Help with sizing BMS

videosystm

10 µW
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Sep 13, 2022
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Hello all, First post. I am building a battery for my drift trike. I have made a 13s6p Samsung 25r battery pack and I need help to get the correct bms. I am using am ME0907 motor 100a continuous and max 300a at 48v. I am using a Kelly controller capable of 72v 100a continuous and 350a max. Any help would be good. I have a daly smart bms 100a but I am second guessing that.
 
Thank you for the response. I totally understand what you’re saying. I see a lot of hat towards Daly and I just wanted a recommendation on what bms you’d buy for my project. Thanks again
 
Seems like finding a 150a or 200a bms is harder than I thought. Could I use the 100a bms that I have just to charge and balance the batteries and then unplug the bms and use the batteries without it? I believe my esc could stop the batteries when I set a low voltage.
 
Thanks again. Your answers are right to the point and I am soaking it all in. I’m thinking I over built the battery but I can use it in many project in the future. Thanks again.
 
videosystm said:
Seems like finding a 150a or 200a bms is harder than I thought. Could I use the 100a bms that I have just to charge and balance the batteries and then unplug the bms and use the batteries without it? I believe my esc could stop the batteries when I set a low voltage.

This BMS stuff is confusing me too ...
Some of the drag racing builders needing high amperage output are indeed only using BMS to charge their packs and bypassing the BMS for discharge.
Also have noticed a bunch of small BMS boards from RC models being used to monitor the charging voltages on large battery builds.
Again ... I'm no expert on the subject ...
 
videosystm said:
Seems like finding a 150a or 200a bms is harder than I thought. Could I use the 100a bms that I have just to charge and balance the batteries and then unplug the bms and use the batteries without it? I believe my esc could stop the batteries when I set a low voltage.
You can, but it cannot protect a cell (group) against overdischarge in the event of enough imbalance between them for one of the cell(group)s to be that much lower than others under a load even though the total pack voltage is still above the ESC LVC.

As long as all your cells are well-matched and equal to each other in capacity and capability, and they are correctly spec'd for the current demands of your system so they are used well-within their capabilities, it shouldn't be a problem.

If any of the cells are mismatched or the pack is aging, or the cells aren't capable of the current demands of the system such that they are stressed by it enough to cause issues, then it could be a problem.
 
PaPaSteve said:
This BMS stuff is confusing me too ...
Some of the drag racing builders needing high amperage output are indeed only using BMS to charge their packs and bypassing the BMS for discharge.
Also have noticed a bunch of small BMS boards from RC models being used to monitor the charging voltages on large battery builds.

I expect that like RC hobbyists, they don't end up using those packs for all that long, relative to people that use their rides daily for commuting, etc. There's always a better cell that can provide more power, so they end up upgrading their packs to new stuff before the old ones really wear out enough to have serious imbalance problems. And the cells are likely also spec'd to be able to handle the very high currents their usage demands, making them less likely to be stressed as badly by the usage.

The racers are also generally much more capable of handling issues arising in the pack than the typical end-user (which are the people the BMSs are designed for), and doing any of the human-level monitoring necessary for their usage, as well.
 
videosystm said:
I will look for a 150a bms and use that for my project. Thank you for all the help.

Something to note about BMS capability: they don't usually state *why* they are rated at a particular current, but if the reason is that the FETs may overheat at currents higher than that, then if your max current is over the rating long enough to heat the FETs enough to damage them, they could fail in a way that still lets current thru them but that the BMS cannot turn off anymore. When that happens, the BMS can no longer protect the cells because it can't prevent overdischarge by turning the output off.

It's a "silent" failure, in that there is no alert or indication this has happened.

The only ways to know are when a cell does go low enough to trigger LVC, and there is either a display (Bluetooth, etc) or LEDs on the BMS the user can see that indicate this, or the cell goes so low it has a problem and fails in a catastrophic way or at least one that prevents the system from operating correctly so that the user will notice, and then test the pack / BMS. Or if the user does during-ride cell-level monitoring, or is testing often enough to see the problem.


Also note that not all BMSs actually shut off on overcurrent, some just have a rating for max allowable current, so before depending on this function, make sure the one you choose supports it.

Most systems use the controller's battery current limit to ensure the pack is not stressed.

On a Kelly controller, it isn't always straightforward how to set that; some use a percentage of the phase current limit, and the phase current limit is a percentage of the max allowable current, making it even more complicated.
 
videosystm said:
I am using a Kelly controller capable of 72v 100a continuous and 350a max.
Note that unlike almost every other controller out there, Kelly controllers are not typically rated by battery current, but instead for motor phase current. It likely has a much lower battery current rating; you can check with Kelly support (Fany, etc) to find out what that is for your specific version.
 
amberwolf said:
videosystm said:
I am using a Kelly controller capable of 72v 100a continuous and 350a max.
Note that unlike almost every other controller out there, Kelly controllers are not typically rated by battery current, but instead for motor phase current. It likely has a much lower battery current rating; you can check with Kelly support (Fany, etc) to find out what that is for your specific version.

I have owned three Kelly KEB models, and a few of the smaller ones.

My "50A contin 100A" phase KEB took 107A from the battery on a peak.

My "160A contin 350A phase" KEB took 280A from the battery on a peak.

My "105A contin 150A phase" KEB took 180A from the battery on a peak.

Point is... They peak high, mine do. Out the battery.

Big BMS is easy to find.. with a real shunt included. I have installed/sold about 10 of these. Never a problem. ON many different systems.

It is kinda big, this BMS. So I run it external to the battery, even though it is not a distributed BMS.It has an inch by quater inch thick shunt inside it.. not a shunt barr, but a real big four wire terminal shunt... 300A contin and 800A softpeak.. and you can easily parallel a pair of identical contactors for 600A contin and 800A soft peak.... Its 150$.

https://www.lithiumbatterypcb.com/product/7s-to-20s-200a-smart-bluetooth-bms/
 
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