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How to select BMS size? Choosing the right fuse

Watzi

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Jan 6, 2022
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Hey folks,

For the past few days i can't wrap my head around on how to select the correct BMS size because of the inverter. The right fuse size is also a huge question for me.

Explanation:

In my camper van setup (12V) I will be using an Victron Multiplus 12/800 inverter. This means the unit can deliver 800W for 30mins (or 700W constant at 25°C ).
However the inverter also has a peak-power specified of twice the nominal power = 1600W (2 x 800W) for 0.5 sec.
The inverter goes into overload and shuts of after these 0.5 sec.

My point:
The system is designed the following way: Batterie -> main fuse -> distribution box --> all devices (charger, inverter, 12V devices)

So if I make quick calculation for worst case (max. inverter load + max. load of all 12V devices (~50A) ) this results in either:

Nominal Power:
800W/12V + 50A = ~120 A (continous)

Peak Power:
1600W/12V + 50A = ~190A (for 0.5 sec)

Sure the peak power is potentially never pulled, because it would mean I would have to connect an electrical heater with 2000W without thinking. But as far as I know the BMS should be your "last safe" which protects the battery itself while the fuses "before" it pop. That is why I personally believe a 200A BMS is the right choice.

Case 1:
I take a 200A BMS and put a 150A main fuse after my BMS the above calculated peak voltage (190A) would trigger the fuse and shut down the whole system.

Case 2:
I put a 200A fuse as main fuse and protect the inverter separately with 150A. All would be fine but in case of a short it would be a game of "who triggers first" between the BMS and the main fuse.

My Questions:
1) Use a 150A or a 200A BMS?
2) Which size should my main fuse be?
3) Should I put a extra 150A fuse in front of the inverter although it has a internal 150A fuse?

Thank you in advance and for your time.

Best wishes from Austria,
Watzi
 
BMS is generally fine to pass a little over rating for a few seconds. I see nothing wrong with 120/150 amp BMS and fuse for 10%-20% more.
 
For the past few days i can't wrap my head around on how to select the correct BMS size because of the inverter. The right fuse size is also a huge question for me.
BMS has an over current protection built in ....... no need for another fuse.
I have a Daly 150A BMS (48V 16cell) running my house with it's internal limit set at 100A.
If I accidentally pull more than 100A it just shuts off (twice so far).

Choose the BMS to run your usual loads with a bit extra to spare.
 
I would not run a BMS without a fuse. Yes, it has overcurrent protection, and that should normally trip before the fuse. However, that doesn't mean the BMS is capable of interrupting a dead short. Calculations for LFP dead shorts run upwards of 10,000A, and it takes a decent size fuse to interrupt that.
 
Size the BMS based on the loads. It doesn't hurt to go bigger on this number. The BMS is not sized (directly) on the battery. However, the battery specs should be kept in mind when sizing the BMS. You wouldn't want to put in a BMS that pulls twice what the battery is rated for (2C).

It's not unusual to put in a big battery with lots of Ah so you have plenty of runtime. I have a 280 Ah battery and a 120 amp BMS. Actually two of those in parallel for 560Ah and 240 amps. Normal operations will never push the 240 amp limit.

Running a BMS on the ragged edge of it's capabilities is likely to reduce it's life.
 
I have to agree on over sizing the BMS a bit. Most BMS units are made in China and use commodity components. Run them at half their rating, and they can last a long time.

My first BMS, managing two strings of cells, is a 200 amp constant unit, with a 350 amp surge rating. My inverter/charger maximum current is 140 amps, but I have not exceeded 80 amps for more than a blip. I ran like that for over a year. I had it fused at 200 amps which is to protect the wiring. I have since added 2 more strings of cells, and each string has another 100 amp BMS, so I now have 400 amps of rated BMS powering my system. But it is all still running through a single 200 amp "Class T" fuse to protect the 2/0 cables from the batter bank up to the inverter that will never exceed 140 amps when everything is working properly.

Fuses are there to ensure the wires don't overheat, even in the worst case failure. When a system is working properly, you should never have a fuse blow or have a BMS go into over current shut down. If that happens, then either something went very wrong, or you did not size your system properly. Programming the BMS to shut down before the fuse blows is not a bad idea as Class T fuses do get expensive, and if it was a simple mistake of turning on a hair dryer while you were microwaving dinner, it is a simple matter of turning off the loads and resetting the BMS. Ideally, the inverter would go into it's over current protect before tripping the BMS though.

I suggest sizing the wiring to handle the maximum power of the inverter + DC loads, and then fusing to protect the wiring. And choose a BMS rated for twice your expected maximum power. 800 watts on a 12 volt system is 80 amps. I would use 100 amp rated wiring and fuses, and a 200 amp BMS. If you go with a "Smart" BMS, set it to 90 amps as the over current limit. Fuses will easily pass double current for a 0.5 second surge, so you don't need to size up for that. I used 800 watts / 10 to get the current as that allows for inverter and wire losses, and gives you a little overhead to eliminate most nuisance fuse pops.

Now that my system has 3 separate BMS units, I can't have them set to protect anything. Each of my 4 battery strings has it's own 100 amps fuse and #2 wire. They all join at a buss bar and go to a 200 amps Class T fuse and 2/0 cable. Any single string can run my system in normal conditions as I just don't exceed 80 amps when things are working right.
 
I have to agree on over sizing the BMS a bit. Most BMS units are made in China and use commodity components. Run them at half their rating, and they can last a long time.

My first BMS, managing two strings of cells, is a 200 amp constant unit, with a 350 amp surge rating. My inverter/charger maximum current is 140 amps, but I have not exceeded 80 amps for more than a blip. I ran like that for over a year. I had it fused at 200 amps which is to protect the wiring. I have since added 2 more strings of cells, and each string has another 100 amp BMS, so I now have 400 amps of rated BMS powering my system. But it is all still running through a single 200 amp "Class T" fuse to protect the 2/0 cables from the batter bank up to the inverter that will never exceed 140 amps when everything is working properly.

Fuses are there to ensure the wires don't overheat, even in the worst case failure. When a system is working properly, you should never have a fuse blow or have a BMS go into over current shut down. If that happens, then either something went very wrong, or you did not size your system properly. Programming the BMS to shut down before the fuse blows is not a bad idea as Class T fuses do get expensive, and if it was a simple mistake of turning on a hair dryer while you were microwaving dinner, it is a simple matter of turning off the loads and resetting the BMS. Ideally, the inverter would go into it's over current protect before tripping the BMS though.

I suggest sizing the wiring to handle the maximum power of the inverter + DC loads, and then fusing to protect the wiring. And choose a BMS rated for twice your expected maximum power. 800 watts on a 12 volt system is 80 amps. I would use 100 amp rated wiring and fuses, and a 200 amp BMS. If you go with a "Smart" BMS, set it to 90 amps as the over current limit. Fuses will easily pass double current for a 0.5 second surge, so you don't need to size up for that. I used 800 watts / 10 to get the current as that allows for inverter and wire losses, and gives you a little overhead to eliminate most nuisance fuse pops.

Now that my system has 3 separate BMS units, I can't have them set to protect anything. Each of my 4 battery strings has it's own 100 amps fuse and #2 wire. They all join at a buss bar and go to a 200 amps Class T fuse and 2/0 cable. Any single string can run my system in normal conditions as I just don't exceed 80 amps when things are working right.
Thanks for the detailed answer.

Regarding the wiring I will keep the cables as short as possible and oversize the cables regarding max. current for sure.

This is exactly the point what I am worrying about, making sure that I do not have to replace the my fuse just because i accidentally plugged something in at the wrong time.

I have done a more exact calculation with datasheets of my components calculating the nominal and worst case.
For standard operation i will never exceed 110A. Most of the time just a few dc loads will be running, keeping the system way below 50A.
As worst case it will be close to 180A (Note: for 0.5sec).

As far as I understand your explanation my fuses will not trip in the 0.5 sec my inverter needs to shut down due to overcurrent and if i am lucky my BMS will also not trip.

My solution based on all the comments will be:
Buy the 200A BMS and a 200A t-class fuse. Configure the BMS to 180A shutdown and use the 200A fuse as intended cable protection.

Thank you again.
 
Thank you for all your suggestions and comments, I really appreciate it.

I have done a more precise calculation for my loads.
Nominal: max. 110A but most of the time below 50A
Worst case for 0.5s: ~180A

My conclusion and way to go will be:

Buy the 200A BMS and a 200A t-class fuse. Configure the BMS to 180A shutdown and use the 200A fuse as intended cable protection.
 
I favor a catastrophic fuse at the battery with another (smaller) sub-fuse for the inverter. Why let the inverter, or any other single device, shut down the whole system?
Calculate the wire size based on inverter 800W rating using voltage drop tables. Fuse to the ampacity of the cable.
Example from life; Battery-> 300 Amp fuse-> 2/0 cable-> BusBar-> 250 Amp fuse-> 2/0 cable-> 2000W inverter.
 
I favor a catastrophic fuse at the battery with another (smaller) sub-fuse for the inverter. Why let the inverter, or any other single device, shut down the whole system?
Calculate the wire size based on inverter 800W rating using voltage drop tables. Fuse to the ampacity of the cable.
Example from life; Battery-> 300 Amp fuse-> 2/0 cable-> BusBar-> 250 Amp fuse-> 2/0 cable-> 2000W inverter.
The inverter has a 150A internal DC fuse, i would double fuse the inverter and if i would take a smaller one (100A) i would kill the inverter before it goes into overcurrent (and shutting itself off the way it is designed).
 
The inverter has a 150A internal DC fuse, i would double fuse the inverter and if i would take a smaller one (100A) i would kill the inverter before it goes into overcurrent (and shutting itself off the way it is designed).
The inverter fuse protects the inverter.
The inverter branch circuit fuse protects the branch wire and hopefully isolates the fault to the branch.
Thus allowing the rest of the system to keep functioning.
The battery circuit fuse is the last line of defense.
 
The inverter fuse protects the inverter.
The inverter branch circuit fuse protects the branch wire and hopefully isolates the fault to the branch.
Thus allowing the rest of the system to keep functioning.
The battery circuit fuse is the last line of defense.
True, to protect the branch it makes sense but in my case we are talking about a cable length from the bus-bar to the inverter from around 0.7m (27 inches). The cable is secured and protected from external force and in case of an issue my system must not be always on. I might still put a fuse there like you mentioned to protect the main battery fuse (since t-class is expensive).
 
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