Bus Bar sizing for 12 new Lithium Batteries?

ligwydligwyd Registered Users Posts: 198 ✭✭
Hello all,

I've got 12 new 100Ah Lithium batts being installed in an coffin style batt box and just figuring out bus bar or cabling size to parallel all 12 batts. Each battery has the potential for 100 A max charge or discharge. 

I have used a serve rack WITH 600A rated bus bars to install 6 of the same batts in the past. I would just use two server racks but would like to utilize the existing batt box and all 12 of the new batts fit into the old box nicely.

I am debating on using two lugs/ cable on each batt terminal and using cable for the bus bar or finding a long 10 feet plus tinned copper bus bar?

I am using one 100A 600V Schneider Controller with the XWpro.

Do I really need to have the bus bar/ cabling be able to handle 1200A (12 batts) and 100 A (max charge amp from controller) and another 100A coming from the XW when the gen is running. Total 1400A of max possible current........

If the 12 batts are running through one 250ADC breaker would that really be all that the bus bar/ cable would ever actually see?
Just hoping for some dialogue on the subject...........


  • BB.BB. Super Moderators, Administrators Posts: 33,260 admin
    Do these 100 AH Li Ion batteries have a BMS, specifically with over current control/shutdown?

    Each battery/battery + cable should be protected against excess current. A BMS with high current shutdown (more or less) replaces the need for a fuse/breaker per paralleled battery connection.

    These (one and two fuse) terminal blocks are nice for compact ba
    ttery to busbar wiring;


    For any cables that leave the bus bar, they should also be protected by breakers/fuses.  Doing that, then you only need to rate the busbars/cables for the current you need and voltage drop limits (plus the over current devices ratings).

    You should also look at the overall wiring to the batteries to ensure that they balance their charging/discharging current:


    Lithium Ion batteries are very voltage sensitive in the sense they have very low internal resistance--So the "current balancing" is "steered" by the overall resistance of the cabling.

    For the bus bar(s), you should also look at some sort of protection against inadvertant short circuits (tools/rings/jewelery/etc.) that can fall into the bus bars (i.e., lock the battery box, and/or have separate covers over the bus bars.


    Don't kid yourself about flame retardant plastics... They may self extinguish if you have a chunk of plastic and remove the flame... But if you have a nice roaring fire--They will burn just fine.

    For wiring inside the box and exiting the box, protecting against sharp edges, a loose connection spinning and shorting to the opposite voltage connection, etc. all is important. Especially (as an example) for the short cable run lengths between the bus bar and a remotely mounted fuse/circuit breaker).

    Then there is the box/surrounding areas. While LiFePO4 batteries are much more resistant to catching fire than most other Li Ion chemistries, I would still avoid flammable materials next to the battery bank (and this really even applies to cables/etc. which have plastic insulation). Using ceramic tile flooring/walls in area (as would be done for a wood stove), concrete backer board, etc.... Basically, you don't want (for example) exposed plywood sub-floor/open 2x4 walls and such next too, or under areas where dripping flaming plastic can fall.

    Other secondary thoughts about Li Ion and fires... Lithium battery fires can create very toxic fumes/debris that the building would need to be scraped down to dirt for "cleanup/decontamination". You are in a bit of a pickle as Li Ion batteries need to be kept over roughly 50F for charging. So inside a conditioned space, but if you ever did have a fire, you would be better off with the batteries in a (for example) concrete block outbuilding or battery shed...

    Using outbuildings for batteries, running gensets, fuel storage is probably always a good idea if possible.

    And some things that catch fire can have very toxic gasses/results. This article from 2019:

    FAIRFIELD, Calif. (KCRA) —

    Fifteen first responders were treated for exposure to toxic fumes after helping battle a house fire over the weekend in Fairfield, officials said Tuesday.

    The Fairfield Fire Department said 11 firefighters and four officers reported feeling sick after being exposed to the fumes.

    Crews responded to a fire at a home on Ash Court around 5 p.m. Saturday. There, crews found 200 pounds of lithium-ion batteries in the garage, the Fairfield Police Department said. According to officials, the batteries emitted fluoride gas after being exposed to the fire.

    That's when the 15 people were exposed to the toxic fumes.

    The fire department also said the incident was so toxic that crews had to throw away all the hoses and boots that were exposed while battling the blaze.

    Investigators are now trying to determine what kind of business the homeowner was running.

    Some Lithium battery fires, when mixed with fire fighting water and known to produce Hydrofluoric Acid (I guess that is the reason for the above Fire department had to toss their equipment--And illness)--Which is some pretty scary toxic effects.


    For any significantly sized battery bank, keeping the bank in a separate building keeps your family safe, and if you have to scrap the building to the ground for cleanup--This helps to limit your losses.

    There are just so many different chemistries for Lithium Batteries (more than 20?)--It is difficult to make wide ranging statements about safety/toxicity, etc. And I am no expert. So, again, due diligence when selecting, designing, and maintaining your systems.
    A couple posts down:

    I know two PV contractors (customers) who will only install large battery banks in an area, not in or attached to the house. Generally, I see simple cinder block structures. You are right Bill, a battery fire can be a huge mess in terms of damage and cleanup.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ligwydligwyd Registered Users Posts: 198 ✭✭
    I will be trying these batts listed below:
    I guess there is caution/ danger, risks and best practice protocol associated with any technology, but I do agree lithium fires would not be nice........ I would only need have one fire and I may never again use lithium. Seems like fairly stable tech now though with BMS, breakers, UL etc

     I build my systems all outside of the home. It would be nice to place batts in separate concrete room for sure......... Have to consider in future projects.

    Just pricing out copper flat bar now and trying to nail down proper sizing (current carrying capacity) of the batts/ system....

    I would like to use big enough bus bar that it could handle  the XW max charging from gen at approx 100A and also one 100 A charge controller in full sun, since if the sun comes out while gen is charging in bulk, the batts could potentially at times, see 200A of current. 

    Also although the bottle neck is the 250ADC breaker in the PDP...... wouldn't it suffice to just simply use bus bar rated for 250A X1.25?......I will however, more than likely, use min 500A or maybe even 1000A material, but going to 1200A plus  charge Amps from both PV and Gen seems overkill..........doesn't it? ......

    Not yet sure if it is necessary to replace the 500A shunt with a 1000Ah shunt or not since the draw from the batts is again, limited to the 250ADC breaker..........

    Customer wants an oversized bank for his own reasons although it is a little over kill..... I guess he will have a few days of coverage with no sun or gen, albeit at quite the cost for batts......

    Always appreciate the insight on the forum and look forward to contributing to the platform through dialogue... :)
    Thanks in advance for you feedback :)

  • BB.BB. Super Moderators, Administrators Posts: 33,260 admin
    I suggest that DC breakers/fuses and such on battery banks are a "different animal" vs what we normally see on AC panels panels for homes.

    Certainly, we need to look at the DC current/voltage ratings for DC circuits--Whereas because AC current is "easier to break" vs DC current (DC breakers and switches appear to be much heavier duty vs AC breakers and switches)--There is the issue that in DC power systems, the battery bank is what really does the actual voltage regulation of the DC power system.

    With an AC power system, if you cut a circuit, or main breaker, etc., the voltage simply drops to zero volts.

    With DC power systems, if you "pop" a DC breaker/fuse to the battery bank, your bus voltage is now (more or less) unregulated. You can have the DC voltage of a bus (with the battery disconnected) double or even more for milliseconds to seconds before the "generator/solar charge controller/etc.) notices the bus over voltage and shuts down. You can also have "inductive kick" where inductive loads (motors, and even the DC wiring) can cause the bus voltage to go "negative" by even 100's of volts. Either (unregulated voltage peaks, or voltage going "negative" peaks) can wipe out your DC connected equipment (AC inverters, DC charging equipment, LED lighting, etc.).

    And when you toss in BMS (battery management systems), you have now added another point of power interruption (battery disconnect by BMS command).

    Now, normally, the systems should run along for years without blowing a battery breaker/fuse/BMS-trip.

    In the USA, the "typical" fuse/breaker is rated to not blow at 80% or less of current flow, and "will blow" at 100% or more current flow (seconds, minutes, hours--Or possibly never).

    As you suggest, I would design the main breakers/fuses/BMS to trip at at least (1/0.80) 1.25x the maximum continuous current (charging or discharging), for several reasons.

    One is because a DC battery trip can cause damage to your DC connected equipment--And second because certain loads, such as battery charging, can cause "over heating" of wiring and such (this the 80%/1.25x factor for "continuous current" operations--Such as gym lighting is designed to pull no more than 80% of branch circuit rating), "over sizing" the breaker/wiring for the system (battery, charging, AC inverter, etc.) reduces the chances of nuisance trips and possible equipment damange.

    I suggest that the design process, Battery Breakers, BMS and such should be the protection of last resort. Charge controllers, BMS monitoring, etc. are set to function (stop charging, disconnect AC inverter, disconnect DC loads) before the BMS/Battery breaker should trip.

    Also, we run into issues where mechanical breakers/fuses have relatively slow trip times--And they can pass several times (or more) current spikes (such as motor starting surge) before tripping. So a 125 Amp breaker on a 100 Amp Charger or 4.8 kWatt AC inverter (100 amps * 48 volts) will be fine even for surging current (i.e., a typical "good quality" AC inverter can surge 2x name plate power rating).

    But, for things with "electronic" protection (AC inverter, BMS devices), their "electronic" circuit protection can be much faster.

    For example the Battery BMS you linked to:


    Performance Components

    • Reliable Built-In EG4 Smart BMS, Voltage, Current, Temp. and Health Management

    • Stress-Free Battery Bank Expansion Capacity

    • Cell & Module Voltage Protection

    • Short Circuit Protection >250A <0.1mS

    • Temperature Protection

    • Operating Power Consumption  <25mA
    And there is the (much preferred) closed loop BMS to Solar Equipment for graceful/accurate operations:

    EG4 Lifetime Lithium, Best In Class Solar Rack Battery

    • Rated for 7000 deep charge & discharge cycles
      • 15-year life with 80% depth of discharge daily, designed for up to 20-year life

    • Tier One, A+ Cell Composition, 99% Efficiency LiFePo4 16-Cell Pack

    • #1 Energy Density Server Rack Battery Form Factor

    • Selectable closed-loop inverter communications including Schneider, Solark, Victron, Growatt, and EG4!

    • Dual Onboard automatic fire arrestors
      • Failsafe operation in high-risk environments and to protect against rare
        hardware failure on high voltage solar charge controllers!

    • Parallel up to 64 batteries
      • Get the most power possible! Up to 327.6kWh while maintaining BMS

    • Have existing V1 EG4-LL Batteries? Enable expanded communication for existing V1 EG4-LL Battery Banks by using a new V2 EG4-LL Battery as your master battery. This feature will be available with a firmware upgrade.
    I have not gone through all the specifications--But dual automatic file arrestors sounds interesting... I would want to find out more to ensure that the overall system design works correctly with these fail safe operations.

    Lots of interesting information in the manual (clip of some of the basics):


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ligwydligwyd Registered Users Posts: 198 ✭✭
    Thanks Bill. Always appreciate you thorough feedback!

    So, when inverting, lets say each batt could discharge 100A max X12 batts = 1200A (although would never happen, but we're covered)

    Now if gen is triggered and charging with one XW at full power, approx 100A at system voltage and sun comes out from behind the clouds and the one 100A controller is also now putting out 100A = 200A of potential current going in to the batt bank, which is no problem since the batt can each take 100A max per batt (although 30-50A id the recommended charge amps per batt) 200A divided by 12 is still under 20A per batt which is fine......Whats my point?...I'm trying to figure that out :)

    Even if the XW was loaded with 12000watts for 60sec (before it trips on its own) 12000 divided by 44.8V (low batt cut out) = 270A Maybe the 250ADC PDP breaker would trip or maybe the XW would trip.....? Either way we are still under 300A......

    Now what happens, at the batt bus bars, with DC current flow if Gen is charging, sun comes out and XW experience max load for 60 sec?
    Does DC current just take the path of least resistance? Does the gen just pass through AC as per XW settings?

    Just simply trying to put some thought and calcs into the bus bar selection for this particular system.....

    Sorry if my question is unclear but I'm sure you get what I am trying to say........

    Appreciate the clarity. I'm in a desk at the front of the room and I'm listening

    Ex. to clarify: What happens at the batt busbars when you have 2 XW's loaded down with 12000 watts max (60sec trip) X 2 =24000watts MAX. 24000watts dived by 44.8V = 535A. Now, with the DC circuit (batt bus-bars) what happens when you would have two 100A charge controller  come on in full sun with more than enough PV???

    You have a max load on the batts, and also have a have the charge controller loaded down charging. A huge load (inverters) and a huge supply (solar). How many amps in theory, will the batt bus bars be carrying.

    I'm sure I am over complicating it, however just want to get a clearer understanding...... :)

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