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G鈥檇ay guys, I have just put 3x 120ah AGM batteries in my canopy they are all close together. I know the cables linking the batteries should all be the same length. But I鈥檓 unsure on what gauge wire to use with linking them together. Thanks in advance 馃憤
 

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I would think you should use battery cable to link them. Then use something like 8mm to draw power for your application.


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The way I look at it.
What鈥檚 you maximum simultaneous draw?
As really unless your running big inverters. Most of use might hit the 30-50amp range at some stage.

I don鈥檛 see why it needs to be much bigger than that amount of load.


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Having different legth cables is not that big of an issue but keep the runs as short as you can within reason. If the batteries are stacked next to each other, fusing is not necessary, but if the cable is run in a path where there is the possibility for it to be damaged, it should be fused.

As for the size cable, I would go with 4AWG (25mm2) for linking batteries next to each other for a system that is running camping accessories. If you have or intend to install a battery link switch to jump start yourself or add more power when winching, i would be running 0AWG.

Keep in mind that most AGM deep cycle batteries will only supply or absorb around 50A/80A before being damaged. This is why i recommend not to self jump start, rather allow half an hour for current to naturally flow back into the main battery and then start (this reduces peak current draw on the aux battery system).

Lastly, if your AGMs are of different age or number of discharge cycles, you may find that they discharge themselves over time when they are linked up in paralell. This will only be an issue if you have charged the batteries to full, and then leave them for a month or so; when you return you may find them complelty flat. When the time comes, one 120Ah LiFeP04 battery will have the same capacity as three 100Ah AGMs at 1/6th of the weight and size for about the same price.

Cheers.
 

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Google victrons factsheet on battery bank wiring. Suggests for charging that positive goes to first battery, negative to last battery. That is don鈥檛 just connect both to first battery. The different cable lengths make a difference apparently.
I鈥檇 have thought 4awg is overkill unless they are 3 m apart (long cables) and you are pulling large currents.
 

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2018 Mazda BT50 Dual cab
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Google victrons factsheet on battery bank wiring. Suggests for charging that positive goes to first battery, negative to last battery. That is don鈥檛 just connect both to first battery. The different cable lengths make a difference apparently.
I鈥檇 have thought 4awg is overkill unless they are 3 m apart (long cables) and you are pulling large currents.
Electricity is like water and takes the easiest path. Small poor grade wire is like using a small straw to drink from.



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But if you鈥檙e only drinking a small amount over a short distance, small straws are fine, right?!
Yes to a certain degree. But he is talking about 3 batteries together. For example if he put 3 x 120 ah batteries together, that鈥檚 360 ah.
Theoretically they need 10% of their capacity to recharge them adequately so that鈥檚 36 ah.
This is why you need decent cable to handle the load and not heat up. Heat equals resistance and chargers see resistance as load so they will pump extra in causing more heat and possible thermal runaway.
Rule #1. Always size your wires correctly, going over size is better than under size


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nissan
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Having different legth cables is not that big of an issue but keep the runs as short as you can within reason. If the batteries are stacked next to each other, fusing is not necessary, but if the cable is run in a path where there is the possibility for it to be damaged, it should be fused.

As for the size cable, I would go with 4AWG (25mm2) for linking batteries next to each other for a system that is running camping accessories. If you have or intend to install a battery link switch to jump start yourself or add more power when winching, i would be running 0AWG.

Keep in mind that most AGM deep cycle batteries will only supply or absorb around 50A/80A before being damaged. This is why i recommend not to self jump start, rather allow half an hour for current to naturally flow back into the main battery and then start (this reduces peak current draw on the aux battery system).

Lastly, if your AGMs are of different age or number of discharge cycles, you may find that they discharge themselves over time when they are linked up in paralell. This will only be an issue if you have charged the batteries to full, and then leave them for a month or so; when you return you may find them complelty flat. When the time comes, one 120Ah LiFeP04 battery will have the same capacity as three 100Ah AGMs at 1/6th of the weight and size for about the same price.

Cheers.

I think if you pull big amps from three AGM batteries at once, you effectively share the load between them so a big amp load won鈥檛 harm them as much as drawing from just one?
 

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Yes to a certain degree. But he is talking about 3 batteries together. For example if he put 3 x 120 ah batteries together, that鈥檚 360 ah.
Theoretically they need 10% of their capacity to recharge them adequately so that鈥檚 36 ah.
This is why you need decent cable to handle the load and not heat up. Heat equals resistance and chargers see resistance as load so they will pump extra in causing more heat and possible thermal runaway.
Rule #1. Always size your wires correctly, going over size is better than under size


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Both Projecta and Redarc 40amp dcdc chargers recommend 6awg up to 5 metres. Both 25amp dcdc, 8awg up to 5 metres.

I would suggest that 8awg, or if you want, 6awg would be fine over 0.2m, unless of course planning to winch, jumpstart or more likely, use a large inverter for a coffee machine!
 

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I went overboard in my camper using 35 mm square



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I went overboard in my camper using 35 mm square



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Going overboard is a better outcome than undercooking it!
I鈥檝e got 2 runs of 3AWG from my under the bonnet to the cargo area - One from the aux battery to the sockets for fridge and 300w inverter, the other from the main battery to the compressors (2x25amp), which is about right I think.
I鈥檝e probably overcooked wiring to rear work light etc, eg 8awg for a 40w led light!
 
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