BMS handling of peak current

Batteries, Chargers, and Battery Management Systems.
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pbert   1 mW

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BMS handling of peak current

Post by pbert » May 15 2019 7:28am

im trying to understand how the following setup would behave:

52V battery built from 5P14S 20700 sanyo with 35A continous.

BMS 120A limit for 14S

the battery would be capable of 175A continous at 1C and at 2C could do burst of 300A.

question im wondering is will the BMS allow bursts of 300A or it blocks it at 120A?

if someone knows of a BMS that allows 120A continous and burst of 300A for Li-ion would appreciate insight.

thanks.
Last edited by pbert on May 15 2019 8:42am, edited 1 time in total.

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fechter   100 GW

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Re: BMS handling of peak current

Post by fechter » May 15 2019 8:13am

If the BMS is rated for 120A it's hard to say exactly at what point it will trip. Most of the ones I've played with trip at a somewhat higher current than the continuous rating. The specifications for the BMS should say what the trip point is.
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pbert   1 mW

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Re: BMS handling of peak current

Post by pbert » May 15 2019 8:30am

ok thanks. If my goal is to allow 300A peak current should i therfore be looking to find a BMS that would allow 300A continous ?

my real goal is to build a battery for an Electric Motion Sport motorbike. They use lipo pouch packs but i want to build a smaller pack out of li-ion. I've built li-on packs before, just not this high amperage. Their pack does 120A continous and 300A peak.

Anyways, any ideas for A BMS that allows 300A peak / 120A continous would be appreciated.

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flippy   100 kW

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Re: BMS handling of peak current

Post by flippy » May 15 2019 10:29am

especially if you are using chinese ones you need to take the nominal power as peak when buying a bms.
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pbert   1 mW

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Re: BMS handling of peak current

Post by pbert » May 15 2019 11:14am

ok for example, this bms with peak current cutoff of 240A after 9ms would be close to doing the job of 300A.

https://www.google.ca/amp/s/m.aliexpres ... 91936.html

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flippy   100 kW

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Re: BMS handling of peak current

Post by flippy » May 15 2019 1:47pm

pbert wrote:
May 15 2019 11:14am
ok for example, this bms with peak current cutoff of 240A after 9ms would be close to doing the job of 300A.

https://www.google.ca/amp/s/m.aliexpres ... 91936.html
no, it can barly do 120A.
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Re: BMS handling of peak current

Post by billvon » May 15 2019 2:00pm

pbert wrote:
May 15 2019 8:30am
ok thanks. If my goal is to allow 300A peak current should i therfore be looking to find a BMS that would allow 300A continous ?
You're not going to get 60A from those cells safely. (Peak rating 52.5 A) You'd likely need to add parallel strings.
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amberwolf   100 GW

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Re: BMS handling of peak current

Post by amberwolf » May 15 2019 2:08pm

THe main thing you need to do is pick a BMS that limits what the *cells you are using* have to deal with so it's within their safe usage range.

If the cells can handle a peak of say, 200A for 10 seconds, under ideal conditions, then the BMS must prevent them from seeing that peak for more than that. If it doesn't, the batteries will be stressed, and probably overheated, whenever these peaks occur like that. If they're allowed to continue long enough, you could damage the cells permanently, and if the heat builds up enough, you could even have a fire.

It all comes down to what hte cells can take, and how they perform under conditions above what they're really capable of (not what they're rated for, because sometimes those are exaggerations, or only under specific lab conditions that the real world exceeds, etc).

If the BMS allows 200A continuously and the cells aren't able ot handle it...you'll need a new pack soon enough.

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Re: BMS handling of peak current

Post by amberwolf » May 15 2019 2:20pm

pbert wrote:
May 15 2019 7:28am
52V battery built from 5P14S 20700 sanyo with 35A continous.

BMS 120A limit for 14S

the battery would be capable of 175A continous at 1C and at 2C could do burst of 300A.
I don't understand the correlation between these three statements. Nor do the statements all even make sense; you have numbers coming from nowhere, that don't compute from the data provided.

First, if the battery is built for 35A continuous, and you use a 120A BMS, and you actually draw anything like what the BMS allows for any length of time, you'll destroy the battery pack very quickly. Might even have a fire if it heats up enough.

Second, if you have a 120A BMS, and a 35A battery, how do you get a number for 175A or 300A?

Third, 1C and 2C refer to the capacity of the battery, and don't have anything to do with a BMS rating, nor anything to do with whatever rating "175A" comes from.

Since you don't state the capacity, I don't know what it would be for sure, but googling "20700 sanyo" shows anything from around 3300mAh to 4300mAh per cell. Can't know what it really is because you don't say which specific cell you're using.

5p means 5x that, so let's go with the 4.3Ah for max capacity.

That's going to be 21.5Ah.

So 1C is 21.5A, and 2C is 43A.

pbert   1 mW

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Re: BMS handling of peak current

Post by pbert » May 15 2019 4:22pm

modified post and repasted text in quoted response below
Last edited by pbert on May 15 2019 4:41pm, edited 1 time in total.

pbert   1 mW

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Re: BMS handling of peak current

Post by pbert » May 15 2019 4:25pm

flippy wrote:
May 15 2019 1:47pm
pbert wrote:
May 15 2019 11:14am
ok for example, this bms with peak current cutoff of 240A after 9ms would be close to doing the job of 300A.

https://www.google.ca/amp/s/m.aliexpres ... 91936.html
no, it can barly do 120A.
have you or someone you know tried this Bms? how do you know its so crappy?
thanks.

pbert   1 mW

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Re: BMS handling of peak current

Post by pbert » May 15 2019 4:39pm

amberwolf wrote:
May 15 2019 2:20pm
pbert wrote:
May 15 2019 7:28am
52V battery built from 5P14S 20700 sanyo with 35A continous.

BMS 120A limit for 14S

the battery would be capable of 175A continous at 1C and at 2C could do burst of 300A.
I don't understand the correlation between these three statements. Nor do the statements all even make sense; you have numbers coming from nowhere, that don't compute from the data provided.

First, if the battery is built for 35A continuous, and you use a 120A BMS, and you actually draw anything like what the BMS allows for any length of time, you'll destroy the battery pack very quickly. Might even have a fire if it heats up enough.

Second, if you have a 120A BMS, and a 35A battery, how do you get a number for 175A or 300A?

Third, 1C and 2C refer to the capacity of the battery, and don't have anything to do with a BMS rating, nor anything to do with whatever rating "175A" comes from.

Since you don't state the capacity, I don't know what it would be for sure, but googling "20700 sanyo" shows anything from around 3300mAh to 4300mAh per cell. Can't know what it really is because you don't say which specific cell you're using.

5p means 5x that, so let's go with the 4.3Ah for max capacity.

That's going to be 21.5Ah.

So 1C is 21.5A, and 2C is 43A.
@amberwolf i realize my post was not very clear. here are the numbers explained:

cell type :
Brand: Sanyo
Model: NCR2070C
Size: 20700
Nominal Capacity: 3500mAh minimum, 3620mAh typical
Rated Capacity: 3475mAh
Discharge Current: 30A Maximum Continuous

battery built will be 5P14S so this would give 58.8V battery (14 x 4.2V) with capacity of 17.5 AH (5 x 3.5AH) with max continous current of 150A (5x30A).

i was wrong on my initial post when i posted 35A continuois, should have been 30A.

so with 150A continuous i am looking for a 120A Bms.

hope that clears up the details.

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flippy   100 kW

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Re: BMS handling of peak current

Post by flippy » May 15 2019 5:21pm

pbert wrote:
May 15 2019 4:25pm
flippy wrote:
May 15 2019 1:47pm
pbert wrote:
May 15 2019 11:14am
ok for example, this bms with peak current cutoff of 240A after 9ms would be close to doing the job of 300A.
https://www.google.ca/amp/s/m.aliexpres ... 91936.html
no, it can barly do 120A.
have you or someone you know tried this Bms? how do you know its so crappy?
thanks.
because they all are. i have never in my life seen a chinese bms with proper mosfets. they all use fake ones that can barely do 25% of their stated rating without exploding.

many people (as i do) either buy proper stuff like a tinybms that can do 160A with just 2 mosfets (real ones) or get a chinese one with fake ones and replace the mosfets with real ones. that way you can get the proper rating without fear of blowing up our bms.

just look at chinese bms-es. they use sometimes dozens of mosfets to get a 200~300A rating but a bms made with real mosfets only use between 2 and 4 for the same rating.

you show me a chinese bms and i show you fake mosfets.

here is some stuff to get down the rabiit hole:https://www.youtube.com/watch?v=wVYMyExlJpU
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pbert   1 mW

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Re: BMS handling of peak current

Post by pbert » May 15 2019 7:33pm

ok i didnt realize that they were that bad. thanks for the info i will read up about this. i have used a chinese bluetooth bms in the past and didnt have any problems yet. been aboit 2 years...maybe just lucky. but i am not currently running any high amps, only running a bbshd at 30A.

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flippy   100 kW

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Re: BMS handling of peak current

Post by flippy » May 15 2019 7:37pm

here i got a pic of a "150A" bms that shat the bed at 70A peak and 30A average:

https://i.imgur.com/1RFiMY7.jpg

but mind you: the battery was capable of delivering litteraly thousands of amps in a short circuit. it would vaporize the entire bms before it could trigger if there was a reason for it. this is why i always put a big megafuse directly after the bms as a fail safe.
Lithium beats liquid dinosaurs.

pbert   1 mW

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Re: BMS handling of peak current

Post by pbert » May 15 2019 9:37pm

wow ya that is a nicely friedboard. you put a big fuse between the battery and bms? what kind of fuse do you put?

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flippy   100 kW

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Re: BMS handling of peak current

Post by flippy » May 16 2019 9:55am

Just google megafuse 300A or something. Its also easy to 3d print a holder for it that contains most of the "exitement" if it pops. If you get cheap ones that tend to explode pretty violently if hit with a instant dead short of thousands of amps. That usally vaporises stuff instead of melting it.
Lithium beats liquid dinosaurs.

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