Breakers and fuses

[chilly2]

Hello,

I have…

- an HQST 12/24/48v 60a scc.
- a Weize 12v 100ah LiFePO4 battery.
- a 12v 3000w psw Inverter.
- pos and neg bus bars.
- 6awg cable for the scc to the bus bars.
- 2/0 awg for the battery to bus bar.
- 2/0 awg for inverter to bus bar.
- 63a din breaker for scc.

I think I need to add a breaker between inverter and bus bar but am not sure what size. 3000w at 12v is 234a. The inverter will surge 6000w for a moment.

I’m a little confused about this as the bms in the battery has 100a discharge / 200a burst. Does that mean I’d actually need two batteries in series to take full advantage of the inverter? Rather that I won’t exceed the amps because the battery bms will shut off?

Do I need a breaker / switch for battery to bus bar if there’s already one on the scc and possibly (depending on answer to above)!the inverter?

What else should I add? I just don’t want it to melt the first time I use the coffee maker.

Kelv

 

[Tomthumb62]

Hello,

I have…

- an HQST 12/24/48v 60a scc.
- a Weize 12v 100ah LiFePO4 battery.
- a 12v 3000w psw Inverter.
- pos and neg bus bars.
- 6awg cable for the scc to the bus bars.
- 2/0 awg for the battery to bus bar.
- 2/0 awg for inverter to bus bar.
- 63a din breaker for scc.

I think I need to add a breaker between inverter and bus bar but am not sure what size. 3000w at 12v is 234a. The inverter will surge 6000w for a moment.

I’m a little confused about this as the bms in the battery has 100a discharge / 200a burst. Does that mean I’d actually need two batteries in series to take full advantage of the inverter? Rather that I won’t exceed the amps because the battery bms will shut off?

Do I need a breaker / switch for battery to bus bar if there’s already one on the scc and possibly (depending on answer to above)!the inverter?

What else should I add? I just don’t want it to melt the first time I use the coffee maker.

Kelv

You don't want a breaker between the inverter and bus bar. Put a FUSE (not a breaker) between battery and bus bar, on the positive cable. I would use a 300A fuse, most people here recommend a T-class fuse (blue seas is a reputable brand), though others use ANL and other type of fuses. You could also put a 300A fuse between the inverter and bus bar, but that would be in addition to the essential battery fuse.

Before using such a fuse, make sure the specific 2/0 cable you have is actually rated for over 300A like this stuff is.

You can put a switch (skip the cheap battery switch that are all over ebay/amazon) on the battery if you use a quality one like this.

As for the 100A BMS issue...you would need THREE of your batteries in parallel to reach 300A output. The 200A burst is usually only for a fraction of a second or several seconds at best.

You might consider if you truly need a full 3000W inverter. It's really pushing the limit for 12v systems. If you went with 2000W (which will power a lot of stuff, all the regular household type stuff but no heavy power tools), then you'd only need two of your batteries.

You do not want to stress a battery to the point that the BMS shuts down due to over-current. Especially not with a budget battery like the Weize (see Will's latest teardown of the 100Ah Weize...it's not great, they have cheapened out on the build quality since he first reviewed it a while back).

 

[chilly2]

Thanks for the in depth response. I appreciate it a lot. I guess I was too much in a hurry to throw stuff together that I didn’t think clearly on the math.

With the single battery would I be okay as long as I limit the loads? It’s only meant for a few basic things in an emergency (fan, lights, etc). Maybe it’ll run a coffee maker or small microwave. I just went overkill on the inverter to cover surge.

I’ll have a look at the fuses.

 

[chilly2]

The cable I got was this. https://a.co/d/6ANL2Tu

The chart on the page lists it as good to 325A assuming I have interpreted it correctly and it’s accurate.

 

[Tomthumb62]

Thanks for the in depth response. I appreciate it a lot. I guess I was too much in a hurry to throw stuff together that I didn’t think clearly on the math.

With the single battery would I be okay as long as I limit the loads? It’s only meant for a few basic things in an emergency (fan, lights, etc). Maybe it’ll run a coffee maker or small microwave. I just went overkill on the inverter to cover surge.

I’ll have a look at the fuses.

No it’s not a good setup. Either get a smaller inverter (1000w inverter will pull 92A) or at least a second battery. For example, running a coffee maker will pull 1500W, converted into DC amps is 1500/12.8/0.85= 137A, which will trip your BMS. If the BMS fails, then you could end up with a battery fire.

You can try to watch your loads, but that’s stressful! How are you to know if you’re getting too close to 100A? Even with a shunt that could monitor this, you’d have to be constantly checking it to know if you’re too close or not. Instead, just size your system properly and you can relax knowing if you try to use too much energy, your inverter will beep at you (not a fire hazard and inverter will shut down).

If it makes you feel any better, we’re too over bought our first inverter. We just sold a 2000W inverter to buy a 1000W one. Our energy needs are low (12v fans, water pump, 75W AC fridge with 800W startup surge, might run an electric blanket for short periods). Even a “small” 700W microwave can use over 1000W operating and over 2000W startup surge.

Last but not least (and the reason why we downsized inverters) is idle draw. This the power consumed by the inverter by being powered on but no loads are present. Our 2000W Renogy sucked down 24W on idle, which was using nearly half our 100Ah battery every day. The 1000W Victron replacing it uses 9W by comparison and 1W on eco mode.

 

[Tomthumb62]

The cable I got was this. https://a.co/d/6ANL2Tu

The chart on the page lists it as good to 325A assuming I have interpreted it correctly and it’s accurate.

Yes that’s good cable for your inverter. But your battery and inverter aren’ta good combo like I said previously.

 

[HRTKD]

The 63 amp breaker between the solar charge controller and the common bus bar is marginal. 1.25 time the rated amps is what you usually want to avoid nuisance trips. So a 75 amp breaker.

 

[chilly2]

No it’s not a good setup. Either get a smaller inverter (1000w inverter will pull 92A) or at least a second battery. For example, running a coffee maker will pull 1500W, converted into DC amps is 1500/12.8/0.85= 137A, which will trip your BMS. If the BMS fails, then you could end up with a battery fire

I'm thinking of taking the second option of a second battery (same brand/spec). The battery I have says 100A discharge, 200A briefly. Two of those in parallel should double it and take things out of the danger zone right? If so would 2/0 suffice to connect the batteries to each other, the bus bar, and the inverter, or do I need higher?

In the meantime I have a spare 800W inverter I can use. The idle draw on it is minimal.

 

[HRTKD]

Wire each battery directly to the common bus bars. Don't run cables between each battery. This way, if you end up with three batteries you don't have to mess with the other two.

2/0 cable is fine. That's what I use on each of my DIY LiFePO4 batteries that can produce 125 amps nominal (limited by the BMS).

 

[Ampster]

Do I need a breaker / switch for battery to bus bar

Yes, because the purpose of a breaker is to protect the wire.

 

[chilly2]

Wire each battery directly to the common bus bars. Don't run cables between each battery. This way, if you end up with three batteries you don't have to mess with the other two.

2/0 cable is fine. That's what I use on each of my DIY LiFePO4 batteries that can produce 125 amps nominal (limited by the BMS).

Great idea. I have space on my bus bar for one more. Each would need their own fuse, right?

 

[HRTKD]

Great idea. I have space on my bus bar for one more. Each would need their own fuse, right?

Yes. There should be a switch between the battery and common bus bar so you can shut down the system easily. You can run the cables from the batteries to the fuse, then to the switch. Some of us cheat and use just one fuse. I used a single 225 amp Class T fuse because my maximum normal loads are well below that. You can also implement a fuse for each battery which is the best way to do it.

Battery (Pos) -> Fuse -> Switch -> Common Bus Bar (Pos)

 

[chilly2]

Yes. There should be a switch between the battery and common bus bar so you can shut down the system easily. You can run the cables from the batteries to the fuse, then to the switch. Some of us cheat and use just one fuse. I used a single 225 amp Class T fuse because my maximum normal loads are well below that. You can also implement a fuse for each battery which is the best way to do it.

Battery (Pos) -> Fuse -> Switch -> Common Bus Bar (Pos)

Thanks for your help

My bus bars have space for four lugs. (BB1,BB2,BB3,BB4).

So to be clear, the above would end up being...

Pos Bus Bar

SCC -> Switch -> BB1
Batt1 -> Fuse -> Switch -> BB2
Batt2 -> Fuse -> Switch -> BB3 (NEW BATTERY)
Inverter -> BB4

Neg bus bar (just the wires in each case, no switches or anything).

Questions

- Assuming wires from BB to inverter are sized for 200A this should mean I'm good for up to 200A of continuous draw, correct?
- How would I monitor state of charge across the complete battery bank in this case?
- Where would you put the one class T fuse in this schematic? Wouldn't it need to be...

Batt1 pos -> bus bar 1 pos
Batt2 pos -> bus bar 1 pos
Bus bar 1 -> 225A Class T -> Bus bar 2 pos
Bus bar 2 -> SCC and Inverter.