Another Fuse and Wire Question: How does BMS max amps factor into wire and fuse sizing?

[jonathan_winters]

Like many others here, I've been putting together plans for a DIY "milk crate" build.

I feel pretty comfortable with the choices I've made so far, though I'm happy to get input (I haven't purchased anything other than the battery yet so there is still time to edit):

• AmpereTime 100Ah Battery
• Giandel 1200w inverter (I will only rarely push it up to even 1000w, but wanted the larger size to have options, even though it means I have a higher no load current)
• BougeRV MTTP 40A solar charge controller
• 2x 100w solar panels (will be adding more after this build is done)
• Various 12v plugs (car charger and USB)
• Blade fuse block to fuse the DC plugs
• DROK (or other similar) AC charger

My main question here is about fuse and wire sizing for the connection from the battery to the inverter.

With a 1200w inverter, I calculated upwards of almost 120 amps when the battery voltage gets to the lower SOC:

• 1200w inverter
• 12v at low voltage
• 0.85 efficiency drop

This gives me 118A.

So I was just about ready to go with 1AWG wire to the inverter (which I think goes up to 150A) and fusing it at either 125A or 150A. Even though I don't believe I'll ever use the full 1200w of the inverter, I want to make sure the wires and fuses are sized for the worst-case scenario.

But then it hit me that this AmpereTime 100Ah battery has a 100 amp BMS (with over-current and short circuit protection). So is it even possible for the battery to discharge more than 100A, even when the battery runs down to ~12v?

If 100A is the absolute max output from the BMS, then it seems like I could size everything based on that 100A max output (even though the Inverter could technically want to draw more), meaning I could go with 2AWG wire to the inverter (or technically even 4AWG based on the chart I was using) and fuse it at 100A at the battery. Does this sound right?

And then the final question (assuming the above is correct about the 100A output at the BMS): Is there any harm or damage that I could do to the inverter or the rest of the system by using a 1200w inverter? It seems like I'll just be limiting the max capacity of the inverter to ~1000w, but is there something else that I should be aware of by using an oversized inverter?

 

[Goboatingnow]

The invertor will be technically capable of triggering the bms overcurrent protection. Not a great idea but may not happen if you don’t stress the invertor. Starting motors or any inductive load will be the issue as there’s big inrush currents

 

[JoeHam]

It doesn’t factor in at all.

Breakers or fuses are there to protect wires, not devices. Period.

A BMS is never a substitute for an OCPD.

If you use 18 gauge wire fuse for that. If you use 1 AWG wire then fuse for that. Otherwise your wire becomes the fuse as it bursts into flames.

Although not terribly likely your BMS can have the FETs fail in the closed position which would then deliver all the current the battery could muster at once if there is a short at the far end. That’s when an appropriate sized OCPD earns it’s keep.

Regarding the oversized inverter, most of them will shut down if inadequate power is delivered on the input side.

Your manual should cover this. Most likely described as an under voltage alarm.

 

[jonathan_winters]

The invertor will be technically capable of triggering the bms overcurrent protection. Not a great idea but may not happen if you don’t stress the invertor. Starting motors or any inductive load will be the issue as there’s big inrush currents

Good thinking! Maybe I'll size down the inverter for now to more closely match the battery's capability.

 

[John Frum]

With a 1200w inverter, I calculated upwards of almost 120 amps when the battery voltage gets to the lower SOC:

• 1200w inverter
• 12v at low voltage
• 0.85 efficiency drop

This gives me 118A.

The inverter low voltage cutoff is 10VDC +-.5 volts.
Is very likely that your FET based BMS could end up trying to disconnect on cell under-voltage at ~10 volts.
1200 ac watts / .85 inverter efficiency factor / 10 volts = 141.176470588 amps

That is a brutal thing to do to the field effect transistors in your BMS.
Field effect transistors have a tendency to failed closed.
This failure will not likely be detected until the next time your BMS should protect your battery but instead the battery is destroyed.

 

[MrThisIsME]

Breakers or fuses are there to protect wires, not devices. Period.

^ This

 

[jonathan_winters]

It doesn’t factor in at all.

Breakers or fuses are there to protect wires, not devices. Period.

Maybe I misstated my question.

I understand that fuses are there to protect the wires -- this is why I always described the current I thought I could expect, then the size of the wire, and then the size the fuse.

Since the expected capacity of the circuit dictates the wire size, knowing what to expect out of the BMS plays a part in that. That is essentially what I'm trying to determine.

A BMS is never a substitute for an OCPD.

This was my understanding, and it makes complete sense.

This is something that struck me as odd about Will's Milk Crate 2.0 video (link is to the time when he does it) -- he didn't fuse the wire between the battery and the inverter - he said both devices will only ever use 50A and the wire can carry 50A, he said he doesn't need a fuse. So it seems like he was doing exactly what you said: using the BMS as an OCPD.

I guess he was taking a calculated risk, or maybe I'm misunderstanding something about what he did/said there.

Although not terribly likely your BMS can have the FETs fail in the closed position which would then deliver all the current the battery could muster at once if there is a short at the far end. That’s when an appropriate sized OCPD earns it’s keep.

Except for a "not terribly likely" failure scenario as you described, would you ever expect to see more than 100A out of a 100A BMS?

I'm basically trying to figure out if I need to size up my wires (and therefore fuses) by a bit to accommodate any kind of surge that would exceed 100A coming from the BMS temporarily (so that I don't trip the fuse for something that is not truly a problem).

Regarding the oversized inverter, most of them will shut down if inadequate power is delivered on the input side.

Your manual should cover this. Most likely described as an under voltage alarm.

Good to know. I don't own this device yet so I'll have to see if I can find the manual online. That is really helpful to know what specification to look for.

Thanks for all of your help!

 

[jonathan_winters]

The inverter low voltage cutoff is 10VDC +-.5 volts.
Is very likely that your FET based BMS could end up trying to disconnect on cell under-voltage at ~10 volts.
1200 ac watts / .85 inverter efficiency factor / 10 volts = 141.176470588 amps

That is a brutal thing to do to the field effect transistors in your BMS.
Field effect transistors have a tendency to failed closed.
This failure will not likely be detected until the next time your BMS should protect your battery but instead the battery is destroyed.

Ah! Ok, so that is good to know. I was obviously using the wrong number here.

So in order to be nicer to my BMS (which I think is probably a good thing to do), is the right answer to size down the inverter?

Based on 10v low voltage cutoff, it looks like an inverter 850w or less would keep me at or below the 100A rating on the 100A BMS (in other words, it sounds like I should be sizing things to try to avoid triggering the protection except in the very rare failure scenario).

 

[John Frum]

So in order to be nicer to my BMS (which I think is probably a good thing to do), is the right answer to size down the inverter?

Yes.
What is the largest continuous load in ac watts that you need to service?

 

[jonathan_winters]

Yes.
What is the largest continuous load in ac watts that you need to service?

In reality, a 600w inverter will be plenty. I was sizing it up only to future proof a bit -- this project is for me to get some practical experience to what I've been learning, so I was thinking 1200w would be better for whatever is next. I could just as easily stick to 600w for now and then really assess my AC needs at that point (and spend the money then, whether that is 1200w or 3000w).

I'd also have the benefit of lower no load current on the inverter if I sized it down, so that seems to be the most reasonable answer at this point.

I really appreciate the input!!

 

[Goboatingnow]

It doesn’t factor in at all.

Breakers or fuses are there to protect wires, not devices. Period.

A BMS is never a substitute for an OCPD.

true to a point , the more correct statement is that a fuse as the OCPD of last resort should be present , but BMS's do ast as OCPDs , many systems have multiple OCPDs

If you use 18 gauge wire fuse for that. If you use 1 AWG wire then fuse for that. Otherwise your wire becomes the fuse as it bursts into flames.

Although not terribly likely your BMS can have the FETs fail in the closed position which would then deliver all the current the battery could muster at once if there is a short at the far end. That’s when an appropriate sized OCPD earns it’s keep.

Regarding the oversized inverter, most of them will shut down if inadequate power is delivered on the input side.

Your manual should cover this. Most likely described as an under voltage alarm.

Typically if you have a suitably rated fuse then your wiring only needs to carry the fault current for a fuse blow length of time, this helps if the nominal current is well below the fault current

 

[chrisski]

I would fuse for 150 amps assuming a 10 volt cutoff and be sure to use wire rated at 100 amps or above.

The panels can give additional power on top of the battery. So your inverter could see more power than the battery applies and the BMS has control of.

In my RV I can use about 75 amps on the 24 VDC side to power a 1800 watt load. This is about 2000 watts from power.

All that power can be pushed by the 2650 watts of solar panels. Nothing from the battery. In fact it’s being charged.

 

[JoeHam]

the more correct statement is that a fuse as the OCPD of last resort should be present , but BMS's do ast as OCPDs ,

Agree to disagree on the definition.

BMS's can and do fail closed.

A proper OCPD fails open.

 

[Goboatingnow]

Agree to disagree on the definition.

BMS's can and do fail closed.

A proper OCPD fails open.

Don’t disagree , a fuse should always be present as a protection of last resort.

But over current protection is commonly a BMS control function. Fuses are too slow to protect equipment as opposed to wires.