Noob BMS Current question

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eiprol   1 mW

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Noob BMS Current question

Post by eiprol » Oct 13 2015 5:27am

Hi guys,

I'm waiting for my VESC, enertion r-spec motor, and single mount kit to arrive (And 3 x ZIPPY 5000mah 3S 20C), so in the mean time I have started to read about BMS systems... I want to build my own laptop-style charging system!

Since I plan to have 9S Battery, I was wondering if I need to find or buy a custom 9S BMS, or using a bigger one (like 12S) and leaving 3 balance pins empty would work.

Secondly, when looking for a custom BMS, I've seen that they ask for the current of that BMS, and this is what I'm not really sure about.
My batteries can provide 5A * 20C = 100A continously (with higher peaks of 150A), but I'm sure that's not the current they are asking me for, since most of BMS I have seen are not above 40A. How much will be my motor demanding and how much should allow the BMS?

Could anyone show me and explain me a little bit (giving my setup) how to calculate the proper current they are asking me for? I really would appreciate, since I want to learn how to do it and understand a little bit more about the electronics behind it!

I will be posting every improvement of my BMS (if there is such a thing :) ) in this thread!
Thank you!

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Nobuo   10 kW

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Re: Noob BMS Current question

Post by Nobuo » Oct 13 2015 6:10am

the max continuous current will be the max current your controller can drain (if the battery / BMS is able to also), (always your motor should be able to admit that amperage and also maximum power depending of the voltage)

The BMS should always cut the current for harming continuous current, or not efficient for your aiming on your battery. That depends for the chemistry you are using. For example for a Li-ion standard NCA cells battery, the BMS should cut around 50% of max continuous current capability or less if you want to secure the max efficiency. Take care, because if your controller + motor drains usually more continuous power than your BMS max, the battery will stop outputting power for some seconds, and that could be annoying.

I recommend to you to never choose a BMS for your controller capabilities, always for the secure discharge of the battery, if you find is not enough, you better make a bigger battery or use other type of cells.

About using a BMS with more series than available: If your BMS is doing its work, it won't let output current through its main wire or wires (B- usually) because he will detect not voltage output on some series and will act as like those paralleled groups are below LVC voltage so it want to prevent you for destroy that cells.

However some BMS's boards from "Bestech Power" for example can work with less series.

The are boards for 9s like those ones

http://www.alibaba.com/product-detail/9 ... image.html

However I would just go with 10s if possible
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eiprol   1 mW

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Re: Noob BMS Current question

Post by eiprol » Oct 13 2015 6:25am

Nobuo wrote:the max continuous current will be the max current your controller can drain (if the battery / BMS is able to also), (always your motor should be able to admit that amperage and also maximum power depending of the voltage)

The BMS should always cut the current for harming continuous current, or not efficient for your aiming on your battery. That depends for the chemistry you are using. For example for a Li-ion standard NCA cells battery, the BMS should cut around 50% of max continuous current capability or less if you want to secure the max efficiency. Take care, because if your controller + motor drains usually more continuous power than your BMS max, the battery will stop outputting power for some seconds, and that could be annoying.

I recommend to you to never choose a BMS for your controller capabilities, always for the secure discharge of the battery, if you find is not enough, you better make a bigger battery or use other type of cells.

About using a BMS with more series than available: If your BMS is doing its work, it won't let output current through its main wire or wires (B- usually) because he will detect not voltage output on some series and will act as like those paralleled groups are below LVC voltage so it want to prevent you for destroy that cells.
Thanks!

So basically my first question is clear now; It has to be a BMS designed for the right number of Cells. I guess I should go for a more standard setup (Maybe 8S,10S,12S) to find a BMS with more ease, or just try to buy a custom one.

As for the second question, I still have some doubts.
I'm using LiPo technology; 3 packages of ZIPPY 5000mah 20C (peak 30C), wich makes a total 5000mah 9S. That is a max continuous current of 100A if I'm wright, and peak of 150A.
The ESC is VESC from enertion, capable of constant 50A drain and peaks of 240A (just for a couple of seconds).
I'm using only a motor, and I'm not sure about it's amperage; it's 2200W 190KV.

Given the fact that my battery is Lipoly (not Li-ion), and my current setup, should I still go for 50% of max battery continuous current? like 50A? Or since I'm using only a single motor I can adjust the current a little bit more?

Thank you!

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Nobuo   10 kW

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Re: Noob BMS Current question

Post by Nobuo » Oct 13 2015 8:40am

If your ESC will deliver a max of 50A cont, your battery will discharge at 10C max cont, that's still a great discharge, it will generates appreciable heat and perform a voltage sag, not good if you want to use those batteries for some years. However it is a common rate for LiPo cells, they are prepared for that. If you install a 50Amax continuous BMS there won't be any problem (BMS's use to admit around a 10% more continuous power than rated)

BLDC motors have specific voltage, amperage and power limits, you must contact seller/supplier or just upload/share a photo
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Ianhill   1 MW

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Re: Noob BMS Current question

Post by Ianhill » Oct 13 2015 11:29pm

As well as discharge currents make sure your charge currents match, the lipos u stated charge best for longevity at 1c/5a so the BMS must have a 5a per cell charge rate and as long as your battery packs nominal discharge current exceeds your BMS max discharge but this figure is lower than the ESC max discharge you should not have a problem with intermittent battery's, if your battery's pack max discharge is equal to the ESC Nominal current then your going to create a lot of heat in the battery's and have a longevity problem under discharge so its important all these figures make sence so the pack doesn't puff up or worse burst into flames under charge or discharge.

eiprol   1 mW

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Re: Noob BMS Current question

Post by eiprol » Oct 14 2015 5:00am

Ianhill wrote:As well as discharge currents make sure your charge currents match, the lipos u stated charge best for longevity at 1c/5a so the BMS must have a 5a per cell charge rate and as long as your battery packs nominal discharge current exceeds your BMS max discharge but this figure is lower than the ESC max discharge you should not have a problem with intermittent battery's, if your battery's pack max discharge is equal to the ESC Nominal current then your going to create a lot of heat in the battery's and have a longevity problem under discharge so its important all these figures make sence so the pack doesn't puff up or worse burst into flames under charge or discharge.
Thank you for the hint about charging current.

So, taking this BMS other user posted:
http://es.aliexpress.com/item/A-22-2V-P ... er=1720416

I guess the charging rate is right, 5A, which is equal to my 5A batteries connected in series, right?

However, about the 10A discharge rate, It's too low, right? What's the right way to calculate intensity for my motor? If it's 2200W and My batterie provides 38V, using W = V * I, do I need to allow I = 2200 / 38 = 58A? At that discharge rate, my batteries would last 6 minutes (58A it's more than 10C).

The thing is that I find a lot of 10A/20A BMS on the internet, but according to this math I should go for a 40-50A BMS, which I can hardly find. That makes me think that I'm doing something wrong!

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Nobuo   10 kW

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Re: Noob BMS Current question

Post by Nobuo » Oct 14 2015 7:25am

5A charging current is max in that BMS (not counting that BMS has a 10A max discharge and is a 6s), that means you charger shouldn't charge at 6A continuous for example, because the BMS charge port will cut the ingoing power.

Depends completely of the power output of your charger (max amps) the voltage should be of course the same as max voltage charge of your cells in series if you are charging your battery at once.

As I wrote in my last comment for an ESC / controller that can pull 50A continuous you need a 50A continuous capable BMS or more if you don't want it to cut the power everytime you hit ~50A for a while. I don't understand how could you question if a 10A continuous BMS is not enough, it is clear is not enough

Forget about motor power rating. If you are pulling with an ESC / controller that won't fry it (so the power output of your controller fits more or less your motor power rating) is this one who will limit the max power output your battery will drain.

For a 50A continuous max ESC, despite the voltage or max power, you need a 50A continuous capable BMS or more.

For example this 10s 60A max continuous BMS:

http://www.aliexpress.com/store/product ... 55141.html
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