Overkill Solar - Wrong BMS Size for my system?

monoxyde

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Jun 2, 2021
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As I'm slowly figuring this out... I think I purchased the wrong BMS size for my system?

Growatt 24v 3000 Watt Inverter
8 * 280 Ah LiFePo4 EVE Cells
Overkill Solar 8s 100A BMS

I am getting ready to buy the wire, was going to use 0AWG wire, with expectation of about 150 A max pull.

Is this going to fry my BMS? Do I need to go ahead and start looking for another one?
 

sunshine_eggo

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3000W/24V = 125A
125A > 100A
BMS will trip at sustained current over 100-110A.

It will not "fry" it, but frequent BMS tripping generally wears them out.
 

monoxyde

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3000W/24V = 125A
125A > 100A
BMS will trip at sustained current over 100-110A.

It will not "fry" it, but frequent BMS tripping generally wears them out.
Good to know. I guess if I'm running full load I may have issues. I'll deal with that later I guess.
 

monoxyde

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Its worse than you think.
3000 ac watts / .85 conversion factor / 24 volts low cutoff = 147.058823529 service amps
Yea, i figured the max I should pull out would be 150 amps. Can't decide if I want to fuse my system at 150 AMP from the battery or at 200.

Either way, that's a catastrophic failure.

To be honest, I don't think I'll quite pull > 100 amps all the time, or a lot?

Most of the stuff I have my battery for, will run at a 15 amp electrical socket.

120 * 15 = 1800 watts.

1800 watts / .85 conversion factor / 24 volts low shutoff = 88.235294117647058823529411764706 service amps.
 
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Yea, i figured the max I should pull out would be 150 amps. Can't decide if I want to fuse my system at 150 AMP from the battery or at 200.
150 service amps / .8 fuse headroom = 187.5 fault amps
2 awg pure copper wire with insulation rated to 105 Celcius and a 200 amp fuse are indicated.
 

Ampster

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Some theories are that you should derate Chinese BMSs to 50%. I do not know the details but perhaps that is for continous load. I am a conservative risk taker and if I were designing a system like you have described I would go with a 200 Amp BMS.
 

monoxyde

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150 service amps / .8 fuse headroom = 187.5 fault amps
2 awg pure copper wire with insulation rated to 105 Celcius and a 200 amp fuse are indicated.

Where are you getting 2 AWG pure copper wire from?

I see lots of different recommendations on wire size, I chose 0 AWG because of the calculator on https://www.fabhabs.com/dc-cable-sizing-calculator.

Maybe 2 AWG will do it, but certainly going overboard with 0 AWG shouldn't cause any issues. Right?
 
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Where are you getting 2 AWG pure copper wire from?

I see lots of different recommendations on wire size, I chose 0 AWG because of the calculator on https://www.fabhabs.com/dc-cable-sizing-calculator.

Maybe 2 AWG will do it, but certainly going overboard with 0 AWG shouldn't cause any issues. Right?
pure copper 2 awg wire with 105C insulation is rated for 210 amps.
I like round numbers and fuses are rated to round numbers so 200 amps.
The rest of the rational was already covered by the math in my previous posts in this thread.
The thing we didn't touch on is voltage drop which is an important consideration.
Would need to know the round trip circuit length to address that.

This is a quick table that I whipped up.
Code:
for pure copper wire with insulation rated to 105 Celcius

ring terminals or ferrules
  16 -  up to 4 amps service current, 5 amp fuse
  14 -  up to 12 amps service current, 15 amp fuse
  12 -  up to 16 amps service current, 20 amp fuse
  10 -  up to 24 amps service current, 30 amp fuse

lugs or ferrules
   8 - up to 40 amps service current, 50 amp fuse
   6 - up to 80 amps service current, 100 amp fuse
   4 - up to 120 amps service current, 150 amp fuse
   2 - up to 160 amps service current, 200 amp fuse

lugs
   0 - up to 200 amps service current, 250 amp fuse
  00 - up to 240 amps service current, 300 amp fuse
0000 - up to 320 amps service current, 400 amp fuse
 
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Forgot to mention.
There is nothing wrong with oversizing the wire.
2 awg with 200 amp fuse is a minimum.
Personally I would probably go with 0 and 250 amp fuse.
 

time2roll

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Start planning for a second battery in parallel. A full size microwave will probably shut you down with BMS over current protection.
 

monoxyde

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pure copper 2 awg wire with 105C insulation is rated for 210 amps.
I like round numbers and fuses are rated to round numbers so 200 amps.
The rest of the rational was already covered by the math in my previous posts in this thread.
The thing we didn't touch on is voltage drop which is an important consideration.
Would need to know the round trip circuit length to address that.

[/CODE]

I bought 30 feet of 1/0 AWG. The absolute "MAX" would be 15 feet each way, but likely, it's going to be more like 5-8 feet. For a total run of ~20 feet.
 

monoxyde

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Start planning for a second battery in parallel. A full size microwave will probably shut you down with BMS over current protection.
Won't be using the microwave. The purpose of my battery bank is comfort and silence.

I did energy audits with a KillAWatt, plugged into 15 amp service panel. Only thing I need is to run the battery bank at night to keep the A/C on, possibly some TV.

Cut the generator off at a reasonable time, and the bank will take care of the rest until the sun rises.

Fridge/Freezer will be powered by propane for the evening, the rest is just to be able to sleep in silence and comfort with a nice cool air blowing. :)

I may in fact add another bank as parallel, but that's another project for another day.
 
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I bought 30 feet of 1/0 AWG. The absolute "MAX" would be 15 feet each way, but likely, it's going to be more like 5-8 feet. For a total run of ~20 feet.

 

ereams65

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I would start shopping for the new BMS for sure. I've had my golfcart trip mine and I could only find 96 amps max draw. Knowing that you need it for "overnight" makes it more important because you'll be waking up to no power overnight and a hot area.
 

monoxyde

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Okay, so this is a dumb question probably, but I want to run it by people with a lot more experience and knowledge than I have.

I bought my 1/0 AWG THHN cable, 30 feet of it.

I've run 1 leg of my cable so far, hopefully I'll get the return feed knocked out tomorrow. So far, I've used ~ 10 feet of cable. The return will be about the same, then I'll have ~ 10 feet of cable to make smaller connections.

Nothing is hooked up, because I won't have a crimper until November 15th, so I'm going to just kind of lay it out with a little extra to work with.

Pardon the text, but this is how it's hooked up.

A/C Side of things:

SHORE CABLE (30 amp, factory trailer plug) ----> Growatt 3000
A/C Output of Growatt (Romex 1/2 Solid Indoor non-metallic Jacket) --> Power junction box of camper --> Breaker Panel

So basically, I inserted the Growatt between the existing plug, and the junction box that feeds to the breaker panel.

This is where I have questions:

D/C Side of things:

D/C Negative Input to GroWatt = 1/0 AWG THHN (about 10 feet) --> Negative Bus Bar --> ( ??? Run?, Cable AWG???) --> Overkill 100 AMP BMS -->( ??? Run?, Cable AWG???) --> Battery Negative Terminal
D/C Positive Input to GroWatt = 1/0 AWG THHN (about 9 feet) --> Battery Cut Off --> (less than a foot of run, more 1/0 AWG) --> 250 Amp D/C Fuse --> (another less than a foot run, more 1/0 AWG) --> Battery Positive Terminal

I know that I have to run the power leads to the battery from the BMS, but as far as the main 2 legs that go in/out from the battery terminal... Do I need to add that in my AMP calculations in the length of run from the battery to the Growatt? I'm assuming I do.

But, I'm not sure if I could run 2 or 4 AWG cable from the negative bus bar to the BMS to the Battery Negative terminal, and then let the heavier 1/0 run the rest of the length.

Growatt <-> Cable run <-> Negative Bus Bar <-> BMS <-> Battery

Rephrased: Size it from Growatt -> Battery, or can I "break" the size, since the BMS is in between?
 

monoxyde

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Jun 2, 2021
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Okay, so this is a dumb question probably, but I want to run it by people with a lot more experience and knowledge than I have.

I bought my 1/0 AWG THHN cable, 30 feet of it.

I've run 1 leg of my cable so far, hopefully I'll get the return feed knocked out tomorrow. So far, I've used ~ 10 feet of cable. The return will be about the same, then I'll have ~ 10 feet of cable to make smaller connections.

Nothing is hooked up, because I won't have a crimper until November 15th, so I'm going to just kind of lay it out with a little extra to work with.

Pardon the text, but this is how it's hooked up.

A/C Side of things:

SHORE CABLE (30 amp, factory trailer plug) ----> Growatt 3000
A/C Output of Growatt (Romex 1/2 Solid Indoor non-metallic Jacket) --> Power junction box of camper --> Breaker Panel

So basically, I inserted the Growatt between the existing plug, and the junction box that feeds to the breaker panel.

This is where I have questions:

D/C Side of things:

D/C Negative Input to GroWatt = 1/0 AWG THHN (about 10 feet) --> Negative Bus Bar --> ( ??? Run?, Cable AWG???) --> Overkill 100 AMP BMS -->( ??? Run?, Cable AWG???) --> Battery Negative Terminal
D/C Positive Input to GroWatt = 1/0 AWG THHN (about 9 feet) --> Battery Cut Off --> (less than a foot of run, more 1/0 AWG) --> 250 Amp D/C Fuse --> (another less than a foot run, more 1/0 AWG) --> Battery Positive Terminal

I know that I have to run the power leads to the battery from the BMS, but as far as the main 2 legs that go in/out from the battery terminal... Do I need to add that in my AMP calculations in the length of run from the battery to the Growatt? I'm assuming I do.

But, I'm not sure if I could run 2 or 4 AWG cable from the negative bus bar to the BMS to the Battery Negative terminal, and then let the heavier 1/0 run the rest of the length.

Growatt <-> Cable run <-> Negative Bus Bar <-> BMS <-> Battery

Rephrased: Size it from Growatt -> Battery, or can I "break" the size, since the BMS is in between?
I'm also asking this, because the OverKill solar has 2 negative leads for input and output. So, since it's (splitting?) the power, can I run a smaller cable.
 
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The overkill 8s bms is only rated for 100 amps continuous.
You can't expect to run 150 amps continuous through it.
 
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