My first off-grid system design, how am I doing?

[TheWanderingJackalopes]

I have 3 questions...

In the process of determining my wire sizes and lengths for wiring my batteries series-parallel, I realized that there are more gaps in my understanding. I have an enclosed space for my LiFePo4 batteries and I want to keep them as close together as possible. They will be laying on their side with the terminals oriented at the top per Valence install specs. As U27 batteries, the cases are 12" on their longest dimension.

I see that it is possible to buy 12" cables, but will they fit given the battery positions I am showing? Or will 12" cables not provide enough "wiggle room"?

I know the wire lengths and sizes need to be identical for all battery to battery connections. Does this rule also apply elsewhere? Can I have heavier gauge wires going from the battery bank to the charge controller?

The wires making my serial connection (+ to -) need to make a 180° turn at the middle. Lighter gauge wire will, of course, bend easier than heavier gauge wire. Given the size of the battery, the configuration, and the MPPT and Inverter, what would be the lightest battery to battery wire I would need and still be safe?

 

[acdoctor]

I calculate 150 amps max. Southwire size calculation shows 3/0 awg.
I would not think 12 inch cables would reach. Something more like 16 to get around the negative terminal in the way.

 

[TheWanderingJackalopes]

I calculate 150 amps max. Southwire size calculation shows 3/0 awg.
I would not think 12 inch cables would reach. Something more like 16 to get around the negative terminal in the way.

Can I ask how you arrive at 150 amps? I am trying to understand all of this better. It seems like ampacity charts and calculators are all over the map. I've seen 4 gauge to 3/0 gauge for the same application which only makes me dig deeper.

3/0 seems like overkill for the battery to battery. My Inverter/Charger only accepts up to 1/0 wire (unless I modify the case) and if I recall correctly, they specified 4 gauge in the installation manual.

 

[acdoctor]

3000 watts inverter out put x 1.20 for conversion inefficiencies is 3600 watts / 24 battery volts is 150 battery amps. Then I use southwire’s wire size calculator ( they have an app ) any length from 1 foot to 20 feet shows 3/0.
Even at 100% efficiency you are looking at 125 amps.

My 3000 watt growatt I am using 130 amp battery breaker ( not tripped so far ) and 2/0 welding cable. Which is a struggle to get into the growatt by the way.

 

[TheWanderingJackalopes]

Great! Thank you! I didn't know about the multiplier for conversion efficiencies so I couldn't understand how you were arriving at 150 amps.

I will be using the GroWatt 24v/3k inverter as well. I think Southwire's calculator is the most conservative of all of the calculators and charts I have looked at. They might lean toward more conservative calculations because they want to sell more wire, but having been in the marketing industry, I'm a little bit cynical.

I'm going to get my hands on some 2/0 wire and see if it will make the bend I need. That's the second most conservative number I came up with.

 

[acdoctor]

I have 2/0 jumpers on my home system that uses 8 inches of welding cable.
They are bent 180 deg. And terminals are about 120 rotated from one another.
The first terminal crimp is easy. The second prebend making sure of terminal orientation the cut to length then orientate the second terminal and crimp.
They will only unbend about half way. So if you made them straight you may be able to bend 90 deg but no more and would stress battery terminals.

 

[TheWanderingJackalopes]

I have 2/0 jumpers on my home system that uses 8 inches of welding cable.
They are bent 180 deg. And terminals are about 120 rotated from one another.
The first terminal crimp is easy. The second prebend making sure of terminal orientation the cut to length then orientate the second terminal and crimp.
They will only unbend about half way. So if you made them straight you may be able to bend 90 deg but no more and would stress battery terminals.

I did get a chance to check some 2/0 wire multi strand today, at a 180° bend it required a minimum of 5 inches of space between my rows of batteries. I could work with that.