Dual BMS on parallel connected packs

[MaximusAnonus]

Hi,
I'm building a few packs all from Grade A brand new cells from Amy that are very closely matched:

Quantity​	Configuration​	Individual cell capacity​	Are these packs connected together in parallel?​
2 pcs​	1P16S​	280Ah​	Yes​
1 pcs​	2P8S​	110Ah​	No​
7 pcs​	1P8S​	280Ah​	Yes​

All pack will be top balanced first before use. Then I will mount a 150A Daly BMS on each pack together with the Daly parallel board (except the 2P8S for obvious reasons) for good measure.
Now the question is. As these BMS can't possibly balance these cells over time (and that's actually not why I want to use them) I need an active balancer like the Daligreen. Is there a nice active balancer besides the Daligreen ($120 a piece) I can use? Something with nice price/performance ratio? Any tips?
I gather there won't be any downsides to using a BMS and an active balancer at the same time on the same pack, correct?

 

[sunshine_eggo]

Hi,
I'm building a few packs all from Grade A brand new cells from Amy that are very closely matched:

Quantity​	Configuration​	Individual cell capacity​	Are these packs connected together in parallel?​
2 pcs​	1P16S​	280Ah​	Yes​
1 pcs​	2P8S​	110Ah​	No​
7 pcs​	1P8S​	280Ah​	Yes​

All pack will be top balanced first before use. Then I will mount a 150A Daly BMS on each pack together with the Daly parallel board (except the 2P8S for obvious reasons) for good measure.

What is a Daly parallel board?

Now the question is. As these BMS can't possibly balance these cells over time (and that's actually not why I want to use them)

This statement is not true.

I need an active balancer like the Daligreen.

Why? If you do this you might as well not bother top balancing. Active balancers tend to work to undo top and bottom balances.

Is there a nice active balancer besides the Daligreen ($120 a piece) I can use? Something with nice price/performance ratio? Any tips?

The JK BMS has built-in active balancing, and it would likely be cheaper than a BMS+balancer.

Other active balancers are discussed on this forum.

I gather there won't be any downsides to using a BMS and an active balancer at the same time on the same pack, correct?

Incorrect. See above.

 

[MaximusAnonus]

What is a Daly parallel board?

See the enclosed datasheet.

This statement is not true.

OK, let me rephrase that. The time needed to balance a pack of 280Ah cells is extremely high to the point it's useless especially since the Daly BMS balances only when charging.

Why? If you do this you might as well not bother top balancing. Active balancers tend to work to undo top and bottom balances.

What about cell degradation? We are talking about years of use. I don't want to have to manually top balance the pack every year.

JK BMS

Is it supported by Home Assistant? If not then it's pretty much useless to me.

Incorrect. See above.

See what above?

 

[sunshine_eggo]

See the enclosed datasheet.

Never seen something like that before. Need for one likely indicates a poorly designed system.

OK, let me rephrase that. The time needed to balance a pack of 280Ah cells is extremely high to the point it's useless especially since the Daly BMS balances only when charging.

You indicated you were going to top balance them first. They should have no issues maintaining that indefinitely provided you purchased quality, healthy, and reasonably well matched cells.

What about cell degradation?

What about it? Cells typically degrade at the same rate within the same battery unless the system is poorly designed or cells are of low quality.

We are talking about years of use. I don't want to have to manually top balance the pack every year.

If your cells are of low quality to the point that they "need" active balancing, you will find that you are plagued by frequent imbalance issues as your charge/discharge currents are likely to exceed the 1-2A the balancers actually provide (all balancers grossly over-state their balance current).

Is it supported by Home Assistant? If not then it's pretty much useless to me.

No idea.

See what above?

I answered the question earlier in my post - active balancers can undo a top/bottom balance.

 

[HRTKD]

OK, let me rephrase that. The time needed to balance a pack of 280Ah cells is extremely high to the point it's useless especially since the Daly BMS balances only when charging.

While I'm in the camp that considers Daly to be cheap crap, I wouldn't want someone reading the forum for the first time to see your post and assume that all BMS work the way you describe the Daly. My Overkill Solar 4s BMS on a 280 Ah LiFePO4 pack balances just fine. Over the past 20 months my cells have stayed balanced so well that I rarely check that any more. Even at 50% state of charge by cells were still so close to each other that it put a smile on my face.

Give up the Daly BMS and move on to something better.

Why are you building a mix of 24v and 48v batteries?

 

[MaximusAnonus]

Never seen something like that before. Need for one likely indicates a poorly designed system.

It limits the current to 15A when plugging the batteries in. I see nothing unreasonable in this.

You indicated you were going to top balance them first. They should have no issues maintaining that indefinitely provided you purchased quality, healthy, and reasonably well matched cells.

What about it? Cells typically degrade at the same rate within the same battery unless the system is poorly designed or cells are of low quality.

If your cells are of low quality to the point that they "need" active balancing, you will find that you are plagued by frequent imbalance issues as your charge/discharge currents are likely to exceed the 1-2A the balancers actually provide (all balancers grossly over-state their balance current).

I'm going by my experience with the current 2P8S 400Ah Winston cells pack I bought directly here in Europe from GWL. I've had it for 6? 8? years now and the older they get the harder the active balancer has to work them (and I cycle them from like 25% to like 75% so nothing extreme).

But it's true I've never top balanced them. The engineer who helped me install my current setup back then (and he is a real electrical engineer who went to college and all has a fancy diploma and a company that installs grid-free setups) never even mentioned top balancing. Maybe it's time to disassemble the whole pack and top balance it.

No idea.

Well then I will have to go with Daly.

While I'm in the camp that considers Daly to be cheap crap, I wouldn't want someone reading the forum for the first time to see your post and assume that all BMS work the way you describe the Daly. My Overkill Solar 4s BMS on a 280 Ah LiFePO4 pack balances just fine. Over the past 20 months my cells have stayed balanced so well that I rarely check that any more. Even at 50% state of charge by cells were still so close to each other that it put a smile on my face.

AFAIK Overkill BMS has the same-ish balancing current as Daly (Overkill is supposedly 50mA and Daly is supposedly 30mA). Both are passive BMS that balance only when charging.

Give up the Daly BMS and move on to something better.

What Home Assistant compatible BMS are you suggesting? I'm open to your ideas.

Why are you building a mix of 24v and 48v batteries?

Because I'm building more than one system?

 

[sunshine_eggo]

It limits the current to 15A when plugging the batteries in. I see nothing unreasonable in this.

Fair enough. Most decide to build a pre-charge and avoid connecting batteries when there's a large voltage delta.

I'm going by my experience with the current 2P8S 400Ah Winston cells pack I bought directly here in Europe from GWL. I've had it for 6? 8? years now and the older they get the harder the active balancer has to work them (and I cycle them from like 25% to like 75% so nothing extreme).

This is actually pretty extreme and likely false. If you're actually operating in the 25-75% range the opportunity for imbalance is near zero with active balancing since you're not operating in the legs, but alteration of the voltage vs. SoC relationship can occur, which might be throwing things off. This is fixed by a single charge to full.

But it's true I've never top balanced them. The engineer who helped me install my current setup back then (and he is a real electrical engineer who went to college and all has a fancy diploma and a company that installs grid-free setups) never even mentioned top balancing. Maybe it's time to disassemble the whole pack and top balance it.

There you have it.

As one with a piece of paper saying I completed a four year university program that awarded me a BS in engineering, I'm saying they can't always be trusted to be competent unless it's truly their field of expertise. EE don't necessarily come out of school with more than a basic understanding of how batteries function in circuits and almost certainly NOTHING pertinent to actual real-world applications of the various chemistries.

There are likely other forces at work. You may find a top balance completely resolves your issues. I wouldn't break it down. You have active balancers. Limit your charge current to that which your balancers can handle, e.g., if they can balance at a true 2A, charge them until 1 hits 3.65V and then reduce charge current to 1-2A. Might take longer, but it minimizes touch time.

AFAIK Overkill BMS has the same-ish balancing current as Daly (Overkill is supposedly 50mA and Daly is supposedly 30mA). Both are passive BMS that balance only when charging.

On the Overkill, this is a configurable option - always or only during charging. These BMSs routinely maintain a top balance when properly configured.

You're operating your batteries in a less than optimal way. If your goal is cycle life extension, one of the most effective means is to charge to lower peak and lower tail current, e.g., charge to 3.45V/cell to a 0.02C termination current. This will make your absorption cycle look like lead-acid with a 5-6 hour total charge time (provided you start charging at 0.2C or greater). This method also allows simple passive balance during charge to maintain top balance.

 

[MaximusAnonus]

Thank you. I will try that.

 

[sunshine_eggo]

I meant to indicate that the 3.45V/cell charge terminating at 0.02C will get the batteries to 98% SoC.

 

[RV10flyer]

My 132kWh bank of cells from Amy and Haley require little to maintain .015V balance. I use 7 brands/10 bms's. They all work. Two Daly.. JK is my favorite. JBD/Overkill second (Add Chargery to second place).

 

[HRTKD]

What Home Assistant compatible BMS are you suggesting? I'm open to your ideas.

Beats me. I don't need BMS integration with my system.

 

[MaximusAnonus]

My 132kWh bank of cells from Amy and Haley require little to maintain .015V balance. I use 7 brands/10 bms's. They all work. Two Daly.. JK is my favorite. JBD/Overkill second.

Thanks.

Beats me. I don't need BMS integration with my system.

Well that's your problem that you either don't have the ability to check your cells or you have to be in the same room.

 

[TomC4306]

Deleted

 

[MaximusAnonus]

There are other QUALITY BMS's supported. And others being added. Go look for yourself. Daly balance current is miniscule, preset voltages potentially damaging. But Hey! Fanboy knows best.

Please link any other Home Assistant compatible BMS. I can still cancel my order as it wasn't shipped yet. Thank you.