I wanted to share with you my wiring of a battery bank for the system I am working on. The system is one that was started some time ago by the homeowner/prepper. They got swindled by a man/huckster giving out false information about solar systems. The short story is this…the folks ended up with a decent all-in-one MPP inverter unit and 24 240w solar panels…and that is it. The homeowner paid to get the panels mounted and then started to wire them up himself.
One evening while they were over for dinner he asked to see my solar system…duh, of course I want to show it off! After 20 mins of viewing he explained his situation to me…and asked if he could pay me to finish his install. Well, I am not licensed electrician, or licensed solar installer, or a professional…so I told him ‘no’…but I would be more than willing to help him install his own system. Big smile time on his part!
His system is installed inside a CONEX and is dry and secure…but cold. So I went with my favorite battery…Trophy. They are incredibly good batteries, well made, high quality components, great features, and built-in heaters. And the heaters are needed for his CONEX that will get down to below freezing throughout the winter.
After doing the math he decided on 3 of the 105ah LifePO4 batteries, at least to start with. He was on a tight budget so no battery rack. But, I had a plan for that too. But without a standard battery rack there would be no busbar for the battery parallel connections. I had a plan for that too.
The batteries sit on the bottom of one of the battery cases, each battery is separated by a ¾” piece of wood running the side depth of each battery. Connecting the batteries together is done with a 1/8” aluminum angle bar with the corner of the piece going on the inside of the connection flange and the battery box.
Each battery is secured to the brace with holes I drilled and screwed with ¼” machine screws with star washers between the head and the battery box flange, another between the aluminum brace and the battery box, and finally another between the nut and the aluminum brace. Why? To break the surface of the anodized aluminum brace and the battery box paint. This ensures an actual connection with the metal surfaces. The brace on the left of the batteries has a 10AWG ground wire back to an earth ground.
As a busbar I used a Victron Lynx PowerIn unit. Its busbar is 1000amp, plenty for my application…and any future expansion of the battery bank.
To protect from high overcurrent I used a 300amp Class T fuse.
For complete current disconnect between the inverter unit and the battery bank I used a marine grade battery switch.
Wire Sizing –
Each of the Trophy batteries’ BMS operates/regulates the battery at 100amps under normal conditions. I am running 41” of cable between the batteries and the PowerIn, so I round that up to 4’. And since calculating circuit runs you use the length of a ‘round trip’ of the current so that means an 8’ run. Using the sizing calculator that would call for a 6AWG, fused at 117a.
But, the BMS is set to shutdown the current flow at 110amps. Using the sizing calculator that would still call for a 6AWG, but now fused at 128a.
But, each battery also has a 150a DC circuit breaker built in for additional protection. Using the sizing calculator that would now call for a 4AWG, but now fused at 175a.
However, the manufacturer recommends “If using a busbar, equal length pure copper 2/0 AWG battery cables” in their manual. Any thoughts as to why?
Well, there are these things called high-current faults. Meaning that the battery is actually capable of extremely high discharge current should a problem occur that is outside of the battery’s built in protection capabilities. We are talking hundreds of amps. High quality, pure copper, 2/0 welding cable can handle 325amps. So using a 2/0 copper cable gives you plenty of safety protection and it also reduces resistance to virtually zero.
When running cable from the battery bank to the inverter I look at the realistic maximum current flow. Even using 3 of the Trophy batteries maximum normal flow will still be only 300amps total. But now you have to figure in the inverter. The MPP inverter is only capable of 6000watts of AC power, about 50aAC. That translates into a maximum DC draw off the batteries of about 150aDC before the inverter will shut down due to an internal ‘fault’.
Since the length run between the inverter and the PowerIn busbar is only 22”, 4’ round trip, that means you could use 4AWG wire. But that isn’t large enough wire to handle the 175a battery circuit breaker.
What?
Remember…a fuse or circuit breaker is designed to protect the wire in the circuit. If the wire is rated for less than the fuse or circuit breaker the wire becomes the fuse. That means the wire becomes overheated, melts, and potentially starts a fire BEFORE the fuse or circuit breakers does its job.
So to handle the potential of the 175a battery circuit breaker the wire has to be up-sized to 2AWG.
However, remember the 300amp Class T fuse providing high current problems????
Yup, that 300amp fuse requires 2/0 cable. But, we also want to factor in resistance…so we up-size the cable again to 4/0 pure copper cable.
Think of it in terms of water pipes…if we have 3 pipes, each are 1”, and they flow water into another 1” pipe we get friction loss…only so much water can flow from 3 pipes into 1 pipe. But, if those same 3 pipes that are 1” each flow into a 2” pipe then we get far less friction loss and much more water flowing through the 2” pipe. Same basic concept with wire/cables.
And FWI…it is pretty much a standard that batteries are connected to busbars with 2/0 cable and busbars are connected to inverters with 4/0 cable. Yes I know, there are plenty of folks out there who hook batteries to busbars with 4AWG wire. But, you will find the quality professional installers using 2/0 cable. But, it is up to you…and your family’s safety.
Last couple of items…
- The cables between the battery and the busbar should ALWAYS be of equal lengths. That means equal lengths from + & – terminals to the busbar and equal lengths for each battery. I have 3 batteries, 41” of cable from each terminal of each battery to the busbar.
- Use only high-quality, pure copper, welding cable and cable lugs.
- Tighten each connection to manufacturer recommendations. Those stats can be found in the owner/operator manual…or call the manufacturer for their stats.
- If you are making your own cables use a high-quality hydraulic crimper designed for the appropriate size cable and lugs. Test each crimp.
- Use heat shrink to protection each crimped lug. Use a heat gun NOT a flame type device to shrink the heat shrink.
- Don’t allow your cables to come into contact with sharp edges.
- Don’t severely bend your cables.
- Periodically I check my cables for heat when the system is running.