Math Question to Electrical Engineers about BMS Capacity...

[eMark]

9.6amp cell energy rating x 4 in parallel = 38.4amps x 10% = 3.84 amps + 38.4 amps = 42.24 amps.
It doesn't cost but a few dollars more for a 50amp BMS.

Before deciding what BMS determine the Controller's amp rating. If you have a 50amp Controller get at least a 55amp BMS or better yet a 60amp BMS. What's the size of your motor (e.g. 500W vs 1000W). Is your riding enjoyment more about casual cruising (cycle life longevity) or raw performance (18amps for 60secs ever so often) ? Provide us with some more specifics about you ebike motor, riding style, terrain and Controller amp rating.

PS: I'm not an EE, but do have a half-decent brain. I'm 78 so my brain isn't what it used to be, but still gets me by - :wink:

 

[amberwolf]

. I'm 78 so my brain isn't what it used to be, but still gets me by - :wink:

More often than not, mine gets by me.

 

[eMark]

From the data sheet:
Max. cell discharge current: 9,600mA = 9.6 amps
Max. cell pulse discharge current 35A for 10sec, 25A for 30sec, 18A for 60 sec.
Bafang BBSHD stock controller max amperage is 30 amp.

I believe the above discharge current spec's are pertinent to my question which is....

Can anybody produce the math/equation that provides the maximum calculated BMS amperage for the described battery configuration?...with perhaps 10% safety factor or another safety factor you choose to assign based upon your schooling or personal experience?

As you know using name brand Grade A cells with amperage ratings you can trust (see bold/underlined above) is #1

Question: The BBSHD stock Controller max amperage is 30amp with max cell discharge of 18A (1.875C) for 1 minute. So 4 x 18A = 72amps (4P). Can your Bafang BBSHD be modified to handle say 50amps or even 60mps for 1 minute ?

Being you still like speed at 68 yrs young :thumb: what, if any, is the downside of using a 60A Controller (70A BMS) with your Bafang BBSHD ... if and when you need an extra burst of speed ... if only for a minute ?

 

[eMark]

When pulling 4000W ... https://www.youtube.com/watch?v=ttVPZBSEW3o&t=5s ... the right side of screen shows two different amp readings when Wattage reaches 4000W. Middle readout is 56A, while the bottom readout is 74A.

Why the two different Amperages and significance when calculating a BMS' protective amp rating? Which readout is used when one goes about figuring the minimum protective BMS Amp rating needed (e.g. 60A, 70A or 80A) ?

Can anybody produce the math/equation that provides the maximum calculated BMS amperage for the described battery configuration?...with perhaps 10% safety factor or another safety factor you choose to assign based upon your schooling or personal experience?

 

[eMark]

The National ebike 'top' speed limit (if there is one) is 25mph. Being the H-D RUSH/CTY ebike (w/belt drive) is rated at top speed of 28mph i'd abide

At 78 ... i'd settle for my Goldilocks on a 28mph H-D Serial 1 RUSH/CTY ... "sweetspot"

 

[eMark]

As to my plan...keeping my 52v 14s x 4p battery, if I up the BMS amperage cut off to say 60 amps, I think I could comfortably set the controller to 50 amps and get 3000 watts or close to it which would likely be the sweet spot without upgrading to a 72v battery.

My thinking was that you would only use 50amps (3000watts) for no more than a minute, if that long in case of an emergency or whatever happens to turn your crank faster ...

What about just upgrading Controller to 40amps with a 50amp BMS? But is it worth the effort/cost with only an increase from a 30amp Controller to a 40amp Controller for your 52v 14s x 4p battery ... if only for infrequent use ?

More and more i'm of the opinion (78yrs) that it's better to leave well-enuf alone. That said it's still a challenge for me not to find something to tinker with (at reasonable cost/labor) ... if only excuse is as a learning experience ...

ENUF SAID! ... (enjoyed reading, learning and reviewing again your insightful posts).

 

[eMark]

Can anybody produce the math/equation that provides the maximum calculated BMS amperage for the described battery configuration?...with perhaps 10% safety factor or another safety factor you choose to assign based upon your schooling or personal experience?

In summary a math/equation doesn't exist because there's too many variables including BMS variations. But there is a general rule of thumb ... depending on whose thumb :wink: . Some believe BMS stands for BatteryMurderingSystem

Here's about as thorough an article as we care to know from an EE, and yet no math/formula ... https://electronics360.globalspec.com/article/17106/fundamentals-of-bms-selection

To further 'clarify' here's a BMS post from an ES member with 7171 posts ...

As long as you know what you're doing the best bms imo is the one you have between your ears. it won't shut you down in the middle of an intersection you need to get across fast because a 20 ton truck is bearing down on you. If you want it just for charging then you can use any small bms and bypass the output with direct connections to the battery pack. If you plan on normal use, I'd get an 80A minimum bms for a 60A max controller.

Here's another BMS post by an ES member with 2351 posts ...

You need a bms that is rated at the maximum rating of the battery.

Do the math on your cell specs and calculate what your pack can deliver safely. That way you can always upgrade the rest of your system without needing to replace your bms.

The entire point of the bms is to protect your pack, not the rest of the system. Look at the limit of your battery, not your specific usage at this point.

Here's another informative BMS post with another perspective by an ES member with 2282 posts ...

I have a 30A bms. It will do more. With enough metal wicking away the heat it will hold 60A. It takes 100A before it considers there is a fault and disconnects. I can't speak for the bestech as one look over and I didn't fit it. However getting one designed for that power level is as much as can be expected of you. Remember it is a fuse of sorts and protects the cabling to the controller, so it shouldn't be too big for their current carrying capacity

Now the 'Rule Of Thumb' is perfectly clear
FWIW ... i don't use a so-called 'smart' tangible BMS :wink:

 

[eMark]

... was looking for shared perspective on this forum and surprised more didn't weigh in.

One reason could be your wording “BMS Capacity”. This is a common mistake among RC Lipo hobbyists who think they should multiply the Ah capacity of their Lipo times the discharge rating (e.g. 25C, 50C, 70C, 130C) which some believe is a continuous discharge amperage. When actually the maximum discharge rating (e.g. 25C, 50C, etc) is only for 2-3 seconds not for 10-15 seconds unless replacing a puffed Lipo after a few 10 sec cycles is not big deal.

Here is another quote from that same ES-BMS thread (3 quotes previously posted) …
https://endless-sphere.com/forums/viewtopic.php?f=14&t=82034&p=1205398#p1205398 ...

I understand that my new direct drive motor (still awaiting delivery) will draw 60 amps.
I plan to build some cushion into my battery pack capacity, which, when completed, could produce as much as 100 amps.

My question is this. Do I choose a BMS based on battery capacity (100 amps), or the estimated draw of the motor (60 amps)?

What he really meant to say was continuous discharge instead of capacity. Is it kind of an oxymoron to say “BMS Capacity” ... perhaps why Amberwolf didn’t want to correct you as he’s by far more tactful than is my nature. Wish i was as polite as is Aw ... :thumb:

How would you answer his question assuming his Controller had a 60amp cut-off? In other words is it necessary for him to get a 120amp BMS or would an 80 amp BMS suffice? Please explain your logic/reasoning. Is there a scenario where either option (120A or 80A is OK ??