Bms in series

maarkmohamed

100 W
Joined
Jan 10, 2019
Messages
103
This might have been touched up on in another thread but hopefully someone can lay it out for me clearly and clear up any confusion. I intended on building a higher voltage battery (40s) and having difficult finding a bms built ready for that. If i found any they are bulky and/or expensive. That being said is it possible to use 2 20s bms in series while still being able to use its lvc and hvc as well as charging? Because the battery will also have a high amp output I was going to use a relay in pair with the bms. The closest thing I found to what I was looking for can be seen in link below, but only 35s. If doable please explain exactly how the wiring would be done. Much appreciated


https://heltec-bms.com/product/350a-relay-bms-3s-4s-32s-3000a-peak-lipo-lifepo4-battery-protection-board-7s-8s-10s-12s-13s-16s-17s-20s-24s-28s-32s-35s/
 
Heltec does go higher. Here's a 40S offering from them:
https://a.aliexpress.com/_m0hy246

Several of the cheaper BMS support cascading and overlapping as well. E.g. buy several and connect the last few p-groups to both the previous BMS and the next BMS.
 
lnanek said:
Heltec does go higher. Here's a 40S offering from them:
https://a.aliexpress.com/_m0hy246

Several of the cheaper BMS support cascading and overlapping as well. E.g. buy several and connect the last few p-groups to both the previous BMS and the next BMS.

The one you listed is also very expensive and to bulky. So for example the bms listed below would it work with this, if so can you explain the exact wiring with a relay? And to confirm I would like lvc and charging to work.


https://www.lithiumbatterypcb.com/product/16s-17s-18s-19s-20s-lithium-or-lifepo4-smart-bluetooth-battery-bms-with-50a-constant-current/

https://ibb.co/1qgrxv8
 
So what I did was quickly drew a diagram for what I believe would be the correct way to wire a single bms to the contactor/relay. If correct, what exactly should be done now when adding a second bms in series?


https://ibb.co/s6xddWq
 
Isn't that a MOSFET based BMS where the full battery power flows through the BMS and is enabled/cut off via the internal fets? I thought you wanted a relay/contactor based BMS where the BMS' measurement of the p-groups just turns the relays/contactors on and off. E.g:
https://a.aliexpress.com/_mPPJjR8

Wiring:
First 24S of 40S -> BMS 1
Last 24S of 40S -> BMS 2
BMS 1 contactor control signal -> relay 1 control input
BMS 2 contactor control signal -> relay 1 signal input
Relay 1 output -> contactor control input
Battery -> contactor signal input
Contactor signal output -> motor controller
 
lnanek said:
Isn't that a MOSFET based BMS where the full battery power flows through the BMS and is enabled/cut off via the internal fets? I thought you wanted a relay/contactor based BMS where the BMS' measurement of the p-groups just turns the relays/contactors on and off. E.g:
https://a.aliexpress.com/_mPPJjR8

Wiring:
First 24S of 40S -> BMS 1
Last 24S of 40S -> BMS 2
BMS 1 contactor control signal -> relay 1 control input
BMS 2 contactor control signal -> relay 1 signal input
Relay 1 output -> contactor control input
Battery -> contactor signal input
Contactor signal output -> motor controller

Yes correct, this is your typical mosfet based bms. The one you linked is along the lines of what I'm looking for, but still they are big and expensive and that one goes up to 32s, I'm guessing if you buy two then it'll get up to 40+s. My question is can it be done with your typical mosfet based bms? They are much smaller and cheaper.
 
Most relays and contactors are designed to be controlled by either 12V or 24V. Normally you'd use a resistor, DC-DC buck convertor, or driver circuit to run them from pack voltage.

I think I did see a Kelly contactor that could be controlled with up to 72V once. So theoretically you could have two, each run by a 20S BMS outputting what each one sees as pack voltage when connected to half the battery each.

Those BMS don't typically have external shunts to measure voltage and external hall sensors to measure current, though. So you'd lose any protection for pack voltage and pack current use to the motor controller. Each MOSFET BMS would just see current to activate the contactor.

Think I'd just roll the dice with this 48S Daly MOSFET BMS at that point:
https://a.aliexpress.com/_mOSAUqq

Their quality is hit or miss, but every weird ass conversion wired in is also something that could go wrong or overheat. Especially high voltage and high current wires. I'll mess with 5V throttles and switches and sensors and whatnot connectors all day, but don't like dealing with pack voltage and motor currents.
 
maarkmohamed said:
Yes correct, this is your typical mosfet based bms. The one you linked is along the lines of what I'm looking for, but still they are big and expensive and that one goes up to 32s, I'm guessing if you buy two then it'll get up to 40+s. My question is can it be done with your typical mosfet based bms? They are much smaller and cheaper.

No, to work in series means all the current must go through each unit.

The BMS must be specifically designed to be wired in series.

Stop focusing on cheap and small, hard enough to get safety and reliability

Cheap BMS easily fail even when used as designed, if you do use that "load current flows through the mosfets" be sure to buy spares and ideally be prepared to replace fets as they burn out.

Personally I would bring balance wires outside the pack and position the BMS so it can be easily replaced.

 
lnanek said:
Most relays and contactors are designed to be controlled by either 12V or 24V. Normally you'd use a resistor, DC-DC buck convertor, or driver circuit to run them from pack voltage.

I think I did see a Kelly contactor that could be controlled with up to 72V once. So theoretically you could have two, each run by a 20S BMS outputting what each one sees as pack voltage when connected to half the battery each.

Those BMS don't typically have external shunts to measure voltage and external hall sensors to measure current, though. So you'd lose any protection for pack voltage and pack current use to the motor controller. Each MOSFET BMS would just see current to activate the contactor.

Think I'd just roll the dice with this 48S Daly MOSFET BMS at that point:
https://a.aliexpress.com/_mOSAUqq

Their quality is hit or miss, but every weird ass conversion wired in is also something that could go wrong or overheat. Especially high voltage and high current wires. I'll mess with 5V throttles and switches and sensors and whatnot connectors all day, but don't like dealing with pack voltage and motor currents.

I haven't come across the one you just linked. I will be contacting them ty
 
john61ct said:
maarkmohamed said:
Yes correct, this is your typical mosfet based bms. The one you linked is along the lines of what I'm looking for, but still they are big and expensive and that one goes up to 32s, I'm guessing if you buy two then it'll get up to 40+s. My question is can it be done with your typical mosfet based bms? They are much smaller and cheaper.

No, to work in series means all the current must go through each unit.

The BMS must be specifically designed to be wired in series.

Stop focusing on cheap and small, hard enough to get safety and reliability

Cheap BMS easily fail even when used as designed, if you do use that "load current flows through the mosfets" be sure to buy spares and ideally be prepared to replace fets as they burn out.

Personally I would bring balance wires outside the pack and position the BMS so it can be easily replaced.

Ok good to know, few ppl have showed me possible bms's for what I'm looking for. I'm going to avoid 2 smaller bms's
 
john61ct said:
maarkmohamed said:
Yes correct, this is your typical mosfet based bms. The one you linked is along the lines of what I'm looking for, but still they are big and expensive and that one goes up to 32s, I'm guessing if you buy two then it'll get up to 40+s. My question is can it be done with your typical mosfet based bms? They are much smaller and cheaper.

No, to work in series means all the current must go through each unit.

The BMS must be specifically designed to be wired in series.

Stop focusing on cheap and small, hard enough to get safety and reliability

Cheap BMS easily fail even when used as designed, if you do use that "load current flows through the mosfets" be sure to buy spares and ideally be prepared to replace fets as they burn out.

Personally I would bring balance wires outside the pack and position the BMS so it can be easily replaced.

Ok good to know, few ppl have showed me possible bms's for what I'm looking for. I'm going to avoid 2 smaller bms's
 
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