U004 Battery: changing BMS

Kaio_

1 µW
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Oct 26, 2021
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I've bought this battery: https://www.aliexpress.com/item/1005002710812223.html?spm=a2g0s.9042311.0.0.21e84c4dy5eKFw

it's a 72v 20s4p battery that currently has a 50A BMS installed. I want to get a 3000W hub motor, and the controllers on most kits are rated 50A as well. From what I understand I need the BMS rating to be higher than that of the controller, then I will need to upgrade the BMS to 60A or even higher.

Is this the right way to go? This bike will be intended for trail riding, and I don't expect to sustain anywhere near 50mph with it, but if the controller is putting out 50A all of the time then it'll burn out the BMS.
Otherwise, if it only spikes to that amperage then I should be fine in moderation? :?
 
Kaio_ said:
I've bought this battery: https://www.aliexpress.com/item/1005002710812223.html?spm=a2g0s.9042311.0.0.21e84c4dy5eKFw

it's a 72v 20s4p battery that currently has a 50A BMS installed. I want to get a 3000W hub motor, and the controllers on most kits are rated 50A as well. From what I understand I need the BMS rating to be higher than that of the controller, then I will need to upgrade the BMS to 60A or even higher.

Is this the right way to go? This bike will be intended for trail riding, and I don't expect to sustain anywhere near 50mph with it, but if the controller is putting out 50A all of the time then it'll burn out the BMS.
Otherwise, if it only spikes to that amperage then I should be fine in moderation? :?

The BMS that comes with the battery is already too much for the pack. The pack is 4P, and the cells are rated at 10A, so the BMS shouldn't be sized greater than 40A. 50A will kill the pack. If you want 60A, but a pack with cells capable of providing 60A, not one made of generic Chinese cells that will struggle to provide 40A.
 
Kaio_ said:
I've bought this battery: https://www.aliexpress.com/item/1005002710812223.html?spm=a2g0s.9042311.0.0.21e84c4dy5eKFw

it's a 72v 20s4p battery that currently has a 50A BMS installed. I want to get a 3000W hub motor, and the controllers on most kits are rated 50A as well. From what I understand I need the BMS rating to be higher than that of the controller, then I will need to upgrade the BMS to 60A or even higher.

Is this the right way to go? This bike will be intended for trail riding, and I don't expect to sustain anywhere near 50mph with it, but if the controller is putting out 50A all of the time then it'll burn out the BMS.
Otherwise, if it only spikes to that amperage then I should be fine in moderation? :?

Well don’t change the BMS. No you won’t trip the BMS at running 50 A. You will only be pulling 50 A for a brief period. That’s your peak amps not rated. Now obviously when A pack is pushed to the maximum current limit you’re not going to get many years out of the battery.

As a reference I have a UPP battery and I have the controller set at the max current and it’s almost one year and it still performs well. You should be able to get at least two years even at 50 A. If you get an adjustable controller you could turn the current down to take some stress off the battery if it’s a concern.
 
E-HP said:
The pack is 4P, and the cells are rated at 10A, so the BMS shouldn't be sized greater than 40A. 50A will kill the pack.

How does a BMS control current? Do BMS in general sense current and shut the MOSFETS off if current approaches what the MOSFETS are rated for?
 
Comrade said:
E-HP said:
The pack is 4P, and the cells are rated at 10A, so the BMS shouldn't be sized greater than 40A. 50A will kill the pack.

How does a BMS control current? Do BMS in general sense current and shut the MOSFETS off if current approaches what the MOSFETS are rated for?

I'm not sure the limit is determined by the MOSFET capability, although the BMS should be designed to work with the hardware. I think the BMS, depending on the features, would have an ASIC or processer chip for the control logic, and something sensing voltage and current, in and out.
My point is that the cells determine the pack's capability, not the BMS. The BMS is there to protect the cells.
 
E-HP said:
I'm not sure the limit is determined by the MOSFET capability, although the BMS should be designed to work with the hardware. I think the BMS, depending on the features, would have an ASIC or processer chip for the control logic, and something sensing voltage and current, in and out.
My point is that the cells determine the pack's capability, not the BMS. The BMS is there to protect the cells.

I've looked at pictures of a few common BMS units (the circuit boards) and it looks to be purely analog. There are no processors. And even if there is some digital chip, for BT perhaps, I haven't seen this over current cutout function mentioned. And if that's the case, I assume a 100A battery with a 100A motor and a 50A BMS in the middle would just smoke the MOSFETs on the BMS.

http://liionbms.com/php/bms-selector.php

"overcurrent" seems to be a function present in a minority of BMSs
 
Most BMSs do not limit current.

Some just let the smoke out if you approach/exceed their rating.

Some are latching type protection, need to be reset.

Maybe some are hiccup, auto'recover, but never seen that.

It is the user's job to buy a robust enough BMS rated for 20% more than what the controller will ever pull.

Throttle restraint also, watching readout maybe.

Or use external contactors rather than flowing the amps through the BMS
 
Comrade said:
I've looked at pictures of a few common BMS units (the circuit boards) and it looks to be purely analog. There are no processors. And even if there is some digital chip, for BT perhaps, I haven't seen this over current cutout function mentioned. And if that's the case, I assume a 100A battery with a 100A motor and a 50A BMS in the middle would just smoke the MOSFETs on the BMS.

http://liionbms.com/php/bms-selector.php

"overcurrent" seems to be a function present in a minority of BMSs

I guess it's possible that magic may be involved. Looks like the Daly BMSs that I see mentioned on the forum a lot, appear to have a logic chip to handle things and have overcurrent protection.
https://dalyelec.en.alibaba.com/product/62553422954-815239262/Daly_3_7V_Rated_Li_ion_Battery_BMS_48V_14S_50A_Li_ion_Battery_BMS_For_Lithium_Battery_E_bike_With_Balance_And_Waterproof.html

I see this one being sold on various ebike sites, and appears to have overcurrent protection as well, per the description:
https://www.alibaba.com/product-detail/jiabaida-bms-20A-li-ion-14s_1600088806433.html
 
"Overcurrent protection" is a very broad concept, many different categories

but all interrupt to device's operation, think fuses and circuit breakers.

Current **limiting** is entirely different, keeping the system going smoothly while the load is trying to pull too many amps

very complex / expensive to implement properly in a device between the load and source, really anywhere other than inherent in the load device itself,

keeping voltage regulated independently of the current is a big challenge.

As stated, this should be a function of the controller.

Also, as a separate issue, basing your design fundamentals off assumptions that vendor claims are accurate

will lead to a lifetime of heartache in these industries, especially in the Chinese markets.



 
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