Selecting BMS approach on my first build

Hello, and thank you in advance for help. I am planning my powerwall build, and I have all the components understood and managed, but I'm having trouble wrapping my head around the correct approach to BMS that I should use for my system.

Design is based around used 18650 cells, all used and tested and sorted by mAh. I am building a 12v system, goal of 7-10 kWh (not sure total capacity, as I have not gotten through all of my cells yet). I have two approaches I am considering, and I would like advice as to what would be the safest.

Option 1: I build 3s8p packs, for a nominal voltage of 12v. Each pack will have it's own, relatively cheap, 3s 60A BMS (AITRIP). I connect each 12v battery in parallel until finished. This way I would be able to add packs to my system as I go. I would of course be careful about SOC when adding a new pack to an existing system. My question is, would the dozens of eventual BMS's compete with each other when all in parallel?

Option 2: I build simple 1s?p large packs, say 30-60 cells per pack, all in parallel, for a nominal voltage of 3.7v per pack. I wire 3 in parallel for a high ampacity, 12v bank. I use a large BMS reading each series for monitoring. This would be good, but in order to add capacity later on, I would only be able to add large banks of 3s at a time. I would also need to make sure that the master BMS has space to add capacity.

If I go with option 2, I am a bit lost on which BMS to select. It would need to have a high-amp rating, and room to potentially add more capacity. I am a bit overwhelmed with what is available, and I was hoping that someone would be able to advise me on which BMS to use, given my plans and requirements.

Thanks in advance!

harrisonpatm said:

I build 3s8p packs, for a nominal voltage of 12v

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Nominal voltage will not be 12V. It'd be 11.4V, which is considered by most lead acid based systems to be a low or dead battery. 11.1V is usually the cutoff voltage. Full charge would be 12.6V, which is considered to be about 50% charged.
Lead acid nominal voltage is 13.5V, btw.

Hopefully before you've gotten too far, why 12V system?
And going 4s isn't any better.

As far as battery construction, if you plan on adding more over time, the easiest would be to build a bunch of parallel packs with equal capacity, what ever your cell count is per parallel pack. Then connect the required packs in series to get the proper voltage.
For example, if you go 24V, that'd be 7s. So you would make 7 packs with equal capacities. Then connect all 7 in series to get the 24V system.
Then you'd have a BMS that monitors the voltage of each pack to make sure they are all equal to one another during charge and discharge.

Option 2 is basically the same as Option 1. Except that you have individual monitors instead of a combined system. Batrium is such a unit that can do both. diyBMS is another, altho this one requires building the boards and flashing the firmware yourself. But it is pretty rock solid, and continuing to grow.

Korishan said:

Nominal voltage will not be 12V. It'd be 11.4V, which is considered by most lead acid based systems to be a low or dead battery. 11.1V is usually the cutoff voltage. Full charge would be 12.6V, which is considered to be about 50% charged.
Lead acid nominal voltage is 13.5V, btw.

Hopefully before you've gotten too far, why 12V system?
And going 4s isn't any better.

As far as battery construction, if you plan on adding more over time, the easiest would be to build a bunch of parallel packs with equal capacity, what ever your cell count is per parallel pack. Then connect the required packs in series to get the proper voltage.
For example, if you go 24V, that'd be 7s. So you would make 7 packs with equal capacities. Then connect all 7 in series to get the 24V system.
Then you'd have a BMS that monitors the voltage of each pack to make sure they are all equal to one another during charge and discharge.

Option 2 is basically the same as Option 1. Except that you have individual monitors instead of a combined system. Batrium is such a unit that can do both. diyBMS is another, altho this one requires building the boards and flashing the firmware yourself. But it is pretty rock solid, and continuing to grow.

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You are right regarding voltage, I should have been more specific. I was rounding up top 12; 11.4 nominal voltage is my plan. My fault.

I don't plan on using lead acid.

Reason for 12v system is that I am working on microgeneration via various methods, some of which won't be able to get up to high voltage. 24v is possible, but I want to work with 12v to accommodate a wider range of smaller charging sources.

You mentioned adding more over time by building parallel packs. Yes, that it what I mean by option one. Let me be specific. Say I have one pack of 3s8p, at 11.4V. I have a small BMS board, for less than $10, wired to this one pack. Call that one unit. How many units, each with it's own BMS board, can I connect in parallel safely? 10? 50? None?

Please clarify for me if I am wrong, but your suggestion of connecting parallel packs of equal capacity, then lining them up in series (say, 3 to get 11.4V), means that every time I want to add capacity to the system, I would need to do it in units of 3 for 12V or 7 for 24V. I was hoping for a more modular approach.

Finally, I can see that options 1 and 2 are similar, but you said, "individual monitors instead of a combined system." I guess what I'm really getting at is, is several individual monitors unsafe? How many can I continue to connect in parallel to add capacity without resorting to a single combined system?

I was mentioning Lead Acid because almost everything is based off those voltage ranges, chargers and such. Very little will work off Lithium Ion, unless it is LiFePO4 cells, which is a direct replacement for Lead Acid voltages.

As I mentioned earlier, 11.4V is considered low voltage by most systems. Chargers and such. With 3s you will be fighting to keep from over charging, and with 4s you'd be fighting to keep from over discharging.
If you absolutely need 12V range for devices, it is highly recommended to use a buck converter from a higher voltage. That will give you the best of both areas.

If each battery string has its own BMS, you can connect as many in parallel as you want "after" the BMS ports. Otherwise you will decrease the capability of the BMS with each unit you add to the parallel strings.

Multiple monitors will communicate with each other with some systems, such Batrium and diyBMS. Other system as long as the voltages stay within proper ranges they'd be fine.

You do not want to charge lithium was standard dumb, or even smart, lead acid chargers or wall chargers. They need to be Lithium capable. If not, you risk damaging the cells and possibly causing a fire. On a 3s/4s system this is very easy to over-charge or over-discharge. With a 7s (24v) system it's less likely to happen. Going with 10s (36V) or 14s (48v) is even safer. The more in series you go, the wider the voltage ranges from discharge to charge window you have to work with. Not recommended to go 96v for beginners as that voltage range is getting into the dangerous territory for shorting things out accidentally, like through your hands.

Korishan said:

If each battery string has its own BMS, you can connect as many in parallel as you want "after" the BMS ports. Otherwise you will decrease the capability of the BMS with each unit you add to the parallel strings.

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Thank you. I do understand charging requirements and the importance of sourcing a charge controller compatible for 11.4V. Multiple charge controllers, as it will end up being.

Each battery string having its own BMS makes it more modular and more upgradable than one big connected system. I understand that the more units I have in parallel, requiring a small BMS for each become less cost effective for each unit added, compared to a singular system. All I'm after now is whether this approach is considered safe.

Dependent on the quality of BMS you use, and how smart it is, yeah, it is safe to do it that way.

I'm not familiar with any 3s BMS systems that are quite reliable/smart enough to do that level of monitoring. Partly because I've never really looked into those. The ones i have seen, and posted most places, are the cheap units that have no balancing and will only disconnect the battery in a fault condition. Then it's up to you to figure out what the fault is.

Thank you again, I appreciate your responses. I understand that this route seems silly,, that it won't have any active balancing for each pack, that it'll disconnect a pack and leave the troubleshooting up to me. However, from what you're telling me, this might seem the best option for my admittedly niche requirements. I like the idea of hands-on management, of a dozen modular packs all with individual safeties working together in parallel. If one disconnects, it can be swapped out with another. And I was always planning on keeping the units small anyway, so a faulty unit won't be a huge inconvenience to either myself or the bank as a whole.

As long as each pack is safe with its own BMS, and that connecting dozens of them in parallel won't be inherently unsafe. I'm not worried about active balancing.

harrisonpatm said:

Thank you again, I appreciate your responses. I understand that this route seems silly,, that it won't have any active balancing for each pack, that it'll disconnect a pack and leave the troubleshooting up to me. However, from what you're telling me, this might seem the best option for my admittedly niche requirements. I like the idea of hands-on management, of a dozen modular packs all with individual safeties working together in parallel. If one disconnects, it can be swapped out with another. And I was always planning on keeping the units small anyway, so a faulty unit won't be a huge inconvenience to either myself or the bank as a whole.

As long as each pack is safe with its own BMS, and that connecting dozens of them in parallel won't be inherently unsafe. I'm not worried about active balancing.

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A key design issue with many independent BMSs / batteries in parallel is to figure out how to monitor them in case 1 or more goes 'bad'. You need a BMS to monitor the BMSs - even if that's only a manual procedure. For example, if you have 6 x batteries in parallel and 2 of the BMSs fail you'll have 4 'active' and 2 that are disconnected. You might (the rest will be overloaded/fail) or might not (less capacity) notice this as the overall power of the battery bank declines - depends on your operational monitoring - e.g. BMS of BMSs.

There's also a matter of scale of independent BMSs to monitor. If you do 80p instead of 10 x 8p you'd have 9 less BMSs per 80p.

Some key goals for my battery bank:
- All or nothing - e.g. I want the entire battery bank to disconnect (shunt-trip) / go offline / cease operation if any part of it malfunctions. This is because a malfunction would more likely be catastrophic than minor and I want to be forced to investigate/fix immediately.
- All automatic - e.g. I do not trust 'manual monitoring' solutions as being effective for overall operations. I tried this in the early 6 months of my efforts and it was a failure / caused damage to some packs.

To give you the vision of multiple batteries in parallel with 1 BMS (1 controller) instead of many independent BMSs.....
Batrium is designed to monitor individual packs, even when they are independently arranged in parallel batteries. This let's me add 'batteries' but I still have 1 BMS. Here's what 6 parallel batteries of 14s100p look like to Batrium and I could add up to 11 more.


There are other systems of similar concept, some much less expensive but much more DIY.

That's a very good point. And if anything, this thread is helping sway me in the direction of something like Batrium. I'm still a ways off from my final build, just wanted to make sure I was getting the right components together, so I have plenty of time to consider the best option.

I will say, regarding a BMS for a BMS: this is actually a feature I wouldn't mind doing myself, manually, at least in theory. Don't get me wrong, I know I'm not actually in that situation yet. But in the example given by OffGridInTheCity, where I have 6 packs total, and 2 BMS fail, leaving the 4 to pick up the slack and potentially overload... I was actually imagining something more like 50 small packs in parallel, 1 fails, and the other 49 easily pick up the slack without straining. I like the concept of modular, redundant components in general, not just in batteries.

But the point is well taken. Some system needs to be in place to make sure each individual pack is working before things get out of hand. What volt meters or visual indications will I be building in each pack so I can easily see when one is dropping out? One master BMS for the whole system has its pros.

If you do voltmeters and get 20 or 40 of them - it will make a jumble of things to read and won't tell you if the BMS is actually allowing the individual battery to connect to the parallel bus. If you put something on the downstream side of the BMS it will be powered by all the other things paralleled. If you put something inline it would have to handle the amps.

Maybe you're best bet is to find a BMS that has an (LED) indicator already built-in and then position them so you can see them at a glance.

Some packs I've torn apart have a switch to enable charge bar LEDs as a health indicator and perhaps you could find/reverse engineer a BMS and run wires out to an overall LED panel so you could toggle a master switch to get a comprehensive view?

Folks don't normally do this kind of thing and I don't know of any ready-to-go solutions. Be sure to let us know what you do - it will be interesting

Ooo, good idea on the master switch. For either the LED or voltmeter. Thank you for that