Can we series BMS?

Hello, I have a question that this community might help me with. So I'm building and electric reverse trike (imagine Vanderhall Venice but electric), and I'm using QS motor's E-car hub motor. To power it, I need a 96v battery pack and the controller have a maximum working voltage of 120v. So I decided to go with a 28s BMS system yet the price seems high and I'd like to make a "modular" battery pack system. I've been thinking if it's possible to series 4 7s BMS OR two 14s BMS to make it 28s? Since I was inspired by Tesla's battery module system which has a 22.2v yet their powertrain is over 350v. I'm thinking of using a Daly 7s 100A or 14s 100A BMS and series it together for the final battery pack. I'm not an electrician and I heard people saying that I might break the mosfets if I series them yet it's safe if we parallel them, maybe another BMS could work? I need your help, so thank you for helping...

Whether it will work or fail depends on the parts used and the BMS design. The problem happens when one BMS shuts off, it now has the voltage of *both seriesed packs* across it's FETs.

There's a number of threads with variations on this question, some of which have better info than ohters, in this list
https://endless-sphere.com/forums/search.php?keywords=series+bms&terms=all&author=&sc=1&sf=titleonly&sr=topics&sk=t&sd=d&st=0&ch=300&t=0&submit=Search

There's other threads with more info but I can't find a single quick search that pops them up, would take manual digging for them.


What I'd recommend is not using the FETs of the BMS to deal with shutoff, but instead use the signal the BMS controls the FETs with to instead activate a relay that turns the controller's "keyswitch / ignition / enable" line on or off.

Yes I wouldn't be using the FETs to shut off the BMS but rather using a relay. Also I'm wondering if a BMS shutting off is caused by having load of greater than the continuous current for too long thus shutting off the BMS to keep it safe and to cool it down? In that way, what if I get a BMS with higher continuous current so I could prevent the BMS from shutting down?

Pegasus195 said:

Yes I wouldn't be using the FETs to shut off the BMS but rather using a relay. Also I'm wondering if a BMS shutting off is caused by having load of greater than the continuous current for too long thus shutting off the BMS to keep it safe and to cool it down? In that way, what if I get a BMS with higher continuous current so I could prevent the BMS from shutting down?

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I find that phrasing confusing.

Turning the BMS on/off is not what I think we're talking about, as in preventing parasitic loads when packs are in storage.

Rather, we want the BMS to trigger external contactors, which close-combine/open-isolate packs from loads and/or charge sources.

In a series arrangement, one module failing must shut down the bank - or at least that string - as a whole.

For paralleled strings, one string dropping out could allow the bank as a whole to comtinue feeding, maybe in "limp home" mode, since voltage is maintained, just Ah capacity reduced.

So precision is required, perhaps graphic schematics to identify which contactors are being open/closed, under which circumstances, by which protective sensor/trigger circuits.

This overall design goes beyond discussion of a single monolithic "BMS", multiple control systems will be involved, e.g. LVC/HVC, cells/group/midpoint imbalance, temperature sense, current level etc, and any number of them may open a given relay.

Only "all systems go" allows operation.

Pegasus195 said:

Yes I wouldn't be using the FETs to shut off the BMS but rather using a relay. Also I'm wondering if a BMS shutting off is caused by having load of greater than the continuous current for too long thus shutting off the BMS to keep it safe and to cool it down? In that way, what if I get a BMS with higher continuous current so I could prevent the BMS from shutting down?

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In practice, I only see BMSes cut power when there's a cell voltage fault (or when something inside goes pop). You don't really want the BMS to do current limiting, because they're not precise about it and they get hot. Controllers usually have a more certain current limit.

Pegasus195 said:

Yes I wouldn't be using the FETs to shut off the BMS but rather using a relay. Also I'm wondering if a BMS shutting off is caused by having load of greater than the continuous current for too long thus shutting off the BMS to keep it safe and to cool it down? In that way, what if I get a BMS with higher continuous current so I could prevent the BMS from shutting down?

Click to expand...

If you're not using the FETs, the BMS isn't going to get hot.

If it does, something is terribly wrong.


A BMS will shutdown for any of the reasons it is designed to. That's different for different units, so you'd have to refer to it's manual or spec sheet for what it's capable of shutting down for, and exactly what those conditions are supposed to be.

This^^ is so true, that really the term BMS is best viewed as a collection of functionality rather than assuming one device can / will do it all.

Very often, with top quality cells and a careful owner "the BMS" becomes the weakest link, and is the **cause** of pack failures, rather than preventing them.

By picking and choosing best-of-breed devices for different subsets, you can put together a system much better and more reliable than anything off the shelf.

amberwolf said:

Pegasus195 said:

Yes I wouldn't be using the FETs to shut off the BMS but rather using a relay. Also I'm wondering if a BMS shutting off is caused by having load of greater than the continuous current for too long thus shutting off the BMS to keep it safe and to cool it down? In that way, what if I get a BMS with higher continuous current so I could prevent the BMS from shutting down?

Click to expand...

If you're not using the FETs, the BMS isn't going to get hot.

If it does, something is terribly wrong.


A BMS will shutdown for any of the reasons it is designed to. That's different for different units, so you'd have to refer to it's manual or spec sheet for what it's capable of shutting down for, and exactly what those conditions are supposed to be.

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Sorry, I have been busy with other work and replying just now. So I will start understanding and learn how the BMS I am trying to use shuts down and see if I can still use that particular BMS or find other alternatives... Thanks for the advice

john61ct said:

By picking and choosing best-of-breed devices for different subsets, you can put together a system much better and more reliable than anything off the shelf.

Click to expand...

An example of this strategy would be appreciated.