Lithium-ion multiple batteries on parallel - same challenge as Lead Acid or not?

[Ampster]

.............. That is why EV manufactures DO NOT TOP BALANCE or ever allow the customer to ever fully charge a lithium battery. They would be sued into bankruptcy if they did on warranty claims and loss of property and life.
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When you say top balance do you mean when the BMS shunts start working or are you talking about putting all cells in parallel and charging that group to something close to max voltage?

[Sunking]

Most of the horror stories about Lithium batteries are LiPo stories.

Wrong again. The bad boys of lithium batteries are Lithium Cobalt, the exact ones Telsa uses, laptops and cell phones. RC planes do not make the news.. Yes LiPo's are dangerous, but RC pilots are fully aware of the dangers and take necessary measures and know what they are dealing with. Consumers are clueless of the dangers.

[nebster]

Yes it does. Struggle a bit with the nomenclature though - is my modules 7s3p or 3p7s or is it the same? When connecting 10 of these in parallel would that be 10p7s3p?

There's unfortunately no standard ordering, at least on the web. I choose left-to-right, smallest-to-largest ordering, because in real life everything is built from small things up to big things. In any event, 7s3p and 3p7s are most certainly not the same if used in the same sentence!

For your proposal, I would call your pack "3p7s10p". If you like to describe the topology from big to small, your way works, too.

The A123 cells in the modules is apparently of a safer design that that of a Tesla module.

They are LiFePO4, which is a relatively safe lithium chemistry compared to some of the blowtorches.

[Roil]

No, you can't have a "BMS per cell" in your scenario. Once three cells are paralleled, they are electrically one. You can put a monitor on each 3p.

Agree It would be a BMS per 3cells

First of all, at issue is what degree of protection you want, and you could, of course, choose to have less protection. It would work fine until it maybe didn't, or until it turned into a blowtorch. Blowtorches are more likely with some lithium chemistries and almost impossible to achieve with others.

The Tesla modules are 74p, and then 6s. Then 16 of those modules are placed in series to make 74p6s16s. Tesla uses a monitor on each of those 96 74p blocks. If you just buy a Tesla module, you would want to implement a similar monitor on your 6 blocks.
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Does that help? .........

Yes it does. Struggle a bit with the nomenclature though - is my modules 7s3p or 3p7s or is it the same? When connecting 10 of these in parallel would that be 10p7s3p?

The A123 cells in the modules is apparently of a safer design that that of a Tesla module.

You are correct that a BMS is at the cell level, but not understanding how to do it. First thing is you cannot define a BMS or what it is. It can be a voltmeter or a Vampire Bleeder Board. A BMS needs to be integrated into the charger.

OK say we have 4S2P as pictured above. Conventional method would require 8 Cell Boards connected with 4-cells per charger. Each string will not charge at the same rate. One would be charged before the other thus requiring a second charger or control which is expensive and a poor solution that does not work well. The Alternate method is what Lithium batteries use paralleled at the cell level, not string level. Regardless if it is 4S2P as shown, or 4S99P only requires 4 cell boards and one charger. Otherwise you would need 396 cell boards and 99 chargers if you used Conventional. Good luck with that DYI.

Sunking - your statment of individual chargers is difficult to understand, individual cell monitoring yes, but chargers?

My assumption is that monitoring all of the 70 cells in a 10p7s3p set of cells with a proper BMS solution that does control both charging and load and that is capable of disconnecting both is a safe solution. The Batrium solution I am considering does have integration with both my Victron Multiplus Inverter and my Victron MPPT and it will trigger a disconnect if cell temp or voltage is out of range.

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[SunEagle]

Is this an RC forum? I don't think anybody in their right mind would put LiPos in a vehicle or even for stationary storage. Most of the horror stories about Lithium batteries are LiPo stories. The few Tesla fires have all been because of severe trauma to the pack. There may be a case of spontaneous combustion of a Tesla pack but I have not heard one.

The LiPo example was to show that even with a high grade BMS charger those batteries can over heat and go off on you. There are other types of Li batteries that are being used very successfully in EV's and Storage systems but they also have expensive and very smart BMS's to keep them happy and cool.

IMO just about any Lithium chemistry battery has the potential to over heat and ignite. Unless you have the knowledge and equipment anyone using a Li battery in a DIY plan is playing with fire. Just be careful and never get complacent.

[Ampster]

I agree if the BMS does it's thing. But even then I have seen LiPo's get very hot and ignite while they are being charged. That is why I use a thermal bag for my RC batteries. You just never can tell when one cell decides to commit suicide.

Is this an RC forum? I don't think anybody in their right mind would put LiPos in a vehicle or even for stationary storage. Most of the horror stories about Lithium batteries are LiPo stories. The few Tesla fires have all been because of severe trauma to the pack. There may be a case of spontaneous combustion of a Tesla pack but I have not heard one.

[nebster]

In just one tiny small job, I use more batteries than you will in a lifetime. I build very large scale projects. Ever design and built a 500,000 watt system before or use up to 48 volt 60,000 amp hour battery plants before? I know the answer is NO. I have over 100 times. I have also built 10 custom EV and a half dozen racing golf carts all using Lithium batteries. I do this for a living for 40 years.

Precisely none of which involve a lithium pack with multiple parallel, managed strings.

Are you done ranting now?

[Sunking]

I'm doing it right here, right now on my ESS bank. Your statements suggest that you haven't run a large bank with multiple paralleled, managed strings before.

In just one tiny small job, I use more batteries than you will in a lifetime. I build very large scale projects. Ever design and built a 500,000 watt system before or use up to 48 volt 60,000 amp hour battery plants before? I know the answer is NO. I have over 100 times. I have also built 10 custom EV and a half dozen racing golf carts all using Lithium batteries. I do this for a living for 40 years.

I'm not sure what this means.

Yes I know, it is over your head and pay grade.

[nebster]

Wrong, dead wrong.

I'm doing it right here, right now on my ESS bank. Your statements suggest that you haven't run a large bank with multiple paralleled, managed strings before.

A BMS is the root cause of Lithium battery failures.

It's far from the only root cause. A cell can have a manufacturing defect. A cell can be subject to too much heat or cold. A cell can be overcharged with no BMS present. And plenty of other things can cause a failure.

No two strings charge or discharge at the same rate. You are on the wrong track semi-right idea. A 100 AH Cell will have roughly half the Resistance of a 50 AH cell. No 2-cells of the same capacity have equal resistance.

I'm not sure what you're trying to say, but let me be very clear: if you place two cells of unequal capacity but identical chemistry in parallel and hold them at an equal charging voltage (which is what "parallel" means), they will each charge in proportion to their capacity and up to the same SOC and the same voltage.

It is this principal that allows strings of series cells to be placed then in parallel and charged with one charger under one charge algorithm, safely and with no issues. As long as the strings have the same chemistry and thus voltage curve, their individual capacities are irrelevant for parallel charging.

Unequal charging is not a problem for any battery type except lithium.

I'm not sure what this means.

Top Balance is a killer of lithium cells and the root cause of failures and fires. All courtesy of the BMS. That is why EV manufactures DO NOT TOP BALANCE or ever allow the customer to ever fully charge a lithium battery. They would be sued into bankruptcy if they did on warranty claims and loss of property and life.

Balancing within a series string is a separate concern and is independent of whether charging several strings in parallel is possible (yes), possible with one charger (yes; contrary to what you wrote), or a foolish decision (no, in some cases; also contrary to what you wrote).

However, for your own edification and perhaps as material for discussion in another thread, I encourage you to go look at what Tesla does with their balancing algorithm. It happens at the top of the charge envelope, not in the middle or at the bottom.

[Sunking]

This is incorrect. The paralleled strings will charge at different rates, proportional to their capacities,

Wrong, dead wrong. Batteries charge rate is determined by the Internal Resistance and SOC. A BMS is the root cause of Lithium battery failures. No two strings charge or discharge at the same rate. You are on the wrong track semi-right idea. A 100 AH Cell will have roughly half the Resistance of a 50 AH cell. No 2-cells of the same capacity have equal resistance.

Unequal charging is not a problem for any battery type except lithium. Top Balance is a killer of lithium cells and the root cause of failures and fires. All courtesy of the BMS. That is why EV manufactures DO NOT TOP BALANCE or ever allow the customer to ever fully charge a lithium battery. They would be sued into bankruptcy if they did on warranty claims and loss of property and life.

[SunEagle]

That's true: if a cell or a string in parallel goes nuclear, all bets are off. In that case, the notion of SOC goes out the window, too, though.

But in normal operation -- which a BMS must be in place to ensure at all times -- paralleled strings will charge and discharge just fine off a single charge bus. Even if the underlying capacities of the strings vary considerably.

I agree if the BMS does it's thing. But even then I have seen LiPo's get very hot and ignite while they are being charged. That is why I use a thermal bag for my RC batteries. You just never can tell when one cell decides to commit suicide.

[nebster]

Without a BMS, parallel lithium cells will charge at different rates and some may exceed their maximum voltage range without a limiter to bring them back. So they may not all reach the same SOC at the same time and some may actually get too hot to stay stable.

That's true: if a cell or a string in parallel goes nuclear, all bets are off. In that case, the notion of SOC goes out the window, too, though.

But in normal operation -- which a BMS must be in place to ensure at all times -- paralleled strings will charge and discharge just fine off a single charge bus. Even if the underlying capacities of the strings vary considerably.

[SunEagle]

This is incorrect. The paralleled strings will charge at different rates, proportional to their capacities, but in so doing they will all reach the same SOC at the same time. It will not be the case that one string is "charged before the other."

Without a BMS, parallel lithium cells will charge at different rates and some may exceed their maximum voltage range without a limiter to bring them back. So they may not all reach the same SOC at the same time and some may actually get too hot to stay stable.

[nebster]

One would be charged before the other thus requiring a second charger or control which is expensive and a poor solution that does not work well.

This is incorrect. The paralleled strings will charge at different rates, proportional to their capacities, but in so doing they will all reach the same SOC at the same time. It will not be the case that one string is "charged before the other."

[nebster]

I understand the need for a BMS per cell. That would mean many individuell cell monitors but they seem to be available for around 12$ a pcs with an 8W capacity. www.batrium.com have a solution for this and they do support cells wired in a string/parallel configuration all the way up to almost 200 cells

Why would I need individuell charger per module?

No, you can't have a "BMS per cell" in your scenario. Once three cells are paralleled, they are electrically one. You can put a monitor on each 3p.

There is no need for a charger per module. That would be an incredibly expensive and even more unusual way to do it.

Finally, there are good reasons to parallel multiple strings of cells in the real world. The classic motivation is to have some degree of sub-pack redundancy: if one cell in one string goes bad, the computer can isolate that string, but the rest of the pack can continue to support critical loads. There are other reasons to go that route as well. But, certainly, it is less common.

Doing that requires more hardware, more expense, and more design consideration. It becomes unrealistic when the parallelism is very high (your example of 20 or 30 or 40 strings). My own pack has 6 parallel strings, and I wouldn't want many more.