MPPT to batteries - pos & neg cables have to be same length?

[basssears]

Have tried searching can't seem to find an answer to this question.


My controller might be moved and might end up much closer to the positive terminal of my three parallel battery bank than the far negative terminal. Do I need to use the same length positive and negative cables (assuming I've already done the correct voltage drop calcs for amperage @ this length etc) or can I have the positive be a shorter cable than the negative to make the install cleaner?


Thanks in advance,


-- Bass

[smac999]

Not going to matter. Just size for total length

[newhaul]

Originally Posted by basssears

Have tried searching can't seem to find an answer to this question.


My controller might be moved and might end up much closer to the positive terminal of my three parallel battery bank than the far negative terminal. Do I need to use the same length positive and negative cables (assuming I've already done the correct voltage drop calcs for amperage @ this length etc) or can I have the positive be a shorter cable than the negative to make the install cleaner?


Thanks in advance,


-- Bass

dc is round trip . The length of each wire makes no difference size wire for total round trip length.

[a64pilot]

Trust me, just go BIG with the wire, you’ll never have a problem then, if you size to just barely work, then as the wire ages and connections oxidize a little etc., you may get some degraded performance, but it will happen so slowly you’ll likely not notice and or assume it doesn’t work as well because the panels are old etc.

[01kiwijohn]

There would have to be an enormous difference to have any effect.

[cjheath]

Ok now i know I'm a long way from the U.S. (Tasmania, Australia - google it!)
but last i checked the fundamental physics of electrical theory still apply; even 'down' here! FYI, i am a licensed electrical technician of 35yrs so I've connected a few wires in my time. The positive and negative cable lengths do not need to be the same length but as mentioned previously bigger cable is far better than smaller cable and most importantly ALL your cable lengths needs to be totaled for your volt-drop calculation. it's a bit like 'how much petrol ( sorry... 'Gas') do i use in my car if i drive to the supermarket to get some milk?' all the way there and all the way back! NOT 'as the crow flies' but the actual distance covered in your car even all the laps you do around the car park trying to find some where to park!


Another very important issue is that if you have parallel batteries (and you've said that you have) you need to have the same cable route length to and from each parallel leg, i.e., each battery, other wise you may encourage uneven charging of each battery due to uneven volt drops in the individual part of the battery circuit. This is why often in a well designed parallel battery system you will see each of the batteries with their own dedicated positive and negative cables of the same length, say for example 600mm (oh, sorry.... 2 feet) going to a common connection point, typically a short piece of heavy copper busbar with the charge/load cable connected there smack bang in the centre of the bar if possible so that each battery 'sees' the same cable length. FYI the Victron website has some very valuable information on all of this and a great downloadable document full of great battery facts and figures. it's not a sales pitch but a very useful text. NOTE: I have no connection financial or other wise with Victron other than to say that i have used their products recently for a customer and will continue to do so because i think they're very good and access to online information is great.



good luck, Chris.

[john61ct]

Intra-bank wiring is a completely separate topic from the OP question, which has been truly answered.

The canonical text for such in a parallel context http://www.smartgauge.co.uk/batt_con.html