BMS Sizing, BMS Amps

sonny93

Solar Enthusiast
I'm unsure if this is explained very clearly in the resources which has made it quite challenging . When purchasing the BMS, I based it upon the an assumption of the likely max amount of current I would draw at any one time.

My understanding is that the Amps of the BMS chosen should corresponds to the max current that is to be drawn from the batteries at any one time.
  • In one of my setups, I will be using 2P4S arrangement of 272aH batteries = a 544aH pack.
  • I plan to use a 2000W inverter, with a few DC appliances including lights, USB power and fridge. I am unlikely to use the full 2000W of the inverter at one time, and more likely to be below 1000W
  • I have already purchased a 200A BMS
I believe I failed to factor in the conversion factor of the Inverter before purchasing my BMS.

Formula
2000 ac watts / .85 conversion factor / 12 volts low cutoff = ~196 dc amps
I assume that this leaves 4amps before the BMS would shut off?
Will this mean I would need to ensure I run my inverter below max load if I wanted to allow room for DC appliances?

However, I highly doubt I would be using 2000W from my inverter while other DC appliances are running. It is likely that the only thing to be constantly running would be the fridge, and maybe lights in the evening. If I'm careful to make sure I do not exceed 200A at any one time, this system will be okay? What would be a user friendly way of monitoring the current being drawn from the batteries?

If not, can I perhaps solve this by reducing my inverter to 1000W?
 

Huck

Watts are Universal
No, you want to over size your BMS. Alot of these Bms's usually do not come with adequate wire sizing. The rated capacity is usually a couple sec to couple minute MAX rating. If you need a 30amp go with a 60amp ..if you need a 150amp go with a 200amp. You can also parerllel the BMS as well. If you need 400amp worth of BMS you can get 2 200amps..
Do not use the BMS as a fuse! You will likely start a fire!
 

sonny93

Solar Enthusiast
Well, that's just as confusing isn't it. I'm not sure if this is explained or mentioned anywhere in the resources section.
So a 100A BMS does not actually mean you pass 100A at any one time, it means you pass 50A at any one time?!?!
What??
 

Huck

Watts are Universal
I'm unsure if this is explained very clearly in the resources which has made it quite challenging . When purchasing the BMS, I based it upon the an assumption of the likely max amount of current I would draw at any one time.

My understanding is that the Amps of the BMS chosen should corresponds to the max current that is to be drawn from the batteries at any one time.
  • In one of my setups, I will be using 2P4S arrangement of 272aH batteries = a 544aH pack.
  • I plan to use a 2000W inverter, with a few DC appliances including lights, USB power and fridge. I am unlikely to use the full 2000W of the inverter at one time, and more likely to be below 1000W
  • I have already purchased a 200A BMS
I believe I failed to factor in the conversion factor of the Inverter before purchasing my BMS.

Formula
2000 ac watts / .85 conversion factor / 12 volts low cutoff = ~196 dc amps
I assume that this leaves 4amps before the BMS would shut off?
Will this mean I would need to ensure I run my inverter below max load if I wanted to allow room for DC appliances?

However, I highly doubt I would be using 2000W from my inverter while other DC appliances are running. It is likely that the only thing to be constantly running would be the fridge, and maybe lights in the evening. If I'm careful to make sure I do not exceed 200A at any one time, this system will be okay? What would be a user friendly way of monitoring the current being drawn from the batteries?

If not, can I perhaps solve this by reducing my inverter to 2000W

You have a 2000w inverter thats 83amps max with 14.6v (will never be there pulling load). What are the watts of DC appliances?
 
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smoothJoey

Everything works if you let it
You have a 2000w inverter thats 83watts max with 14.6v (will never be there pulling load). What are the watts of DC appliances?

2000 ac watts / .85 conversion factor / 12 volts low cutoff = ~196 dc amps
~196 dc amps / .8 fuse headroom = 245 fuse amps.
As a matter of policy I spec this very conservatively and ignore the dc loads unless they are significant.
 

Huck

Watts are Universal
Well, that's just as confusing isn't it. I'm not sure if this is explained or mentioned anywhere in the resources section.
So a 100A BMS does not actually mean you pass 100A at any one time, it means you pass 50A at any one time?!?!
What??
100amp means you can pass 100amp but only for a small amount of time unless specified. The Chinese manufactures are selling you bare minimum and non less safety. You need to give yourself a safety cushion. If you have a total max current usage of 200amps you would want the BMS and wires at min. to accept 20% greater load
 

sonny93

Solar Enthusiast
2000 ac watts / .85 conversion factor / 12 volts low cutoff = ~196 dc amps
~196 dc amps / .8 fuse headroom = 245 fuse amps.
As a matter of policy I spec this very conservatively and ignore the dc loads unless they are significant.
Thanks smoothJoey, I do not believe the dc loads would be very significant. Could you explain what you mean by fuse amps and fuse headroom? what does 245 fuse amps imply?
 

sonny93

Solar Enthusiast
100amp means you can pass 100amp but only for a small amount of time unless specified. The Chinese manufactures are selling you bare minimum and non less safety. You need to give yourself a safety cushion. If you have a total max current usage of 200amps you would want the BMS and wires at min. to accept 20% greater load

So it should be fine so long as I choose not to draw over 200amps right?
 

Huck

Watts are Universal
Yes, Max output at any given time. however I must stress again that it is likely momentary.
Also, charge input current may differ. For example, I have a Daly BMS it can handle max output of 60a but has a max input of 30a. This is likely do to internal construction. Will I ever push 60a output.. a big NO.. thats because it only came with cheap 10AWG wire
 
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smoothJoey

Everything works if you let it
Thanks smoothJoey, I do not believe the dc loads would be very significant. Could you explain what you mean by fuse amps and fuse headroom? what does 245 fuse amps imply?
The orthodox rule is to multiply the max expected current by 1.25 which is the same as dividing by .8 so that your fuses and breakers don't nuisance trip.

As for bms sizing there are few factors.
1. The cheap Ebay BMSs exagerate their ratings so experienced folks have added some fudge factor. The polite term is de-rating.
2. Its not just current alone that matters. Its current over time that matters.
3. Since FET based BMSs have a habit of failing due to heat stress its good practice not to run them to hard so that they last longer.
4. FETs have a nasty habit of failing closed. That means when the BMS fails it still appears to work, it just won't protect the battery when it needs to.
 
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sonny93

Solar Enthusiast
I don't understand what you mean by "well your inverter yes" - I am asking if the whole system is using under 200amps at one time, it is okay to use a 200A BMS.

So if my DC appliances, which would be LED lights, fans, and a fridge and AC appliances are all drawing under 200amps, it is okay to use a 200A BMS with a 544aH battery array.
 

smoothJoey

Everything works if you let it
I don't understand what you mean by "well your inverter yes" - I am asking if the whole system is using under 200amps at one time, it is okay to use a 200A BMS.
Probably not, for the reasons I stated above.
A cheap and cheerful fet based bms rated for 200amps probably won't survive long at 200amps.
There are workarounds though.
 

Huck

Watts are Universal
I don't understand what you mean by "well your inverter yes" - I am asking if the whole system is using under 200amps at one time, it is okay to use a 200A BMS.

So if my DC appliances, which would be LED lights, fans, and a fridge and AC appliances are all drawing under 200amps, it is okay to use a 200A BMS with a 544aH battery array.
I fixed comment #9
 

Fatty

New Member
Tere are workarounds though.
I have seen the Victron Battery Protect or whatever it is called, but it is only rated for 100a. What other options are there for a system that wants to push 200a? Get a 500a Daly? Or are there other products that could work better?
 

smoothJoey

Everything works if you let it
I have seen the Victron Battery Protect or whatever it is called, but it is only rated for 100a. What other options are there for a system that wants to push 200a? Get a 500a Daly? Or are there other products that could work better?
1. My favorite is to "remote control" the inverter via solid state relay and let the all the rest of the system be directly swtiched via the bms.

2. Use a contactor controlled by the bms to switch the whole system.
This is more expensive but is possible.

I run the first setup and if you are interested I can explain how neat and easy it is.
 

Fatty

New Member
2. Use a contactor controlled by the bms to switch the whole system.
This is more expensive but is possible.

Yeah, there are some cheap ones out there, but again, who knows their longevity or power consumption.

I run the first setup and if you are interested I can explain how neat and easy it is.

Yes, please. I think I have a general idea, would you direct connect the inverter to the battery then use the output of the BMS to turn the inverter off? Would you do something similar with the charger?
 

smoothJoey

Everything works if you let it
Yeah, there are some cheap ones out there, but again, who knows their longevity or power consumption.



Yes, please. I think I have a general idea, would you direct connect the inverter to the battery then use the output of the BMS to turn the inverter off? Would you do something similar with the charger?
This illustrates the point.
A real world setup is ssr connects to fuse block via 1 amp fuse
The downstream side of the ssr goes to the inverter.
Some inverters are easier than others to integrate.
The final bit is the negative lead of the inverter goes direct to the battery bypassing the bms.
The rest of the system is still bms protected.
What inverter do you have?
 

Fatty

New Member
This illustrates the point.
A real world setup is ssr connects to fuse block via 1 amp fuse
The downstream side of the ssr goes to the inverter.
Some inverters are easier than others to integrate.
The final bit is the negative lead of the inverter goes direct to the battery bypassing the bms.
The rest of the system is still bms protected.

Yeah, that makes sense. That was one of the videos I haven't yet seen 😁

What inverter do you have?

I am still in the dreaming phase. But I like the Victrons.
 
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