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BMS supply voltage question
#1
I’m currently building a 72v 45ah 21s pack for an Escooter project using Lg Chem pouch cells (sub modules) harvested from a Chevy volt EV with a nominal voltage of 3.75v

My BMS which is a Batmon BMU 12s system from Ev power Australia requires a supply voltage between 9v - 30v

To obtain a suitable supply voltage within this range Am I right in thinking I can wire a 7s 3p configuration from the 21s pack to give me a 26.25v supply while spreading the BMS load across all cells in the pack.

Is there something I’m missing?

The Batmon system is modular comprising of the BMU + 2 x 12s balancers boards and a configurator. Essentially the balancing system will be wired with a 10s & 11s stack controlled by the BMU. Charge and discharge load is controlled by the BMU via a Gigavac 400A contactor .

This is my first Battery build so any guidance welcomed and appreciated.
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#2
Use a Buck converter instead. The other One just gets your battery out of balance
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#3
If you mean "can you power a device from part of the pack string" - no, this leads to out of balance cells fast.
If from separate small pack(s), yes.

EDIT: found the info: 2x CBM parts are powered directly off the cells & are isolated.
The 1x BMU running the lower part of the pack can likely be run of a non-isolated step down converter.
Look up Daygreen on AliExpress, they have 72V rated step downs.
Note your 21s pack might reach 88.2V (@4.2V/cell) when fully charged so rate everything accordingly.

Be super careful to prevent shorts. Higher voltages like this have Very Large amounts of energy stored.
Use suitably rated breakers/fuses. They will (help) save your ass one day.
Please read & then read more about safety first.
Running off solar, DIY & electronics fan :-)
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#4
Thank guys, I’m not looking to use part of the 21s string rather wire a 7s 3 p to get the required supply voltage but still spread the load evenly across all cells.

I do have a 72-12 buck converter already but I’m hesitant to use it for the BMS as it seems silly to have the fail safe rely of a $5 Chinese buck converter
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#5
Hi there!
I'm having a similar problem as Lootsef. I want to rebuild the battery for a used Segway which batteries are all dead. Those cells were Li-FePO4 cells (3.2V/3.6V charged up) and I want to replace them with Li-ion cells. The sign on the old pack says that the voltage with 24, 3.2V in series is 76.8V. That, charged up would be 86.4V. Now I'm planning to replace this with a 21S3P pack, 3.6x21= 75.6V, which is right under the nominal voltage of 76.8V. But as soon as I charge it up, I have 88.2V which is 1.8V above the fully charged state of the old pack. What should I do now? Should I just use a step down converter, as daromer said?
Thanks!
JG
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#6
Dont Charge it Up to max then. Stop att 4.13 or something.....
Crimp Daddy likes this post
The Ultimate DIY Solar and build place
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Current: 10kW Mpp Hybrid | 4kW PIP4048 | 2x PCM60x | 100kWh LiFePo4 | 20kWh 14s 18650 |  66*260W Poly | ABB S3 and S5 Trip breakers
Upcoming: 14S 18650~30kWh
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