2 pairs of 6v Batteries - wiring best practice?

Tim Good

Tim Good

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Is there a best way to wire two pairs of 6v Trojan T-105 batteries? See my diagram below. The layout in my battery boxes is the same as the diagram.

IMG_8893.JPG

1) my existing 12v batteries.

2) proposal 1 for new Trojans

3) proposal 2 for new Torjans

Does it actually matter how long each cable length is providing they are wired series pairs in parallel?
 
None of the above!

Put two batteries in series. Call them Bank 1 and Bank 2,

Try to get the same cable length in each circuit so run the negative to bank one and the positive to bank 2.
Connect negative B1 to neg B2 and positive B2 to positive B1.

That will give the same resistance in each leg.

Tony
 
Jabs said:
None of the above!

Put two batteries in series. Call them Bank 1 and Bank 2,

Try to get the same cable length in each circuit so run the negative to bank one and the positive to bank 2.
Connect negative B1 to neg B2 and positive B2 to positive B1.

That will give the same resistance in each leg.

Tony
Click to expand...

Very difficult to get the same cable length as I have two battery boxes with two batteries in each.
 
northcave said:
That's the same as my Version 3 apart from cable lengths.
Click to expand...

Trying to get fairly equal cable lengths will help to ensure that all the batteries get charged with a similar voltage, which is why Trojan recommend that method. In practice, as long as you use fairly meaty cable, it won't matter too much.
 
Provided that most of the heavy loads go out on the same side as the charging comes in, and you have adequate cables, I can't see it making a big difference.
You might start to get some odd effects if you drew current for an inverter from the port bank, while charging on the stbd side, on a habitual basis.
 
As mentioned in the previous thread, I decided to connect each pair in series and then connect the two pairs in parallel. Advantage is that if one battery goes down then that pair can just be isolated by removing the link or, it could even be a switched link for belt and braces. I haven't so far done anything about trimming battery or solar cables to balance lengths and notice the BM1s (still have both connected) sometimes show a discharge or charge difference of 1-200ma.
 
Ok so going by the Trojan diagram this is the only way to do it whilst trying to keep cables of a similar length in two seperare battery boxes. Unforuantely will mean make more holes in the boxes but hey ho.

IMG_8897.JPG
 
northcave said:
Ok so going by the Trojan diagram this is the only way to do it whilst trying to keep cables of a similar length in two seperare battery boxes. Unforuantely will mean make more holes in the boxes but hey ho.

View attachment 69923
Click to expand...

No Your scheme 3 is almost correct. The short links between the two batteries in each pair are fine. Just make them the same length as each other

Make the other two links the same as each other too

But move the negative connection to the opposite pair to the +ve and yu will have what Trojan, jabs and pvb suggest

Northcave batteries.png

BTW your existing wiring is incorrect because you have both positive and negative connections on the same battery.
 
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VicS said:
No Your scheme 3 is almost correct. The short links between the two batteries in each pair are fine. Just make them the same length as each other

Make the other two links the same as each other too

But move the negative connection to the opposite pair to the +ve and yu will have what Trojan and pvb

BTW your existing wiring is incorrect because you have both positive and negative connections on the same battery.
Click to expand...

Ok how about this.

Both the same but V2 maybe simpler to wire?

IMG_8903.JPG
 
VicS said:
Yes they are Ok. V2 is the same as my edited version of your original scheme 3 anyway.

V1 is Ok provided the + to + link is the same length as the - to - link and the two + to - links are the same as each other
Click to expand...

Ok thanks team!!! V2 it is then.
 
ghostlymoron said:
My pal had no room for two Trojans next to each other so the leads to one are a couple of metres longer than the other. Will these lead to problems?
Click to expand...

Not if they're fairly chunky cables. The problem is obviously worse during high current charging, but the volt drop in the cable reduces as the charging current reduces, so as the batteries approach full charge the potential cable effect is virtually nothing.
 
pvb said:
Not if they're fairly chunky cables. The problem is obviously worse during high current charging, but the volt drop in the cable reduces as the charging current reduces, so as the batteries approach full charge the potential cable effect is virtually nothing.
Click to expand...

What sort of cable thickness is ideal?

Just got back from the battery place with new cables for V2 diagram as discussed but they are a gauge down from what was already in the boat. The new ones are 40mm2. I asked him to over spec it but I'm thinking maybe it should be up a grade now?
 
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