Wiring for batteries (storing leccy from solar panels)

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Wiring for batteries (storing leccy from solar panels)

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OtleyLad

Joined: 13 Jan 2007
Posts: 2737
Location: Otley, West Yorkshire

Posted: Thu Aug 01, 13 12:18 pm Post subject: Wiring for batteries (storing leccy from solar panels)

I currently have 2 x 12v x 110ahr batteries linked in series (I wanted the 24v as input to the inverter).
When I get hold of some money I will get another 2 of the same batteries.
So the general idea is to wire the 2 new ones in series and then wire both pairs in parallel, so maintaining the 24v.

Here the planned wiring:

Does this look right?
Is there a better way (given that the inverter needs 24v input).

Just to add that all cables are 35mm csa rated at 240amps.

dpack

Joined: 02 Jul 2005
Posts: 45521
Location: yes

Posted: Thu Aug 01, 13 1:24 pm Post subject:
diodes?

OtleyLad

Joined: 13 Jan 2007
Posts: 2737
Location: Otley, West Yorkshire

Posted: Thu Aug 01, 13 3:32 pm Post subject:

dpack wrote:

diodes?

Where would you put them?(without being rude)

dpack

Joined: 02 Jul 2005
Posts: 45521
Location: yes

Posted: Thu Aug 01, 13 4:23 pm Post subject:

between the panels and batteries in case the panel voltage is lower than the battery voltage,(in the dark for instance),which is a bad thing cos back current cooks the panels

many panels have them built in but tis worth checking

iirc richard is the chap for good advice re pv rigs,i know a bit about small kit but anything beyond running a mobile/ led lights/ electric fence is a known unknown to me .

i found a fully charged 120 a/h through an inverter ran my lappy for about 7 hrs

RichardW

Joined: 24 Aug 2006
Posts: 8443
Location: Llyn Peninsular North Wales

Posted: Thu Aug 01, 13 10:53 pm Post subject:

Forget the diodes as you have a controller that should handle back feeding.

I would change your wiring slightly.

You have the controller & the inverter connected at different points. That is wrong.

1, All connections to the battery bank should be at the same + & - points.

2, You have a balanced - feed to the inverter yet have the + feed from one battery terminal (and the other but by an extra wire). See point 1 above.

So either make a + & - busbar & connect each 24v battery to it & then also connect the controller & the inverter to the busbar

or

connect each batteries + terminals together then each bats - terminals together, then connect the + for the controller & inverter to bat ones + terminal, then connect the - for the controller & inverter to bat two's - terminal. That way the current flow has the same number of wires & connections to all batteries. Try to keep all wires as short as possible & matched in length for the + & -

Oh & do not go by the rated amp carrying capacity of 12v cable (in this instance). You need to be more concerned with voltage drop. Also do not use the standard 3 to 6% allowance that you would use for mains voltages. Think sub 1%.

A very good quick method to work out low voltage dc cable size is:-

peak amps x total cable distance both ways in meters / 12 (I use 10 as its easy to do in your head & gives even lower V drop).

So for example an inverter that needed 100amps & was 2 meters from the busbar would need:-

100 X 4 (round trip remember) / 12 = 33mm2 CSA cable not 100amp cable

RichardW

Joined: 24 Aug 2006
Posts: 8443
Location: Llyn Peninsular North Wales

Posted: Thu Aug 01, 13 10:55 pm Post subject:

dpack wrote:

many panels have them built in but tis worth checking

The diodes fitted to panels are not back feed diodes but bypass diodes to help minimise shading losses.

dpack

Joined: 02 Jul 2005
Posts: 45521
Location: yes

Posted: Thu Aug 01, 13 11:38 pm Post subject:
i learn more ,thanks

OtleyLad

Joined: 13 Jan 2007
Posts: 2737
Location: Otley, West Yorkshire

Posted: Fri Aug 02, 13 7:08 am Post subject:
Thanks for those ideas. I guess what you're saying is that A will feed the batteries unequally and B will feed them equally (assuming equal cable lengths)? Here's another plan with 2 bus bars - Blue is for +ve and Green -ve: None of the wires are long - all less than 1 metre. Is it getting better?

RichardW

Joined: 24 Aug 2006
Posts: 8443
Location: Llyn Peninsular North Wales

Posted: Fri Aug 02, 13 2:23 pm Post subject:

Yes in A bat one will supply 66% of the power & get 66% of the charge if the - is also wired the same way to the same bat.

Just by changing the - to the same but on the other bat will balance two bats. It gets more complex when you have more bats & try to never have more than 8 in parallel at any time. If you need more storage get bigger bats.

B is ideal.

You can save on two wires by moving the multi connection point / busbar + to bat one terminal (as in "A") & the - to bat two terminal & still be 99.9% ideal.

What size inverter is it?
Does it also charge?
How many amps is the controller?

OtleyLad

Joined: 13 Jan 2007
Posts: 2737
Location: Otley, West Yorkshire

Posted: Fri Aug 02, 13 2:38 pm Post subject:
Here's the inverter. 1500w (3000w peak). I didn't get the hairdryer with it though No it doesn't charge. The charger is here and it's up to 60amp.

OtleyLad

Joined: 13 Jan 2007
Posts: 2737
Location: Otley, West Yorkshire

Posted: Fri Aug 02, 13 2:50 pm Post subject:
Is this what you mean - without busbars:

RichardW

Joined: 24 Aug 2006
Posts: 8443
Location: Llyn Peninsular North Wales

Posted: Fri Aug 02, 13 4:14 pm Post subject:
Yep thats it. You can make one more change if the layout allows it. Wire the solar charge controller to the inverter terminals. Keep the mains charger going direct to the bats. At your 1m max cable length your 35mm2 cable is fine for the inverter. Last edited by RichardW on Fri Aug 02, 13 4:35 pm; edited 1 time in total

RichardW

Joined: 24 Aug 2006
Posts: 8443
Location: Llyn Peninsular North Wales

Posted: Fri Aug 02, 13 4:28 pm Post subject:
Now you just need to look at DC fusing, N & E bonding & RCD's.

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