Proper rating for DC breaker for battery ?

[meetyg]

Hi there.
I am planning a DIY solar generator, using Topband 25ah Lifepo4 cells (from BH).

I have a 12v 1500w inverter. I would like to power loads up to this rating (around 120A at 12.8v).
I have 12 25Ah cells that will be arranged in 4S3P configuration (75Ah total). Since these cells should be able to handle 2C discharge rate, this should give me the ability to run 150A loads maximum.

The reason I chose 12v is so that I would be able to use the inverter with some other ready-made 12v batteries too.

Anyways, I'm looking for a 150A DC breaker that will go between the battery and inverter (and all the other loads, like 12v sockets etc...). This will be not only for protection, but also as a main "ON" switch for the solar generator (so fuses don't help here...)

I know that the cheap Aliexpress "marine type" breakers are not recommended because they are not reliable and probably can't handle their rated amperage (especially with those little 6mm studs).

So I was looking at these Bussman DC breakers.
I came about the datasheet (attached) and was stunned!
These breakers need about 200-300% of their rated load for about 10-15 seconds in order to open/break.

This doesn’t sound good to me, because that could fry the wires easily. I want to use lowest diameter wire (highest AWG) necessary to handle 120A (maybe a bit more, but not 200%).

So, should I actually get a lower rated DC breaker (say 60-80A), so that it will break faster when 120A is exceeded? Would that be safe?

Would you recommend another type of breaker ? Like maybe this DIN rail types used for solar (if there are any at this amperage rating...) ?

Thanks in advance...

BTW: Do you really think the Bussman DC breakers are better than the Aliexpress ones? Some of them look very similar and some also use the 6mm studs...
So I'm wondering if it's really worth getting the Bussman? I mean either the Chinese are doing a great job copying these (but less reliability/quality) or maybe the Bussman are made at the same Chinese factories and are just rebranded and cost twice as much?

 

[meetyg]

Anyone?

 

[John Frum]

Over-current protection(fuse/breakers) protect the wire.

1500 ac watts / .85 conversion factor / 12 volts low cutoff = 147.058823529 service amps
147.058823529 service amps / .8 fuse headroom = 183.823529412 fault amps.

That means minimum 2 awg pure copper wire with insulation rated for 105C with a breaker rated for 200 amps.
The 187 series are quality breakers and from memory the series tops out at 200 amps.

Check the AIC rating.
Because the AIC rating will be pretty low you probably should go with an ANL or MRBF fuse and a disconnect switch.

 

[Johnson]

I replaced my breaker with a fuse after the cheap breaker went up in smoke. Like you I am interested in recommendations.

 

[John Frum]

"Interrupt Rating: Main Breaker Protection Interrupt Rating (5,000A@ 14Vdc, 3,000A@ 28Vdc and 1,500A@ 48Vdc)." -- the pdf you linked
An MRBF fuse is rated for 10,000 amps at 14 volts -- https://www.bluesea.com/products/5187/MRBF_Terminal_Fuse_-_200A

 

[meetyg]

Over-current protection(fuse/breakers) protect the wire.

1500 ac watts / .85 conversion factor / 12 volts low cutoff = 147.058823529 service amps
147.058823529 service amps / .8 fuse headroom = 183.823529412 fault amps.

That means minimum 2 awg pure copper wire with insulation rated for 105C with a breaker rated for 200 amps.
The 187 series are quality breakers and from memory the series tops out at 200 amps.

Check the AIC rating.
Because the AIC rating will be pretty low you probably should go with an ANL or MRBF fuse and a disconnect switch.

First, thanks for the clarification.
The AIC rating at 14v is 5000A. Would that be enough? How do I calculate which AIC rating I need?

 

[John Frum]

First, thanks for the clarification.
The AIC rating at 14v is 5000A. Would that be enough?

What is the internal resistance of the cells?

How do I calculate which AIC rating I need?

I've not seen anything more than an educated guess based on the internal resistance of the cells and the joinery between them.
For the big blue prismatic cells that are popular here the WAG is >10,000 amps for 4s and >20000 amps for 8s.
@Hedges may be able to shed some more light on this.

 

[meetyg]

"Interrupt Rating: Main Breaker Protection Interrupt Rating (5,000A@ 14Vdc, 3,000A@ 28Vdc and 1,500A@ 48Vdc)." -- the pdf you linked
An MRBF fuse is rated for 10,000 amps at 14 volts -- https://www.bluesea.com/products/5187/MRBF_Terminal_Fuse_-_200A

The reason why I want a reset able breaker is because let's say I accidentally go over 1500w, or in case of a surge (inverter can handle up to 3000w surge), I don't want to blow a fuse just for that...

I want to protect not only the wiring, but also the battery/BMS. Because I don't have a high capacity battery, I wouldn't want to go over 2C discharge (150A). I know that the BMS can protect against this, but I would rather cutoff the load before the BMS needs to handle this.

 

[John Frum]

The reason why I want a reset able breaker is because let's say I accidentally go over 1500w, or in case of a surge (inverter can handle up to 3000w surge), I don't want to blow a fuse just for that...

The surge rating for a commodity high frequency inverter is ~20 milliseconds.
The typical trip curve for a breaker like the one you are interested in is something like 150% rating for 3-5 minutes.
So no danger there.
Remember the fuse protects the wire so I specked 200 amps anyway.

I want to protect not only the wiring, but also the battery/BMS. Because I don't have a high capacity battery, I wouldn't want to go over 2C discharge (150A). I know that the BMS can protect against this, but I would rather cutoff the load before the BMS needs to handle this.

What is the continuous rating for the BMS?

 

[meetyg]

What is the internal resistance of the cells?

I've not seen anything more than an educated guess based on the internal resistance of the cells and the joinery between them.
For the big blue prismatic cells that are popular here the WAG is >10,000 amps for 4s and >20000 amps for 8s.
@Hedges may be able to shed some more light on this.

Here are the specs for the cells.
It says internal resistance is lower than 4mOhms. Does that sound correct? Sound to little for such a small cell. These will be 4S3P as mentioned.

 

[John Frum]

Here are the specs for the cells.
It says internal resistance is lower than 4mOhms. Does that sound correct? Sound to little for such a small cell. These will be 4S3P as mentioned.
View attachment 77514

That is a lot higher internal resistance than the big blue cells that are popular here.
From memory they are ~.15 milli-ohms.
I guess that the breaker's AIC is adequate.

 

[meetyg]

I haven't got the BMS yet. I was planning on a JBD 150A.

 

[John Frum]

This is my opinion on breakers.
Tripping an high amp DC thermal breaker under load is not to be done capriciously.
Its my understanding that the contact points get pitted and once they get pitted the chance of them welding shut goes up significantly.
I have a breaker in my system but it is only used as a means of disconnect.
If it ever tripped on over-current I would figure out why, fix the problem and buy a new breaker.

 

[meetyg]

That is a lot higher internal resistance than the big blue cells that are popular here.
From memory they are ~.15 milli-ohms.
I guess that the breaker's AIC is adequate.

Cool...

 

[meetyg]

This is my opinion on breakers.
Tripping an high amp DC thermal breaker under load is not to be done capriciously.
Its my understanding that the contact points get pitted and once they get pitted the chance of them welding shut goes up significantly.
I have a breaker in my system but it is only used as a means of disconnect.
If it ever tripped on over-current I would figure out why, fix the problem and buy a new breaker.

What about those thermo magnetic ones? Would that be more suitable?

 

[meetyg]

From your suggestions, I think I will add an ANL type fuse just in case the breaker gets welded...

 

[John Frum]

From your suggestions, I think I will add an ANL type fuse just in case the breaker gets welded...

Why not MRBF?
They are smaller and have a higher rating.

 

[John Frum]

175A: MRBF-Series: Blade Fuses | Waytek

Bolt-down fuses provide protection from 30A to 600A and are ideal for battery and alternator circuit protection. Order from Waytek today!

www.waytekwire.com

 

[chrisski]

Why not MRBF?
They are smaller and have a higher rating.

Wish I’d considered MRBF. I’m wondering how much space it could save compared to a class T and a ANL fuses.

The ANL fuses are pretty big in the holder and the Class T is even bigger. THe Battery box pictured is 2’ X 2’ and has two 125 amp class t fuses and two 250 amp busbars.On the top right of the pic, there is an ANL fuse. That is about as wide as my fist. Space is a premium on an RV build.

Not pictured is the 225 amp class t fuse for after the batteries are combined. THat is slightly bigger than the two class T fuses pictured.

 

[John Frum]

The bluesea mrbf carriers fit a 3/8"(10mm) stud.
Littelfuse makes a terminal carrier that fits an 5/16"(8mm) stud.
I don't know of any carrier that fits a 1/4"(6mm) stud.