How do you determine what size BMS to use?
- Thread starter RobOfYork
- Start date
You will need a 4S BMS (4 cells in series for 12V).
The current rating (A) you need depends on the maximum power that your system will draw - i.e. if you’re going to use, say, max 1000W at any one time, then you would need a BMS rated to at least 1000W/12V = 84A. Just remember that if you’re powering via an inverter, there will be a power loss due to inverter efficiency, so your power use will be higher and this needs to be allowed for.
What are you using the system for?
The current rating (A) you need depends on the maximum power that your system will draw - i.e. if you’re going to use, say, max 1000W at any one time, then you would need a BMS rated to at least 1000W/12V = 84A. Just remember that if you’re powering via an inverter, there will be a power loss due to inverter efficiency, so your power use will be higher and this needs to be allowed for.
What are you using the system for?
Steve_S
Offgrid Cabineer, N.E. Ontario, Canada
There are two criteria basically.
1) What amperage demand will your system have. Constant current demand as well as Peak Surge potentials.
12V/1000W draws typically 84A (uncorrected) or 100A corrected for 20% efficiency loss.
Example: A 1200W Inverter Microwave will pull 145A from a 12V Inverter.
Peak Surges occur when a Fridge, AC. or Well Pump motors starts, they are typically Double Amperage for a second or two to startup.
2) The Capacity of the Battery Cells and what they can handle.
The EVE-280AH Cells can discharge up to 1C-Rate or 280A for a max of 1 hour (if fully charged to 100% SOC)
They can take a Burst Surge Rate of up to 3C (840A) for MOMENTARY ONLY - This NEVER gets dipped into for ESS use. It's a Safety Thing.
They can ONLY take up to 0.5C Charge Rate which translates as 140A Charge Potential.
* FUSE according to Max Capacity of the BMS. So if the BMS is a Max of 250A the fuse should be the same. Each Pack should be fused independently.
For a 12V/280AH Battery Pack, a BMS which can handle an output of 250A for discharging & a Max of 140A for charging would be suitable and within spec for the capabilities of the cells.
Any FET Based BMS for such an application should be overrated by at least 15% - 20% for safe margin. They are typically rated/spec'd to the "edge" of their performance range.
Relay/Contactor controlled BMS's rely on the particular Relay/Contactor specs for Amp Handling. Relays/Contactors can handle up to 1000A provided those are chosen. THESE BMS' tend to be more expensive but are more reliable. There ARE Caveats to be aware of as well, as Relays & Contactors consume power, so there are Energy Saver Relays and Solid State Contactors and more which use considerably less standby power.
A WORD OF CAUTION
Many BMS' can function in Parallel, so you can have multiple battery packs in Parallel which increases the Storage Capacity in Amp Hours but not Voltage. Most BMS' will NOT work in Series because they are Voltage Specific. 12V/100AH+12V/100AH in Series gives you 24V/100AH. If one pack cuts off for any reason the other attempts to shoulder the 24V and will fail.
IF at some point you wish to upgrade to 24V system, the packs will need to be reconfigured with 8 cells each & an 8S BMS + fuse for each..
Hope it helps, Good Luck
1) What amperage demand will your system have. Constant current demand as well as Peak Surge potentials.
12V/1000W draws typically 84A (uncorrected) or 100A corrected for 20% efficiency loss.
Example: A 1200W Inverter Microwave will pull 145A from a 12V Inverter.
Peak Surges occur when a Fridge, AC. or Well Pump motors starts, they are typically Double Amperage for a second or two to startup.
2) The Capacity of the Battery Cells and what they can handle.
The EVE-280AH Cells can discharge up to 1C-Rate or 280A for a max of 1 hour (if fully charged to 100% SOC)
They can take a Burst Surge Rate of up to 3C (840A) for MOMENTARY ONLY - This NEVER gets dipped into for ESS use. It's a Safety Thing.
They can ONLY take up to 0.5C Charge Rate which translates as 140A Charge Potential.
* FUSE according to Max Capacity of the BMS. So if the BMS is a Max of 250A the fuse should be the same. Each Pack should be fused independently.
For a 12V/280AH Battery Pack, a BMS which can handle an output of 250A for discharging & a Max of 140A for charging would be suitable and within spec for the capabilities of the cells.
Any FET Based BMS for such an application should be overrated by at least 15% - 20% for safe margin. They are typically rated/spec'd to the "edge" of their performance range.
Relay/Contactor controlled BMS's rely on the particular Relay/Contactor specs for Amp Handling. Relays/Contactors can handle up to 1000A provided those are chosen. THESE BMS' tend to be more expensive but are more reliable. There ARE Caveats to be aware of as well, as Relays & Contactors consume power, so there are Energy Saver Relays and Solid State Contactors and more which use considerably less standby power.
A WORD OF CAUTION
Many BMS' can function in Parallel, so you can have multiple battery packs in Parallel which increases the Storage Capacity in Amp Hours but not Voltage. Most BMS' will NOT work in Series because they are Voltage Specific. 12V/100AH+12V/100AH in Series gives you 24V/100AH. If one pack cuts off for any reason the other attempts to shoulder the 24V and will fail.
IF at some point you wish to upgrade to 24V system, the packs will need to be reconfigured with 8 cells each & an 8S BMS + fuse for each..
Hope it helps, Good Luck
They decide based on cell capability.
But average one amp to one amp hour of capacity.
But average one amp to one amp hour of capacity.
John Frum
Tell me your problems
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Based on the c-rate.
What Is A Battery C Rating & How Do I Calculate C Rate - Power Sonic
Power Sonic look at what a battery C Rating is and how that C Rate is calculated. Includes battery C Rate charts, formulas and examples.
www.power-sonic.com
chipdouglas
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I also ordered a Daly 150 from Ali express. Now this has me concerned. Courious what was your experience with this transaction. Did you get a refund? Good luck with your setup. I’m a newb so I can’t comment directly to thread topic.
kshopper2084
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My inverter is basically the same, I landed on the JBD 200A Smart BMS.
Spec sheet was posted in the BMS thread: https://diysolarforum.com/threads/jbd-4s-200a-bms.27012/post-387089
I ordered mine from the manufacturer's website, price was $93USD shipped to Canada.
4S 200A BMS Lifepo4 Battery 3S 12V Li-Ion Battery Smart Balance Protection Board With Bluetooth Uart - LLT POWER ELECTRONIC
Features and Characteristics : Characteristics : this is a smart BMS with Bluetooth and PC communication which will be used to protect and monitor your battery status visible from the computer and your android APP phone .which can support UART or UART and RS485 communication .the charge and...
www.lithiumbatterypcb.com
I ordered it on Nov. 25th, and got a shipping notice this morning (7 days). They estimate shipping will take 10-18 days.
EDIT> AFAIK this brand/manufacturer is the same one that Overkill Solar uses, though I've read that they upgrade the connecting wires and tweak the default configuration on theirs. I also don't see a 200A version on their website.
Last edited:
RobOfYork
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When I finally got fed up with their canned responses that it is on the way. They then wanted me to place a second order and pay for it. After that, they would refund the first purchase. I let them know that I was going to my credit card company and I would let them handle it. The same day, I get a tracking number sent to me. They shipped a new one express and it arrived a few days later. The one they claimed was shipped months before never arrived.
