MPPT to Battery cable size

Boris7

Boris7

Full Member
Posts
4,179
Likes
12,471
I’m fitting a Victron 75/10 MPPT that will draw power from a 125w panel.

This will connect to an EcoFlow and to the vehicle starter battery, the main aim is maintain the starter battery during long periods of non use.

What sized cable should I use between the victron and the battery?

The run is short (1m max)
 
Nabsim said:
Solar panels usually come with 4mm or 6mm cable on them, I haven't seen bigger than that but that doesn't mean there aren't any. 6mm is overkill but will certainly be enough
Click to expand...

I gather the important bit is the bit from the controller to the battery which is the question here. Thats where it should be thicker especially if its a fair distance (more than a couple of feet). Least thats what Wizard Merlin taught me. :)
 
barryd said:
I gather the important bit is the bit from the controller to the battery which is the question here. Thats where it should be thicker especially if its a fair distance (more than a couple of feet). Least thats what Wizard Merlin taught me. :)
Click to expand...
Do you think there is a massive increase in power after the controller? You would need a very long run of cable before 6mm isn't enough of a 125W panel :)

Edit: Bit of reading for you Barry

joca-cable.com

Choosing the Right Solar Cable: 4mm or 6mm for Your Needs

Discover how to choose the right solar cable for your installation. Learn when to use 4mm vs. 6mm solar cable based on current carrying capacity and amp requirements.
joca-cable.com joca-cable.com
 
Cabling from the solar panel is already in place, tiniernet tells me I should use awg8, which I already have what’s making me think is this
1749323910550.png
 
Personally with lead acid I think the voltage drop at maximum current should ideally be no more than 50mv, it's not so critical with lifepo4 because they are less fussy WRT charge voltage, so with lithium up to 100mv.
A 125w panel will theoretically give a max charge current of around 8.5A but in reality will probably be around 7A.
With a 2m run of 4mm cable (1m there and 1m back) you'll get 65mv drop at 7A, OK for LFP but if you can get 6mm into the terminals I'd use that, especially if I were charging Lead acid.
There's a useful voltage drop calculator here

Voltage Drop Calculator | 12 Volt Planet Cable Calculator

Use our Voltage Drop Calculator to select the correct Cable for your campervan, motorhome or boat wiring application. Cable Calculator for Auto and Marine Electrical Components from 12 Volt Planet - Cable Size Calculator
www.12voltplanet.co.uk
 
merl said:
Personally with lead acid I think the voltage drop at maximum current should ideally be no more than 50mv, it's not so critical with lifepo4 because they are less fussy WRT charge voltage, so with lithium up to 100mv.
A 125w panel will theoretically give a max charge current of around 8.5A but in reality will probably be around 7A.
With a 2m run of 4mm cable (1m there and 1m back) you'll get 65mv drop at 7A, OK for LFP but if you can get 6mm into the terminals I'd use that, especially if I were charging Lead acid.
There's a useful voltage drop calculator here

Voltage Drop Calculator | 12 Volt Planet Cable Calculator

Use our Voltage Drop Calculator to select the correct Cable for your campervan, motorhome or boat wiring application. Cable Calculator for Auto and Marine Electrical Components from 12 Volt Planet - Cable Size Calculator
www.12voltplanet.co.uk
Click to expand...
Thank you
 
Nabsim said:
Do you think there is a massive increase in power after the controller? You would need a very long run of cable before 6mm isn't enough of a 125W panel :)

Edit: Bit of reading for you Barry

joca-cable.com

Choosing the Right Solar Cable: 4mm or 6mm for Your Needs

Discover how to choose the right solar cable for your installation. Learn when to use 4mm vs. 6mm solar cable based on current carrying capacity and amp requirements.
joca-cable.com joca-cable.com
Click to expand...
It's not really about power loss Neil it's about communication. If the charger (be that solar or whatever) doesn't 'see' an accurate representation of the actual battery voltage at it's terminals then it'll stop charging sooner than it should. This slows down charging time.
We've all seen the slight differences in bulk/ absorbtion voltages on gel, flooded and AGM batteries, sometimes varying by just 100mv. It's completely pointless trying to be that accurate if you've already got say 300mv of error between battery and charger!
On the charger INPUT side though you're dead right, it's all about power loss. When I did the calcs for Robs van he had over 800W of solar and the cable runs were very.long, probably over 5m, 6mm was more than good enough,
 
Last edited:
You could have fitted a votronic mppt which charges the starter batt, or run a thick wire to a switch/relay from hab batts to starter.
 
You must log in or register to reply here.

Users who viewed this discussion (Total:45)

Back
Top