Yet another battery cable sizing question...

bigdogeat

I have read a bunch of the threads on the subject but it seems almost every system is somewhat unique and apparently my math sucks...

You guys were super helpful last time I upgraded.

I am currently upgrading, (I hope it's an upgrade anyway), my batteries. I currently have 8 Trojan T-105  6V batteries. This is the weak point in my system. I really put the hurt to them.

 I just acquired an industrial battery set that I hope will cure that.

It is 4 GNB Industrial Power 3-100G27 VRLA-AGM Batteries 1460Ah 6 Volts. Actually it is 12 individual 2V batteries. (https://www.solar-electric.com/gnb-industrial-power-3-100g27-1460ah-6v-battery.html) I want to rig them in series to make it 24V. Originally it had solid inter-terminal connectors but they are missing. It has 2 negative and 2 positive terminals per battery. All the photos I have seen show them using all the terminals when connecting them. (i.e. when in series there are two cables to each side of the battery instead of the usual one) Honestly not sure if that is entirely necessary but it is that was from the factory so there is probably a reason.

Question is, are there any suggestions as to an economical option for linking them together with solid connectors? Or, if not, what size cable would be recommended between the batteries. (Almost all will only be 6" long, longest would be 12")

Thanks guys!

Outback FM80
AIMS 6,000W split-phase inverter (24V) 
22 BP 150W 24V Solar panels, (3,300W) (Wired 48V into the Outback controller)

softdown

Four connectors per battery indicates an industrial battery with movement and vibration being accounted for. Those lugs are kind of expensive, they are long and thick from what I have seen. I'm guessing here  - 2/0 copper might work alright for the cabling. 

One would need plentiful data to fine tune your cabling. Even then 10 experts might recommend 6 sizes. 

bigdogeat

Thanks softdown. I kinda expected a variety of opinions. Last time I got beat up pretty good on a variety of "safety concerns" from some experts on my attempt to go off grid. However, they were probably, (totally), right and their points were well taken. They would be happy to know that I now have all the right breakers installed in all the right places, (mostly I think), and all the wiring is now adequate gauge.

That Is why I decided to just ask about the wire size.

I researched all I could about these batteries and you are correct, there was a lot of technical info on stacking based on seismic activity, etc. Probably not an issue here in Missouri. So do you think it is required to link all four per battery terminals or is just one positive and one negative enough?

mcnutt13579

What size are the existing battery cables and how did you come to that size?  Not familiar with AIMS equipment.

I would use solid copper bus bars as shown in the sideways stack picture.  If there are 2 bars per battery then each bar should be 1/2 the ampacity that the battery cable is required to be.  Cross sectional area in free air.

bigdogeat

mcnutt13579 said:

"What size are the existing battery cables and how did you come to that size?"
Not sure how that pertains to my question about the new setup, but I am using 1/0 cables between the Trojan T-105 batteries.

"Not familiar with AIMS equipment."
Does the brand of equipment change the cable size?

"I would use solid copper bus bars as shown in the sideways stack picture. "
According to the specs they original connectors are "Lead-Tin Plated copper intercell connectors". I get that point that that is what I should use, my question was if there was any advice on economically sourcing them. 

"If there are 2 bars per battery then each bar should be 1/2 the ampacity that the battery cable is required to be."
Again, my question is what is that number relate to in physical size of connector or cable?

"Cross sectional area in free air."
No clue what that means...

On that subject, I have read some material about DC current travels around the outside of cable ve AC current going through the middle. Not sure how that applies or if it is true then why would the manufacturer use solid connectors? Just a side note.

I appreciate the comment but it doesn't really answer my questions.

softdown

Everyone else uses two connections per battery - that works fine. There are much better uses of solar money than duplication connections. I have some train batteries - made perfect sense to duplicate connections due to train vibration. 

Safety is a big issue in the solar community and for good reason. I have joked that one best plan on turning their 401K into a 301K if they follow all of the safety precautions.  Many are just common sense. Some seem a bit like money grabs unless the owner is on the wealthy side. 

Cabling and safety equipment costs plenty. A lot of people think cheap panels = cheap energy = no brainer. I would be happy to pay $10,000 for grid power when I build again. No way was I going to pay the $50,000 that I heard quoted at this location. 

BB.

Cross sectional area in free air--Just means that bus bar is in open air and can be cooled by air flow/convection/etc... Large hollow tubs have more surface area and cool better.

DC current flows through the whole cross section of cable/wire/bus bar.

AC current, because of the fluctuating AC current flow, actual forces the current to flow towards the "skin" of the conductor. A hollow tub vs a copper bus bar of the same diameter carry about the same amount of current because the center does not carry AC current. Note that the skin effect is frequency sensitive, and at 60 Hz and below, has almost no relevance on the size of copper conductors we would every use in our systems.

https://en.wikipedia.org/wiki/Skin_effect

-Bill

bigdogeat

11 cables vs 22 cables would save a fair amount of $. Wasn't sure if the four terminals had anything to do with the spreading the load within the cells or something.

mcgivor

@bigdogeat said

"Cross sectional area in free air."
No clue what that means...

On that subject, I have read some material about DC current travels around the outside of cable ve AC current going through the middle. Not sure how that applies or if it is true then why would the manufacturer use solid connectors? Just a side note.
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Free air is a rating of a conductors amperage carrying capacity when not in a raceway "conduit" such as in overhead runs, many tables will have ratings for not more than 3 conductors in a raceway with derating for more than 3. Free air would apply to battery links, so calculate by measuring width by thickness io get cross sectional area, use metric as it's much easier then use a cross reference chart to convert to AWG. So a bar that measures 20 mm × 3mm = 60mm^2 Cross reference  53.5mm^2 is 1/0 so close enough, below are some resources you may find useful to gain some insight.
https://www.calculator.net/voltage-drop-calculator.html
https://www.powerstream.com/Wire_Size.htm
http://wiresizecalculator.net/tables/maxampfa30.htm

AC travels on the outside of conductors especially at higher frequencies, known as skin effect, 60Hz is very low frequency so not really applicable in house wiring, DC travels through the entire cross section.

bigdogeat

So I'm still unclear how big the cables linking batteries in series should be. I don't really know how it is calculated. Is it calculated based on 6,000W that my inverter is capable of? Or is max output? (18,000W)  Is it based on the 240V from the inverter? 6,000W at 240V is only 25 amps. That is like 12AWG. That doesn't seem right.

Is it 6,000W at 24V? That is 250 Amps. 3/0 according to the chart. That sounds right from the battery to the inverter but, I'm just unclear how it is calculated withing a series of batteries...... Is that whole load going from cell to cell? Not really looking for a science lesson or anything but I don't want to fry stuff. 

If it is 2V at 6,000W then that is 3,000 amps, right?... so that probably is not the case...

BB.

Assuming you have one string of batteries to give you 24 volts battery bus (i.e., 12 batteries in series)... Then the maximum current of the AC inverter is the maximum load on the battery bank.

6,000 Watts is "a lot" of power from a 24 volt battery bank... * would suggest around 2,400 to 3,600 Watts is "practicable" from a 24 volt battery bank for a "reasonably" wired system (i.e., not working on a submarine battery bank).

And based on your battery bank of flooded cell lead acid batteries with 24 volt @ 1,460 AH would be (this is based on our rules of thumbs to give a "reliable" power system):

• 1,460 AH * 500 Watts 1/100 AH battery bank capacity (24 volt rule) = 7,300 Watt for AC inverter max (and good rough maximum for charging power too).

Anyway, we use ~11.5 volts as the 50% capacity for a 12 volt FLA battery under load. And 10.5 volts is the "cutoff" voltage for a typical AC inverter (12 volts). For 24 volts, it is 2x that amount (~23.0 volts 50% battery capacity, 21.0 volts for cutoff).

That gives us (at 24 volts) a supportable voltage drop of 2 volts... And I would suggest designing for a 1 volt drop at 6,000 Watts, so that you will have a maximum of 2 volts drop at 12,000 Watts (if you want to support a 12,000 Watt surge load).

Now, we have two calculations to make... First based on allowable voltage drop for your 24 volt battery bus. Shorter cables, can be smaller AWG (and cheaper/easier to work with). Let's assume 4 feet from inverter to battery bank:

• 6,000 Watts * 1/085 AC inverter eff * 1/21.0 cutoff = 336 Amps (24 volt battery bank).

Using a simple voltage drop calculator with 4 feet of cable (one way run for this calculator) and 1 volt maximum drop, we get:
https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=0.8152&voltage=24&phase=dc&noofconductor=1&distance=4&distanceunit=feet&amperes=336&x=64&y=31

Using 4 AWG cable, we get:
Voltage drop: 0.67
Voltage drop percentage: 2.78%
Voltage at the end: 23.33

And we need to look up what size cable will carry 336 Amps safely... There are two major standards that we can start with... NEC (National Electric Code) which is pretty conservative... I suggest a 1.25x NEC derating for inverters and battery chargers (a bit more conservative):

• 336 Amps * 1.25 NEC derating = 420 Amp rated branch circuit and breaker/fuse

Looking up in a (simplified) NEC code table for wiring:
https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

That is an ~400-600 cmil cable (big stuff), or several smaller cables in parallel... Note: you probably will be running the cable in "free air" (not in conduit), which helps keep the cable cooler (better air circulation).

And there is the ABYC (marine) wiring standards:

https://acbsphl.org/Tips_and_hints/ABYC_Wiring.htm

And they suggest around 3/0 or 4/0 cabling would work (note that SAE wire gauges are slightly smaller than AWG wire gauges).

If your AC loads are typically much less than 6,000 Watts (or you never use more than 4,000 Watts, as an example), you can always use the lower current to do your designs. (and why for "very large" systems, going to a 48 VDC battery bank is suggested).

If you are going to use bus bars, then you can figure the cross section area and compare to AWG cross section area.

https://www.rapidtables.com/calc/wire/wire-gauge-chart.html#chart

Does this help?

-Bill

mcnutt13579

To answer your question about my questions.

The battery cable size is sized per the inverter manufacturer's specification.  The intercell connectors need to be the same ampacity as the rest of it.

softdown

My vote for practicality and economy is this:
4/0 from inverter to batteries. Or two separate 2/0 cables - for a total of four cables connecting inverter to batteries. 
2/0 for serial battery cell to battery cell connections.

bigdogeat

Thanks Bill, that helped a bunch! 

I actually used to run all the 120V in my house off of just an 1,800W 24V inverter. Unfortunately, my house was definitely not designed with alternative energy in mind. It has lots of 240V in it. The only thing I really "needed" 240 for was the well. Because of where I live it is pretty deep. 425' That is the reason I went with the 6,000W, 240V split phase inverter. I decided to stick with the 24V inverter because of cost. The inverter was about $1,200 cheaper than the 48V version and I would have had to upgrade my whole battery bank as well. At least $2,000. 

I am like softdown in that I don't have a ton of $ to devote to the system all at once so I have had to make due with what I could afford and it has led me to my current state.

I think this new battery set will make a huge difference by tripling my capacity and get me pretty close to my end state for the system.

Thanks for all the help everyone!

bigdogeat

Probably should have mentioned it earlier, but I have a 250A breaker installed between the current batteries and the inverter and it has never flipped. 

BB.

The deep well pump is one of those "gochas" in solar... May not run more than 1 hour per day, but has high current/power (i.e., need a 2 HP motor to supply the GPM needed and run 40 PSI at the house).

An alternative is to use a much smaller pump to a cistern/holding tank. And use a small RV style (or a bit larger) that simply draws water from the cistern and pumps to 40 PSI (use a pressure tank to buffer, if wanted/needed).

Put in a 1 HP pump in the bottom of the well and run it for multiple hours per day at much lower peak current/wattage. Much easier for you and easier on your solar system.

But--Everything costs money... An alternative is (possibly) a large cistern that can hold (for example) a week of water, run the RV/small pump for pressure, and run the deep well pump on a big old cheap gasoline (or other handy) genset for a couple hours once or twice a week.

Anyway--Just other ways to look at the issues. May not work in your case.

-Bill

mike95490

The water pump was a huge factor for me.   I went with elevated tanks for pressure anytime, and a timer to run the pump only during good daylight hours (I have to manually switch OFF with cloudy weather and remember to turn back on)
But to get enough gallons daily, I needed larger pump, which drove me to larger inverter and larger battery voltage

bigdogeat

BB, i am leaning that way. I have the inverter already and I don't want to mess with the well pump, so I think I am going to get a couple IBC containers for my well pump to empty into and then use a small 110 pump to pressurize my current plumbing. Should be much easier on the deep well pump not having to pressurize the system. A spring project... I've already bought the small pump and found a local guy with food grade IBC containers for $40 each.

Because of the cold weather, (single digits this week), I can't really have a tower outside for pressure so I'm putting the tanks in a storage room in my basement.

littleharbor2

I have a few friends who use these pump with pressure tank units from harbor freight. They seem real happy with them.