What size battery cables?

[Flagbag]

With all the voltage drop calculators and discussions of wire size in these great forums I still can't figure-out how to 'figure-out' what size cables should be used in the battery bank. I have twelve 6V golf-cart batteries, in series/parallel for a 12V system (diagram and further info below).

What size cable to go from one battery terminal to another? The average length of cable from one terminal to the next is about 13inches. The length of cable from the CC to the batteries is 70inches. The cables from the batteries to thew inverter is 30inches.

Any guidance appreciated.

Batteries: 12 Energizer GC2 107 (208Ah each) 6V*208Ah*12V=14.9kWh
Panels: Imp 45A (Isc 49A) 12V
Charge Controller: Xantrex C60 Charge Controller
Inverter: Xantrex Freedom 458 Inverter
Inverter Mode:
Input 12V DC 200 Amp at rated output
Output 120 VAC 60Hz 2000 VA continuous
Load is 3500KW/Day general household including 10cf refrigerator, lighting, tv, etc.

[solar pete]

Howdy Flagbag, I am no battery expert, but Igota say I dont understand why you are running a 12v system with those specs.

[Logan005]

You have the batteries needed to run a 48 volt system, why are you considering running a 12 volt system?

[Logan005]

I would configure those batteries as, 8 in series for a 48 volt bank to run an inverter from, and another 4 in series for a 24 volt system to power a 24 volt chest type refrigerator. The less you actually need the inverter the better. a 12 volt inverter is a dangerous toy.

[Mike90250]

In my .sig is a link to a spreadsheet with a great cable length sizer.
BUT, first you determine what amps are going through the cable, and you use a Wire Gauge Look-Up chart, to see what the smallest wire is that can be used safely for Power Transmission. http://www.powerstream.com/Wire_Size.htm
200A needs at least OOO (triple ought cable, 0.4096" di.) for safety, the spreadsheet will tell you what gauge you need to keep voltage loss below 3% for the length your run is,
Good luck, you are also going to need a large hydraulic cable crimper to put lugs on that cable

[Flagbag]

You have the batteries needed to run a 48 volt system, why are you considering running a 12 volt system?

It is an existing system that has been operating for years. I am trying to just improve it and make it safe with a limited budget.

[Flagbag]

Howdy Flagbag, I am no battery expert, but Igota say I dont understand why you are running a 12v system with those specs.

Solar Pete. I seriously considered changing to a 24V system... please see and PLEASE HELP my topic (https://www.solarpaneltalk.com/forum...battery-system).

[Flagbag]

I would configure those batteries as, 8 in series for a 48 volt bank to run an inverter from, and another 4 in series for a 24 volt system to power a 24 volt chest type refrigerator. The less you actually need the inverter the better. a 12 volt inverter is a dangerous toy.

I would like to learn more about what you mean about a 12V inverter is dangerous. It is in this system here built by my late step-father. Could you, or someone, please elaborate?

[Flagbag]

In my .sig is a link to a spreadsheet with a great cable length sizer.
BUT, first you determine what amps are going through the cable, and you use a Wire Gauge Look-Up chart, to see what the smallest wire is that can be used safely for Power Transmission. http://www.powerstream.com/Wire_Size.htm
200A needs at least OOO (triple ought cable, 0.4096" di.) for safety, the spreadsheet will tell you what gauge you need to keep voltage loss below 3% for the length your run is,
Good luck, you are also going to need a large hydraulic cable crimper to put lugs on that cable

Thank-you Mike90250... I see the calculators that you mention but don't know how many amps ARE going through the cables in this system. The inverter has a max of 200A, do I use that number, or??

[Mike90250]

OK, I looked at your other thread, and the panels you listed, add up to a bit over 40A So with a PWM controller at 12V, a 400ah battery is about the normal limit.

.....

	Pm	Vmp	Voc	Imp	Isc
Panel #1	100W	17.5V	21V	5.71A	6.4A
Panel #2	100W	17.5V	21V	5.71A	6.4A
Panel #3	205W	18.4V	22.8	11.15A	12.1A
Panel #4	205W	18.4V	22.8	11.15A	12.1A
Panel #5	205W	18.4V	22.8	11.15A	12.1A

The issue with 12V systems, once they grow just a little bit, is the current density in the conductors. It takes a lot of copper and good cable terminations to prevent heating and meltdown in the wiring, And some of the 12V inverters are not well made, and at high power levels, they fail or sometimes have burned,

The easy solution is see, with the panels you have, is a new 24V inverter, and rewire some panels. The C60 is PWM charge controller, well built and rugged. It can manage a 24V system.
The panel rewire would be
Parallel the two 100W panels to effectively make them into a 200W panel. This would closely match the other 3 205w panels. You now have effectively, 4 - 200W panels (give or take)
Wire 2 in series, and then the other 2 in series. Then take and parallel the 2 series strings:
-100+
-100+ making this into a 200W panel then:

-200+ -205+
-205+ -205+

2 panels in series and then parallel. The panel volts and amps are all pretty close, and will not have much loss, You now have 20A @ 24V for charging a 200ah 24V bank, which is simply 4 golf cart batteries in series. (6V, 200ah each golf cart battery) upgrade to a MPPT controller like the Midnight KID and you can get a couple more amps out of the PV array.
Expect 2-4 years life on the golf cart battery, unless you are really loading them hard. Adding more batteries will not properly charge them, and their life will still suffer.

[Flagbag]

OK, I looked at your other thread, and the panels you listed, add up to a bit over 40A So with a PWM controller at 12V, a 400ah battery is about the normal limit.


The issue with 12V systems, once they grow just a little bit, is the current density in the conductors. It takes a lot of copper and good cable terminations to prevent heating and meltdown in the wiring, And some of the 12V inverters are not well made, and at high power levels, they fail or sometimes have burned,

The easy solution is see, with the panels you have, is a new 24V inverter, and rewire some panels. The C60 is PWM charge controller, well built and rugged. It can manage a 24V system.
The panel rewire would be
Parallel the two 100W panels to effectively make them into a 200W panel. This would closely match the other 3 205w panels. You now have effectively, 4 - 200W panels (give or take)
Wire 2 in series, and then the other 2 in series. Then take and parallel the 2 series strings:
-100+
-100+ making this into a 200W panel then:

-200+ -205+
-205+ -205+

2 panels in series and then parallel. The panel volts and amps are all pretty close, and will not have much loss, You now have 20A @ 24V for charging a 200ah 24V bank, which is simply 4 golf cart batteries in series. (6V, 200ah each golf cart battery) upgrade to a MPPT controller like the Midnight KID and you can get a couple more amps out of the PV array.
Expect 2-4 years life on the golf cart battery, unless you are really loading them hard. Adding more batteries will not properly charge them, and their life will still suffer.

Mike--I really appreciate your time and analysis of this syestemm!
Now wondering won't I loose too much storage capacity? I mean, now we have twelve 6V Golf cart batteries if we drop to just four that is only one third of our current storage? We probably use 3-4kWh/day...?

[inetdog]

Mike--I really appreciate your time and analysis of this syestemm!
Now wondering won't I loose too much storage capacity? I mean, now we have twelve 6V Golf cart batteries if we drop to just four that is only one third of our current storage? We probably use 3-4kWh/day...?

It does not help to have a very large battery bank if you do not have the panels needed to charge it.

[Mike90250]

Mike--I really appreciate your time and analysis of this syestemm!
Now wondering won't I loose too much storage capacity? I mean, now we have twelve 6V Golf cart batteries if we drop to just four that is only one third of our current storage? We probably use 3-4kWh/day...?

And that's why you have 12 dieing batteries. To recharge / harvest that much in winter, with only 3 or 4 good sun hours, you should have 3x as much solar panels as you have. There is no way you keep that many batteries healthy with that small of a PV system. You could run a small generator for an hour every other day in winter (got to have one anyway for the cloudy days anyway) and that would keep overloaded batteries charged a bit better. You just can't use more power than your harvest, or you never get the batteries charged fully in the short winter months. Even in summer with 5 hours of good sun (usable solar hours, not dawn to dusk), your current array is undersized,
I think you need to take a close look at your loads and maybe measure some of them with a kill-a-watt meter (about $20 on amazon) to see what you need to design for. Just "throwing" batteries into the system is not going to help, except the fellow selling the batteries to you,

[Sunking]

It is an existing system that has been operating for years. I am trying to just improve it and make it safe with a limited budget.

12 volts is anything but safe or inexpensive.