Bus Bar sizing for 12 new Lithium Batteries?
ligwyd
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...........
Comments
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;
https://www.bluesea.com/products/5191/MRBF_Terminal_Fuse_Block_-_30_to_300A
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:
http://www.smartgauge.co.uk/batt_con.html
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.
https://www.professionalplastics.com/FireRetardantMaterialsPlastics
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...
https://forum.solar-electric.com/discussion/comment/421928#Comment_421928
And some things that catch fire can have very toxic gasses/results. This article from 2019:
https://www.kcra.com/article/15-exposed-to-toxic-fumes-after-responding-to-fairfield-house-fire/27933317
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.
https://www.suu.edu/erm/eohs/chemical-specific-protocols/hydrofluoric-acid.html
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.
-Bill
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:
https://signaturesolar.com/eg4-ll-lithium-battery-48v-100ahd
Performance Components
EG4 Lifetime Lithium, Best In Class Solar Rack Battery
hardware failure on high voltage solar charge controllers!
communications!
Lots of interesting information in the manual (clip of some of the basics):
https://eg4electronics.com/wp-content/uploads/2023/07/EG4-LL-48V-100AH-Manual-2.0.0.pdf
-Bill