that we don't currently consider using in DIY.

Webinar: Battery Management System Impacts on Energy Storage - Nuvation Energy

Battery management systems impact the utility, effectiveness, and life of an energy storage system. Passive balancing is less expensive than active balancing, but weaker cells or modules may need to be replaced near the end of the ESS’s lifespan. Active balancing transfers energy from stronger...

nuvationenergy.com

Thanks, now I get it!

 

[hwy17]

It seems like there's a turning point where the community advice is to balance in absorption and then as soon as that gets tight there's a fork in the road, where you either change the charge strategy to give passive more time somehow or you say we need to fit in this window so we need active.

And a lot of the community is moving towards the latter fork now with JK making active a standard feature.

 

[rhino]

2) JKBMS Active Balancing starts at 4.200

I assume that was a mistype and you meant 3.2? Don't want to confuse noobs

 

[hogback]

I assume that was a mistype and you meant 3.2? Don't want to confuse noobs

Or perhaps the meant 3.42?
As an aside, there's no reason why you couldn't have a 4A passive balancer or a 0.1A active balancer......

I always figured that Li float meant the V you hold the batteries at with a scc. I believe this should be the open circuit potential once the charging is done. That way, you are essentially avoiding including any overpotential, which would imply charging, and for the sccs, you're just trying to match loads to solar production.