Running 2 or more BMS in series

I was thinking...dangerous I know!!!

Anyway, the idea is this. use three 12S or two 16S BMS in series to get to 32S or higher. They are ridiculously cheap compared to a 32 or 36S BMS. Most of the cost in a BMS is in the mosfets. Since a 36S BMS needs 200 volt mosfets that adds significantly to the cost. If you think of a battery pack that includes a BMS as a single unit, you have a positive and negative wire that you connect to the outside world. Why not put two or 3 complete packs in series? I'm going in a couple of directions here.

1. The first one is to make modular battery packs that are all identical...say 6S or 12S blocks.
2. Be able to use the same packs interchangeably in everything.
3. Make each pack removable by simply undoing a few screws or cover and pulling out the pack.

So this gets me back to running them in series. 12S is pretty limiting IMHO. I have a scooter that runs on 12S LION, but honestly I max out the power band too quickly. I also have a scooter that currently runs at 20S and motor torque keeps going and going up to a much higher RPM. So depending on if I need a setup that is 12S, 18S or 24S or whatever, I can string multiples of these smaller packs in series to get higher voltage. I would also run them in parallel as well. Obviously there would need to be a back plane of some kind where the individual packs can be connected together in series and parallel to get the voltage and current requirements I need. I'm thinking of how easy something like this could be implemented with say 10000mah turnigy LIPO packs. I would need to make an end cap that contained the power connections and then hide a 6S or 12S BMS inside the end cap too. The whole thing would plug in end first. This same idea could be implemented in 18650 packs as well.

So then the real question is this...
Why not use several small BMS in series? Lets say, each individual pack can deliver 15-20 amps. There's lots of 6S or 12S BMS that can do that...no big deal. What kind of issues am I likely to see by running multiple smaller BMS in series? They are cheaper than a single larger BMS since there are no high voltage/high amperage components.

I've been looking at the Mitsumi MM3474 LION control chip. It is capable of handling 3 to 5 cells depending on how you set its two select pins. It detects HVC, LVC and over load conditions. I've seen implementations of this chip where they are used in up to 5 in series on the same BMS. Each 3474 only ever sees a maximum of 5S or 20.5 volts. Since the 3474 can handle a maximum of 30 volts, this is quite workable. Each 5 cells/3474 is daisy chained to the next 5 cells/3474. I see no reason why you couldn't use this little controller at 200S since each one only ever sees it's 5 cells. The total voltage of the entire pack is divided among these groupings of cells.

So this gets me back to running BMS in series. Since it is commonly done on the same BMS at the control ship level but is using the same switching mosfets in common for the entire BMS, why can't a daisy chain several BMS in series and get the same result with several lower cell count BMS's ? What's really the difference between two 12S BMS in series and one 24S BMS?

The only thing that came to mind that might be an issue is the mosfets. If I used 12S BMS as my standard and lets say I have 3 BMS in series, then that's effectively 36S. Each BMS has the 148 volts divided equally into thirds so it should only ever see 12S or 49.2 volts. My thought was, what if I have a 3 X 3 arrangement. IE: Three sets of 12S packs in series of 3 and in parallel at their end points. The idea is I need 30,000mah and 148 volts made out of 10,000 mah turnigy packs. What happens when a single 12S pack shuts off? The other two strings of three 12S packs have to take up the load obviously. There's another implication too. Current through that 3 pack string with the down BMS is going to drop to close to zero, but the two remaining packs in the string will now have 74 volts across them. Will this fry the remaining BMS's? I think as long as nothing goes wrong that multiple BMS in series will be OK. It's when things go wrong that concerns me. Every EV I've ever built used multiple BMS on separate battery packs in parallel, but there were no BMS in series. It has worked well, when you need more current delivery than a single BMS can deliver.

What do you think? Is running BMS in series doable and OK?

Caught the title. Not read the whole post yet.
I'm working on similar project for Volt 12s users.
Might adapt to other batteries.
Pcb plugs directly into 12s Volt pack. Xh output to 12s bms. P- connects to pcb with small wire.
Pcb has isolated outputs that can be connected in series to any number,of modules.
Also has isolated "enable" input that powers the circuitry. When disabled only balance leads are connected.
The Daisy chained outputs from each board control ac or dc ssr.

Hope it can work with any configuration of series/parallel 12s modules.

Boards will be in tomorrow for first test.

When a BMS shuts off, it turns off the FET, which opens the circuit. This means that the FET could then have a potential voltage across it dependent on the series circuit voltage.

You can test this scenario easily enough to verify.

If it does end up being true, then the FETs will need to be rated for the full potential voltage of the pack, including any extra voltage the system could create as regen or otherwise generate from the motor in overspeed (like a downhill run, etc).

amberwolf said:

When a BMS shuts off, it turns off the FET, which opens the circuit. This means that the FET could then have a potential voltage across it dependent on the series circuit voltage.

You can test this scenario easily enough to verify.

If it does end up being true, then the FETs will need to be rated for the full potential voltage of the pack, including any extra voltage the system could create as regen or otherwise generate from the motor in overspeed (like a downhill run, etc).

Click to expand...

Yeah! That's a good point, but easily remediated by using high voltage mosfets. Lets say the 16S BMS uses some kind of 100 volt mosfet. If I replace them with IRF4115's which are 150 volt mosfets, then at 32S or 131 volts, they will survive nicely.

Inwo said:

Caught the title. Not read the whole post yet.
I'm working on similar project for Volt 12s users.
Might adapt to other batteries.
Pcb plugs directly into 12s Volt pack. Xh output to 12s bms. P- connects to pcb with small wire.
Pcb has isolated outputs that can be connected in series to any number,of modules.
Also has isolated "enable" input that powers the circuitry. When disabled only balance leads are connected.
The Daisy chained outputs from each board control ac or dc ssr.

Hope it can work with any configuration of series/parallel 12s modules.

Boards will be in tomorrow for first test.

Click to expand...

It sounds like you have the same idea as I do. My idea is to make modular battery solutions. It would be cool to have a plug-in box that you can drop your packs in, ride that EV and then pull the packs and drop them in another EV. I currently have 2 functional EV's and that over time will grow to several more. Batteries are the big expense in an EV. Swappable batteries would make this into a 1 time purchase that covers everything. I'm thinking 2 module sizes are most probable. 12S expands to 24 and 36S easily. 16S expands to 32 and 48S. Most folks are not building higher than 200 volts so this seems pretty reasonable. Regardless of what you standardize on, after that it's a matter of just rinse, wash and repeat the battery holder that takes the fixed size modules. Even 6S is an option, but BMS in 6S tend to not be very beefy. I've found a decent 16S BMS that I like a lot that has no monitoring abilities. I have 2 of them and 5 of the 20S version. They work consistently well. I also found a 16S smart BMS that looks promising. A friend of mine is redoing the software for it so that it's not super buggy Chinglish. It will run on your android phone. He has several of these BMS and I am looking into getting a couple myself. They upgrade for more current easily.

I lost interest.
I have these boards to give samples, if someone wants a place to start.



Chevy Volt specific but adaptable.

Resurrecting this long dead thread...

To answer my own question becasue I've now done this...

If you can use the balance connections and B- only on the BMS, then daisy chaining BMS's is quite doable. Ignore the mosfets and don't use P- or C- at all. Now the BMS's are just balance boards. As long as you can live without being able to shut down the BMS (turn off the mosfets), this is a very workable solution for balancing only.

If you need 64S or 48S or whatever...each BMS only sees the cells it is connected to and therefore is not in danger of self destructing.

Scenario:
Lets say you have an abundance of 7S BMS and you have 49 cells in series. As long as each BMS ONLY sees its 7 cells, it can't be damaged by the total pack voltage.

Lets fast forward to the top most 7 cells in a 49S pack. The 7S BMS connects its B- connection and its P0 balance pin to the negative end of the 7th cell from the positive end of the pack. Each balance lead there after connects to the next interconnect between cells with the final wire in the balance cable going to PACK+. The BMS never uses it's mosfets since P- or C- don't get connected to anything and each 7S BMS only sees 7 cells. It's best to electrically isolate each BMS.

Ive done this exact scenario and it worked great with this BMS.



This is 5 BMS from my first test, but you get the idea.



Since that worked so well and I had the above 32S LIPO pack, I took 2 dumb 16S BMS and did the same thing. The pack charged and balanced and discharged just fine. The individual BMS's never shut themselves down or in any way gave me reason to believe they were doing anything other than working.

I'd say...yes...very doable...just don't use P- or C-.

If you still need the pack to shutdown on LVC or HVC, then you can "AND" the two gate signals from the BMSes, and have them drive a single relay (contactor) on the pack positive (or negative, whichever works for your system), via whatever small electronics is necessary (transistor driver, probably).

Then both BMSes have to be "on" for the pack to work, meaning neither one is in the HVC or LVC state.


(depending on how their outputs work--if they have separate charge and discharge ports you can use two separate relays, each controlled by the appropriate ANDed outputs of both BMSes. )

amberwolf said:

If you still need the pack to shutdown on LVC or HVC, then you can "AND" the two gate signals from the BMSes, and have them drive a single relay (contactor) on the pack positive (or negative, whichever works for your system), via whatever small electronics is necessary (transistor driver, probably).

Then both BMSes have to be "on" for the pack to work, meaning neither one is in the HVC or LVC state.


(depending on how their outputs work--if they have separate charge and discharge ports you can use two separate relays, each controlled by the appropriate ANDed outputs of both BMSes. )

Click to expand...

Thanks for adding that!

Large relay contactors are readily available but they cost a lot. I have some 50 amp 3 pole relays and they cost me $40 each. Paralleling the contacts works OK. I bet under serious loading the contacts in them would NOT last long since they are exposed to the air. You really want inert gas filled contactors and relays so the main contacts survive. Those buggers are NOT cheap!

The rest of the circuitry is so simple to implement if you know basic logic gates. An alarm that's highly noticeable when the mosfet gates get turned off wouldn't be an automatic cut-off. It would let you know to stop whatever you are doing and pay attention to the battery pack. Flipping off a manual contactor would do the same job for far less money.

Edit:
I just looked at Tesla battery contactors. I have no idea what they are rated for, but they cost around $145 each for used ones. That's the old style that died all the time. The new style in used are $245 each. I can buy quite a few manual contactors for that money!

ElectricGod said:

If you can use the balance connections and B- only on the BMS, then daisy chaining BMS's is quite doable. Ignore the mosfets and don't use P- or C- at all. Now the BMS's are just balance boards. As long as you can live without being able to shut down the BMS (turn off the mosfets), this is a very workable solution for balancing only.

Click to expand...

I'm not sure I fully get how you wire these?
Just balance leads only but where would I connect the BO or negatives? Pair them up and connect to my main - wire, or splice in somewhere else?
Thanks

Also I'm already bypassing the discharge on my bms's, what would be any drawbacks to bypassing the charging as well?
Cheers

If your BMS supports using just the balance leads and NOT the P- connection, you can run BMS's in series. You'd be using the balance function only. I've tried this on a few BMS's and it works fine. You won't have shut-off functionality...aka what the mosfets do, but if that's not important to you, then this is viable. If you need 48S, 3 16S BMS's using just their balance leads can do the job for balancing for your pack.

You can find out if your BMS balances without P- easily enough. Just don't charge through P- or C- and let a cell get a bit over voltage. Stop charging and see if the BMS drops voltage after a while. Some BMS need B- connected and whatever is the batt- pin in the balance cable. This is OK since no significant amount of current needs to pass through B- for balancing. Just connect B- and pin 0 in the balance cable together at BATT- on the pack.

What BMS were you using in that photo of the five stacked? And do you have a small display next to it or is that something separate?

Hi ElectricGod

Unfortunately I can no longer see your pictures. I'm having trouble figuring out how you wired in a charger to charge your batteries if you do not use C- or P-

I'm trying to create a 12s battery with four 3s BMS wired in series.

I think your method would work however I'm confused as to where to wire in the charge port. I also don't want to dis-charge through my BMS and simply want to use the BMS to balance charge my cells.

As you can tell we have a very similar common goal, I would love more feedback from you regarding how your endeavor has worked out and obviously my above request regarding the charger wiring.

Kind regards

electricmanSA said:

Hi ElectricGod

Unfortunately I can no longer see your pictures.

Click to expand...

He hissy-fit-deleted everything and stormed off the forum a long time back, because he didn't want people to talk about anything except what he wanted, in threads he was in, and since he couldn't control everyone, he got mad and left (actually had to be banned due to behavioral issues at the end). So all the stuff that is gone is just gone.

Unfortunately I don't know the answers to your questions, but someone else that has done this may come along to help with that part (I just didnt' want you to sit there waiting from an answer from EG when it wont' come).

Thanks for the response. I will be waiting patiently.

I havent read anyones answer to this but i can tell you this.

I have 4 x 36v (42V fully charged) 15ah battery packs.
I have paralleled them in pairs, doubling the amps effectively to 30ah.
THEN i put the two pairs in series. so i doubled the amps and turned it into 84V 30ah battery power.

ALL KEEPING THE 10S BMS for each pack
things you should take seious not of
DO NOT RUN the power from the BMS, if you use the battery
you take the BLACK directly from the battery negative
and you take the positive red from the end of the series.
you do NOT USE THE BMS TO CONTROL DRAIN
If necessary then remove the bms's that are in series and put one for the entire battery,
so remove the two 10s bms and put a 20s on there.

ALSO IF YOU ARE KEEPING THE BATTEERIES WITH THEIR 10s BMS DO NOT CHARGE THE ENTIRE
BATTERY ARRANGEMENT WITH ONE CHARGER. Meaning that if my battery pack in series went from 42 to 84v,
DO NOT use an 84v charger.
You must have separate charging positions for each bms. you can have two 42v chargers, one for the first 10s bms and then when thats done charge the other bms. or get two chargers and connect them as if you were charging the series battery packs individually. DO NOT STUFF IT UP.
the BMS needs a ZERO point, the first BMS will probably work with an 84v charger while in series but the secind one will see 42 to 84v , not 0 to 42. essentially i would imagine the second BMS wouldnt allow charge as you are already over the cut off high point. and because of that the 84v wont be evenly distributed and send itself entirely to the first BMS, which would be very nasty. This same scenario in a way occurs if you use the dischage via the BMS boards in series.

You can though without issue charge multiple bms packs in parallel off a single charger, it will however take twice as long to charge obviously.
Usually the BMS in battery packs wont allow high discharge and cut out before the lets say motor controller cut off will kick in. and will also cut you off super early from discharge if the load draws too much initial power to drop volts below the bms cut off momentarily. This is a headache which made me never discharge through a BMS. BTW my batteries have never lost balance not having the BMS control discharge. Even with a 20s battery used for another project which is 54ah.

So all up yes you can use them but charge them individually not in deried. BTW they cam still be connected in series while you charge each one but move the charger to the correct location of the negative and positive of each BMS. And dont go through the BMS discharging. Or remove the BMS boards in series and connect the entire series with a single sized correctly.
REMEMBER IF USING MULTIPLE CHARGERS, this example is 42v chargers that THE NEGIS CONNECTED TO THE BMS negative AND THE POSITIVE IS CONNECTED TO THE END OF THAT BMS string so the 10s is physically comnected to the positive of that individual pack. i say this BECAUSE YOU ARE NOT PUTTING THE CHARGERS IN SERIESYOU CANNOT GET THE FIRST CHARGER RED AND CONNECT TO THE 2nd CHARGER BLACK ONLY. THEY MUST TOUCH THE PACKS OR YOU HAVE TURNED THE CHARGERS INTO A 84V CHARGER,you want everyb bms to see from as charger 0 to 42v.