Wiring Batteries in Parallel

[Craig]

Guys,

there are a number of articles about how to wire batteries in parallel.

I have 3 house batteries. They have been wired as follows:-

Red lead from shunt to battery 1, the red from Bat 1  to red on battery 2 and red from Bat 2 to red on battery 3. Black from Shunt to Bat 1, Black from Bat 1 to Bat 2 and black from Bat 2 to Bat 3.

I have been advised that it is better to have the red wired as above but the Black from the Shunt to Bat 3, then to Bat 2 then to Bat 1.

I also have a solar charger that I have been advised should be wired with the red to Bat 3 and the Black to Bat 1.

In other words, Bat 1 should have one lead going to the Shunt and the other through the other 2 batteries before connecting to the shunt.

Has anyone any thoughts on this, or is this just over the top thinking about losses between terminals.

Craig
"Shirley Valentine"
Gold Coast
Australia

[Lyra]

Unless I do not understand what you mean by "shunt" something is not clear to me - a shunt is typically a resistor in series between the battery and the loads, so I can not see how it is connected both to the red (I assume you mean "positive") and  black (negative) of the battery bank.

It is true that on multiple banks it is advised to connect the leads to the bank at opposing sides. As battery number increases there are more elaborate connection configurations but for 3 units I think this is good enough and this is how I connect them in my boat.

[tiger79]

I wouldn't worry too much with only 3 batteries; if you had 5 or 6 parallelled it might be different.  With decent connections, typical charging currents and batteries close together, the advantages of wiring to opposite ends of the bank are absolutely minimal.  It's correct in theory, but the practicality is virtually absent.

[JEN-et-ROSS]

Unless I do not understand what you mean by "shunt" something is not clear to me

Surely the "shunt" is the remote sensor linked to an ammeter or battery monitor. We have a similar setup linked to a NASA Battery Monitor (BM1).........Very useful it is too !!...........Bill

[Yngmar]

Without trying to decipher your wiring description (sometimes a sketch is quite handy), all you need to know about battery bank wiring and losses on the cables between them (which can be significant when drawing high loads or during charging) can be found here: http://www.smartgauge.co.uk/batt_con.html

I've found the middle battery in my bank of 5 AGMs had cooked out and died while drawing high loads (big inverter) due to poor choice of bank wiring by the original (professional) installer.

The shunt is typically on the negative (black) battery bank take-off and between it and everything else. Nasa's BM manual has a nice description of this (with sketches).

[Craig]

The article Yngmar refers to is one of the better ones I've seen and shows a substantial difference in the draw between batteries, even with good wiring between terminals.

The resistance between batteries is not the wiring but the internal battery resistance as mentioned in the article.

I've attached a diagram of how I now have the batteries wired, similar to method 2 in Yngmar's article. Prior to this method of wiring, I had Bat 1 connected to the Shunt/Control panel, with Battery 2 connected to Battery 1 and Battery 3 connected to Battery 2. Previously, it was always Battery 1 that failed first. I would have thought that in the wiring shown in my diagram the middle battery would fail last, not first.

I also have the solar panels wired so that the negative wire goes to Battery 3 and the positive to Battery 1. Of course the charging through the alternator or shore power would occur in the opposite direction.

Sorry to confuse people with my terminology. The shunt does measure resistance and thus where the battery monitor is attached to and is where the negative wiring goes before the control panel.

I merely wanted to check how other owners wired their boats to see if the theory in Yngmar's article is what owners see in practice.

Craig
"Shirley Valentine"
Gold Coast
Australia

[Craig]

sorry attachment follows

[tiger79]

The resistance between batteries is not the wiring but the internal battery resistance as mentioned in the article.

Internal battery resistance is irrelevant with batteries wired in parallel.

Also note that the Smartgauge article you mentioned has arithmetical errors in the calculation of link resistance.

[MarkTheBike]

Not that I fully understand the technical side of this topic but I find it utterly fascinating. I've got my 2x140aH AGM batteries disconnected at the moment. Normally, the live goes from bat1 to bat2 and the neutral does the same. Is there any mileage in me doing a quick reorganise and having the live (or neutral) do the opposite, i.e. from bat2 to bat1 (or would it be so little advantage that it's pointless)?

Tiger - genuinely, I don't understand why connecting in parallel renders the internal resistance irrelevant. Is it easy to explain without long mathematical formulae?

[Yngmar]

I merely wanted to check how other owners wired their boats to see if the theory in Yngmar's article is what owners see in practice.

It's definitely what I've seen on mine. My boat had 5x 130 Ah AGMs, wired like Method 1, except with +/- take-offs at the middle battery out of the five. That battery was therefore doing most of the work on its own and one day when I worked on some other wiring in the battery locker, I found it was very warm. Further investigation revealed it wasn't holding charge anymore, so I popped the seals and valves and had a peek inside - several cells were dried out and severely sulphated. The sulphation may be a sign of age (and there are desulphators that can be installed to reduce and/or prevent it), but the fact that they had dried out meant they had been either charged or discharged too rapidly and the pressure relief valves had let the water escape (in the form of hydrogen and oxygen). I've tried refilling them, but the plates had also suffered damage, so the battery was recycled. I've tested the remaining four batteries and they were absolutely fine - having done far less work in the same time.

As the capacity of the remaining four was sufficient with the solar system I'm currently adding, I decided to keep them as it is instead of replacing the entire bank. I changed the wiring to Method 2, which means that the outer two (and so far least hard worked) batteries share the highest load between them and haven't had any problems since (including when using the big inverter). Method 3 would be the ideal, but there wasn't really much room in the locker for an extra set of posts and it would've meant rewiring the entire battery setup. I might do that when the time for replacing the rest of the bank comes, but for now they're fine.

As Chris Gibson writes:

There really is no excuse whatsoever (apart from, perhaps, incompetence or laziness) for using the first example given at the top of this page.

[tiger79]

Not that I fully understand the technical side of this topic but I find it utterly fascinating. I've got my 2x140aH AGM batteries disconnected at the moment. Normally, the live goes from bat1 to bat2 and the neutral does the same. Is there any mileage in me doing a quick reorganise and having the live (or neutral) do the opposite, i.e. from bat2 to bat1 (or would it be so little advantage that it's pointless)?

Tiger - genuinely, I don't understand why connecting in parallel renders the internal resistance irrelevant. Is it easy to explain without long mathematical formulae?

You guessed - minimal difference so pointless.

When connecting batteries in parallel, the potential problem is resistance in the interconnecting cables.  With decent sized cables and properly made terminals, the resistance is absolutely minimal.  If you're parallelling 5 or 6 batteries, there's nothing to lose by connecting to opposite ends of the bank.  Remember that any slight imbalance in the charge/discharge of individual batteries will be evened out in time, because they're all interconnected.

[MarkTheBike]

OK, that's clear. Thanks, Tiger.