Choosing a BMS for self made battery

tampano

Finding my (electric) wheels
Aug 18, 2017
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Hi everyone,
while studying the theory of self-building a battery pack, there's one thing not clear to me, coming from some ignorance about electricity: what should be the Amp value for choosing a BMS?

I'd like to do my first build with a Vruzend kit, not wanting to go straight away into a spot welder.

So, the series and parallel theory is absolutely clear to me in terms of Volts to be reached:
-10s using 18650 batteries to reach the 36v required
- 3p or 4p or 5p to reach a certain level of Ah for capacity.

What is not clear to me is the absolute Ampere value of batteries, how this is affected in the series connection, and how this final value determines the choice of a BMS.

I have a 36v 250 Bafang motor.

I'm undecided between using Samsuns 25R (2500mAh 20A) or Samsung 30Q (3000mAh 15A)

Let's say I do a 10s4p with any of those. I'd reach a 10Ah with one and 12Ah with the other.

BUT

Do I reach also a total of 80A with the first and 60A with the second? And what this value defines in the choice of my BMS? Let's take as example the Vruzend page on BMS...which one should I get?


I hope my question makes sense and sorry if it's too dumb. Everything else is pretty clear to me but not the Amp thing.

Thanks in advance
 

Nealh

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Cells groups in series will carry the same current from the first cell group to the last.
The 25R will indeed have a current rating of 80a & the 30Q 60a, but you won't want to try and put that through a Vruzend kit a sthe ends caps can't handle much more then 3.5 - 4a. Current will be wasted and converted to heat instead of out put watts/power.
For BMS current rating you are guided by the max current draw of the controller, ideally select a BMS with a slightly higher continuous current rating so that is doesn't struggle from the demand of the controller.
No use having a 80a rated Bms if the controller is only 25a rated.
 

tampano

Finding my (electric) wheels
Aug 18, 2017
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London
Cells groups in series will carry the same current from the first cell group to the last.
The 25R will indeed have a current rating of 80a & the 30Q 60a
That's great. At least this part I understood it well :)

but you won't want to try and put that through a Vruzend kit a sthe ends caps can't handle much more then 3.5 - 4a. Current will be wasted and converted to heat instead of out put watts/power.
I respect a more informed opinion, I'm here for that...but...the latest model of Vruzend, they advertise it as rated for 20A per cell/cap. Does this address your concern? I'm very undecided between caps and spot welder, the cost difference is not enormous and it's an interesting project for me...so I'll get all the opinions I can.

For BMS current rating you are guided by the max current draw of the controller, ideally select a BMS with a slightly higher continuous current rating so that is doesn't struggle from the demand of the controller.
No use having a 80a rated Bms if the controller is only 25a rated.
Last time I opened the controller "box" on my bike I can't remember to have seen any sticker about Amp rating of it, and VoltBikes documentation is ridicolous. Can I apply here the calculation I found on a website (again, please forgive my ignorance on electrical measures) that says that a 250w motor powered at 36v should draw 6.9A? Does this mean my controller does draw something around that, considering some higher values for some peaks?

Thanks a lot again for any input.
 

Nealh

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Your volt controller should have a label with info on it.
Most controllers will be at least 12a max rating and this can vary right up to 20a for 250w systems.

Where do you get 20a from ? There Q& A on the site still say's 3.5a.
To handle 20a then they will have to use all copper contacts and interconnectors.

Looks like the v2 can handle 20a according to Micah Toll, copper is apparently used now.
 
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anotherkiwi

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If you have a 15 Amp controller you want a 20 Amp BMS and if you have a 20 Amp controller a 30 Amp one. The little bit of overhead prevents the BMS from starving the controller in very rare critical situations (it has only ever happened to me a handful of times).

15 Amps / 4 (4P10S) each series string needs to supply 3.75 Amps but unless you are climbing a 15 km long mountain pass you will not be drawing 15 Amps constant. The Vruzend kit works OK in 5P10S and above, even the copper bar kit is more designed for high parallel count batteries, the connecting element on the cap is still stainless steel.

I had a kit but sold it unused, I would have needed two kits to get enough P groups for my needs. I am about to buy a welder and will be building a 6P12S battery made up of two 6P6S bricks in series. I won't be using a BMS but will balance charge with my "LiPo" chargers (which can also charge many other battery types) and the battery will be "protected" from over discharge by alarms.
 

Nealh

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If the the Buss is still S/S then they can't handle 20a current flow let alone 3a or so., the ideal scenario would be to make up copper Buss to replace them.
Nickel optimal current transfer is 4.6a/1mm2, Steel is half that.
Typical nickel Buss 7.5mm x 0.15mm is 1.125 mm2 so about 5.1a.
 

tampano

Finding my (electric) wheels
Aug 18, 2017
17
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50
London
If you have a 15 Amp controller you want a 20 Amp BMS and if you have a 20 Amp controller a 30 Amp one. The little bit of overhead prevents the BMS from starving the controller in very rare critical situations (it has only ever happened to me a handful of times).
Unfortunately my crappy controller has no indication of its amps rating. I'm attaching the picture I have of it, there's nothing else but those digits, that return nothing specific on the internet.
Would you go for 20 or 30 amps BMS?
 

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RossG

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I believe that controller is made by Dapu Motors. If so then it's the 15 Amps max 7 Amps continuous version.
 

vfr400

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It's a 36V 14 amp (max) controller. A 20 amp BMS would be appropriate for that controller, though I'd go for a 25 amp one in case you want to upgrade the controller later.

If you're going to use a Vruzend kit, I think it would be a good idea to add a second sense wire connector so that you can keep an eye on cell balance because the inter-cell connections can break down at any time. To be honest, I wouldn't use a Vruzend kit on an ebike. They're OK for static batteries, but I can imagine that vibration will play havoc with the connecttions.
 
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