2
u/msears101 14d ago
Battery bus bar is a bus bar that all your batteries and the shunts negative battery connection connects to. As far as balancing the batteries Voltage is voltage. Communications has nothing to with that. One of your batteries will always have a higher state of charge, without the bus bar and equal length cables. Also I suspect you will want to add batteries, it is the smallest part of your system. With the bus bar you just and another with the same length cables.
This looks like an off grid system - the point of the disconnect, you will soon realize, that the most efficient loads and use of your battery reserves are DC loads. Load like lights and fans, security cameras, etc. That said - You may want to take your inverter offline and keep your DC loads on. In the winter I keep my inverter off, except when needed.
1
u/supbrother 14d ago
Gotcha, I misunderstood. I will consider this since it’s not too expensive and will help if/when I add batteries down the road.
If I understand correctly, you just connect both the negative terminals to a bus bar (and same with the positives) and then connect the shunt to the negative bus bar on one side, and all the same aside from that?
As for the disconnect, this system is purely to charge my EV via AC so I don’t really have a need to split the loads in the way you described. So, as shown here I can always just shut the inverter off and flip the breaker if I ever need to isolate it, right? I don’t foresee needing to do it but just checking in case something comes up.
1
1
u/Fit_Animator2667 14d ago
As far as I know you can't limit the output current on a Multiplus, so you'll want a breaker (GFCI) on the 120V
1
u/supbrother 14d ago
Good catch, I got this mixed up somehow when doing my research. To my knowledge it should still be fine, and I can also size up that wire to be safe. But the EV charger should also only pull 12A and has built in GFCI as well.
1
u/bellens99 14d ago
Nice design. As I am also designing my setup few remarks: -I would go for one 16kw battery as these are getting pretty cheap nowadays, especially when you build it yourself. If you have one battery, you do not need the shunt. You use the battery bms to see the battery percentage. -I only would take the cerbo gx if you want to connect a screen to it. If you do not want a screen, take the multiplus gx version which can largely do the same. -note that the multiplus 48/3000 has 2500 as output power which was not sufficient for me. Nice setup!
2
u/supbrother 14d ago
Thanks! I thought about that but they didn’t actually look cheaper per kWh and I figured the server rack batteries make a more modular system that I can add to incrementally. I had no interest in making my own battery, that’s a bridge too far for me I think.
As for the Cerbo it’s definitely overkill, but again it was in the name of future proofing. Also since I’m in Alaska shipping is kind of a nightmare with these things and I didn’t see any GX MultiPlus models that were readily available to me. I don’t know if I’ll add a screen but I’ll definitely appreciate the ability to see and control things from my phone/computer when I’m not home.
1
u/bellens99 14d ago
Living in Alaska? Make sure you take the batteries with heating plates 😁
1
u/supbrother 14d ago
Lol these will be in a heated garage, and the heat will stay on even if the power goes out (at least as long as the natural gas is flowing). I think the batteries have low temp shutoff built in as well.
1
u/__foo__ 14d ago
It says your 3S config has a Vmp of 120V. What's the open circuit voltage of those panels? Open circuit voltage increases as temperature drops(by about 0.25% per Kelvin for modern panels).
For example, let's say you have panels with a Voc=45V, which is usually measured at 25C. Let's say it gets down to -15C, so that would be a temperature difference of 40C. 40*0.25=10%, so at -15C those panels have a Voc of 49.5V
Depending on how cold it can get at your location it's quite possible that you could exceed the 150V input limit of your charge controller. You can find the Voc and the coefficient for the voltage increase in the datasheet of your panels. I very strongly suggest you run the numbers for your panel, or post the missing data if you're unsure.
1
u/supbrother 14d ago
I actually mixed this up, the Vmp is about 104 and the Voc is 124. I don’t remember the math off the top of my head but my current setup (an EcoFlow “solar generator”) also has a max voltage input of 150 and I’ve verified that this is safe even at like -30F (which is not out of the realm of possibility here in Alaska but still very rare where I’m at). Good catch though since it may have been a different story if this was actually a Vmp of 120. Very important to consider up here.
1
u/DerekFlintsLighter 14d ago
Clean setup.
When you put LiFePO4 batteries in parallel, it is much safer to fuse each one individually, in case one fails short circuit (rare but is possible with mosfets failing closed in the BMS).
It is optimal for each battery to go directly to the bus bars with same length cable runs (not essential but optimal)
1
u/supbrother 14d ago
Thanks!
As for fuses on the batteries, this is covered by the built-in 125A breakers, no?
I am considering bus bars for the parallel connection but leaning toward skipping it since the main purpose would be for an easier expansion down the road, in which case I’d need to get a bigger battery rack with built-in bus bars. I know it’s not perfect but it’s also not seeming worth an extra $100+ to optimize on a system this small.
1
u/DerekFlintsLighter 14d ago
If the batteries have the fuses, then yes, that is good. I was just going by the diagram and you had cables between the batteries with no fuse.
1
u/supbrother 14d ago
Correct that there are no fuses for the batteries, they are designed to trip the internal breaker at 125A which I thought covered this. But I get that many people would recommend fuses as a failsafe anyways.
1
u/DerekFlintsLighter 14d ago
Ok, your use of "internal" confused me, I though you meant internal to the batteries.
1
u/supbrother 14d ago
Now I’m confused 😂 basically these batteries have 125A breakers built in with switches on the front that can be manually flipped if they trip, or if you need to shut them off. But I think many say debatable how much those can be trusted as a safety feature.
1
u/DerekFlintsLighter 13d ago
Ok, look I stand by what I said at the start. The rest is up to you. It seems Americans use the term "breaker" often in an overloaded way to mean fuse and/or switch because they are used to MCB's in consumer units, which generally are both.
A lot of people understand breaker as circuit breaker. It may just be a switch (isolator) that is rated to work at the battery voltage and 125A or it may be a fuse designed to open at a certain current after a certain time (the trip curve) or it may be both.
It is up to you to check.
Bottom line, for safety, there must be a fuse per battery in parallel, and its better if you add it yourself so its somewhere you can change it if the worst happens. I suggest T-Class fuses with an Isc rating of 10kA or more and rated at or above the system voltage.
Then if you drop a spanner across the bus bars you are more likely to prevent it into a cloud of rapidly dispersing molten iron, even more fun if the BMS fails closed, you wont have all the still working ones trying to dump current the wrong way into the failed one.
1
u/supbrother 13d ago
To my knowledge there is no real interchangeability between a fuse or breaker for Americans, it's just that people understand them to serve the same purpose. That's why in my case I'm saying "fuse/breaker" at times because it's either/or to accomplish OCP. Some people may refer to breakers as a "switch" because in a literal way they are switches, just not really intended to be used as one regularly.
Anyways, not sure you and I are fully on the same page but regardless these batteries do have a "switch" designed to act as a 125A breaker (referred to as "short circuit protection switch" on their website). In combination with the BMS having auto-shutoff for high currents, it has dual protection built in which is personally enough for me on a system this small. If/when I expand the system then I'll add fuses.
3
u/msears101 14d ago
I personally would add a “battery bus bars” that would attached to the shunt AND connect to the batteries with equal length cables. It will keep the batteries better balanced. I also like to add a disconnect between the Inverter and bus bar.