BMS confusion

Manooti

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Oct 13, 2021
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Im trying to find a correct bms for my set up. Not sure if its to BMS each 4 cells or bms all the cells?
Using the battery hookup 3.2 cell packs. charging them now cautiously without a bms in parallel to 3.6-3.65 and then switch them to series. 3.25 currently while charging.

Im aiming for 4 batteries in series parallel for 24v 200ah but since theyre cells not sure which bms to use.
Would I need 4 BMS on each battery (4 cells in series 1 bms) or a 16s? Wouldnt the 16s limit me on ah to 100 vs 200?
If i do a 4s bms on each pack would that still allow me to do the final wiring in series parallel?
this is the toughest part, im stumped
 

upnorthandpersonal

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16s would be 48V pack, with one BMS. You could do 2 x 8s with two BMS for a 24V system.
Stop looking at the Ah number without taking voltage into account, and turn it into Wh. 200Ah at 24V is the same amount of energy as 400Ah at 12V - 4800Wh in both cases.
 

Rocketman

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Don’t charge/discharge cells in series without a bms attached to monitor each cell.

To find the correct bms for you here are some questions to consider.

What voltage do you want/need your system? (Higher voltages with lower amps are almost always better)

What is the highest amount of power you will use? On a continuing basis and also as a surge. Can your bms handle it? Most mosfet based bms’s(the bms that switches the power on/off inside the bms), are very optimistic on the amps they can handle - many people derate them by about 50%)

Do you want to monitor each cell? (Yes if possible).

What communications do you want/need from the bms? Display on Bluetooth/computer?

Do you want it to control charging? Discharging? (Sometimes it is needed)

What other items do you want/need the bms to do? (Relays, etc).

If you are going with 24 volt system you need a 8s bms. (Many 4s bms’s don’t like to be serial connected).

There are two main types of bms:
The cheaper ones (like Overkill, Daley, etc.) use mosfets for switching the power inside the bms.

More expensive kinds like Batrium, Orion, Rec-bms, use a contactor or breaker to kill the power. These can allow the full power from the batteries to flow.

Which kind depends on your needs. For your basic list above most people would setup two 8s 24v batteries- each with its own 8s bms. The Overkill 8s bms is 100amps, so running two batteries in parallel- that will give you 200amps @24v. (Remember many people will derate that a bit). Will that be enough?

Let us know a bit more about your system needs and maybe we can help steer you into products that can work for you.
 

Manooti

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Oct 13, 2021
Messages
20
16s would be 48V pack, with one BMS. You could do 2 x 8s with two BMS for a 24V system.
Stop looking at the Ah number without taking voltage into account, and turn it into Wh. 200Ah at 24V is the same amount of energy as 400Ah at 12V - 4800Wh in both cases.
Ah i see now, same Wh. So the same in 48v @100ah, 4800wh? But the voltage difference from 120 is 2.5 so less draw on batteries? Dont know the correct terminology on that.

Don’t charge/discharge cells in series without a bms attached to monitor each cell.

To find the correct bms for you here are some questions to consider.

What voltage do you want/need your system? (Higher voltages with lower amps are almost always better)

What is the highest amount of power you will use? On a continuing basis and also as a surge. Can your bms handle it? Most mosfet based bms’s(the bms that switches the power on/off inside the bms), are very optimistic on the amps they can handle - many people derate them by about 50%)

Do you want to monitor each cell? (Yes if possible).

What communications do you want/need from the bms? Display on Bluetooth/computer?

Do you want it to control charging? Discharging? (Sometimes it is needed)

What other items do you want/need the bms to do? (Relays, etc).

If you are going with 24 volt system you need a 8s bms. (Many 4s bms’s don’t like to be serial connected).

There are two main types of bms:
The cheaper ones (like Overkill, Daley, etc.) use mosfets for switching the power inside the bms.

More expensive kinds like Batrium, Orion, Rec-bms, use a contactor or breaker to kill the power. These can allow the full power from the batteries to flow.

Which kind depends on your needs. For your basic list above most people would setup two 8s 24v batteries- each with its own 8s bms. The Overkill 8s bms is 100amps, so running two batteries in parallel- that will give you 200amps @24v. (Remember many people will derate that a bit). Will that be enough?

Let us know a bit more about your system needs and maybe we can help steer you into products that can work for you.
Bluetooth would be very beneficial without a computer. Going to be powering some lights, laptops, modem, router, tool batteries (if needed) and looking for maybe a 12v fridge/freezer or the never ending fridge calculation lol. My new chest freezer uses like 800 watts a day, my older one uses almost 2k. Testing the thermometer I purchased off amazon to see if I can at least convert it to a fridge if needed and see how much power i can minimize it to.

these are what im using and the battery designer
 

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upnorthandpersonal

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Ah i see now, same Wh. So the same in 48v @100ah, 4800wh?

Yes.

But the voltage difference from 120 is 2.5 so less draw on batteries? Dont know the correct terminology on that.

Think of it this way:

watts.png

The amount of power (Watts) on you battery side equals the amount of power on the 120V AC side (minus some losses from the inverter). So, if you pull 1000W on the AC side (8A) you pull 1000W on the DC side as well, meaning 84A if it were a 12V system, 42A on a 24V system and 21A on a 48V system. Power = Voltage x Current.
 

Rocketman

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Messages
761
What inverter are you planning on using?

Just make sure you can open those batteries and place the bms leads on them?
Do they already come with a 12v bms?
Look at the Overkill manual - read it all very well!
My guess (without all the info) that would be a good one for you. But get to know it - then you can compare it with any others you want to research.

Also, make sure you have a good battery monitor that counts amps in and amps out. (I like Victron Smartshunt). The battery monitor in the overkill software is ok - but (IMHO) not good enough .
 

Manooti

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Joined
Oct 13, 2021
Messages
20
Yes.



Think of it this way:

View attachment 73602

The amount of power (Watts) on you battery side equals the amount of power on the 120V AC side (minus some losses from the inverter). So, if you pull 1000W on the AC side (8A) you pull 1000W on the DC side as well, meaning 84A if it were a 12V system, 42A on a 24V system and 21A on a 48V system. Power = Voltage x Current.
lol thanks im trying not to overthink it or overcomplicate it.
What inverter are you planning on using?

Just make sure you can open those batteries and place the bms leads on them?
Do they already come with a 12v bms?
Look at the Overkill manual - read it all very well!
My guess (without all the info) that would be a good one for you. But get to know it - then you can compare it with any others you want to research.

Also, make sure you have a good battery monitor that counts amps in and amps out. (I like Victron Smartshunt). The battery monitor in the overkill software is ok - but (IMHO) not good enough .
is the overkill bms any good?
the batteries has a bunch of cells and i think the one they offered had 4- and 4+ so they would connect to the terminals already crimped there
 

Rocketman

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Messages
761
For the cheaper bms’s I like my Overkill. But just because I (or anyone else on the internet) like something doesn’t mean it’s right for you.

Remember YOU are the system engineer. It is your job to make sure stuff works together safely and correctly.

To use the 8s overkill bms - you will need to attach a small wire to each cell. Because you will be spanning two batteries with one bms - it could be easy to mess up- if you mess up those cell wires you will destroy the bms. So be sure and read the manual well before ordering.
 
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