Battery Connection

[Fred Mc]

In my bus I have 3 group 31 batteries connected in parallel for start batteries.
Both the positive and negative cables come off one of the batteries with the other two batteries connected in parallel to the first one. Is this OK or should the positive come off the first battery and the negative off the last?

Thanks

Fred Mc.

[Jeremy]

Electrically it can make no difference, assuming the resistance of the leads connecting the batteries together is negligible - which it will be if the cables are thick and short

Jeremy

[Sean]

Actually, it makes a huge difference.  Parallel batteries should always be connected "on the diagonal" as you propose, and all the jumpers should be as close to identical as possible.  This will keep the usage profile of all the batteries more or less the same.  Connected on one end, as you have it now, will cause differential usage and will lead to premature battery failure.

We've discussed this subject several times in the past, in great detail and with links to supporting information.  Check the archives.

-Sean
http://OurOdyssey.BlogSpot.com

[Jeremy]

Hmm...Ok...I have perhaps misunderstood the question then, because I see no possibility of a 'huge difference' in the set up I am envisioning. I did have a quick scan through the 17 posts that come up on a search of the archives on "parallel battery connection", but could see nothing about this - but by all means point me in the right direction if you wish, as I'd like to understand the logic here - as I expect would all the other people who must be scratching their heads about now.

Jeremy

[Sean]

Sure -- here is one such thread:
http://www.busconversions.com/bbs/index.php?topic=15592

The link at the bottom explains the logic, although I have since determined that he makes at least one error, as I described in this thread on the other board:
http://www.busnut.com/bbs/messages/11/41241.html

-Sean
http://OurOdyssey.BlogSpot.com

[Jeremy]

Ok, interesting:- so to summarise the explaination in my own language (and correct me if I'm wrong), the issue is that that on those occasions when very heavy currents are being pulled from the house batteries, the negligible resistance of the cables inter-connecting the batteries ceases to be negligible, as very small voltage differences between the individual batteries will lead to significant differences in the current being drawn from them.

So I've learnt something new today. When I was taught about electrical circuits it was all 1.5v batteries and little light bulbs. No one said anything about how sucking 100amps might change things...


Jeremy

PS. I never go to the BNO board, where the discussion took place about the 'Smartguage' explanation. I see someone there is converting a British double-decker. Cool.

[Sean]

Jeremy, that is a good summary, but also remember that it is not just the resistance of the cables themselves but also the additional resistance imposed by the mechanical connections at each battery post.

-Sean
http://OurOdyssey.BlogSpot.com

[belfert]

For my house batteries I used ground and positive cables all the same length.  I ran them all to three good quality binding posts where I then connected the cables running to the fuse panels.  (I used three posts as I have a center tap for 12 volt with a Vanner.)

I can't say the same for my chassis batteries.  Dina used cables of all different lengths.  I think some of the interconnecting cables aren't even rated high enough for the load.  I have replaced one of the cables, but not all yet.  They used 2/0 cables for the ground and positive, but then used like #2 cables to tie the batteries together.

[Tony LEE]

For start applications where heavy draw is only going to last a couple of seconds, I can't see that not diagonally-connecting them is going to do much more than slightly reduce the total CCA because the second and third batteries are delivering through slightly more resistance. Could make enough difference in marginal situation but not smart to be in that situation anyway.
As for leaving the three batteries unbalanced - yes, obviously a theoretical consideration but aren't they going to equalise themselves within minutes as the third and then second batteries - being more fully charged - deliver charge to the least charged batteries. Assisting this will be the fact that the alternator will be delivering charge to the battery with the least resistance - the first battery. As charging rates drop towards zero, lead resistance becomes a no event so batteries end up near enough to equally charged.

Diagonal connection may actually involve a longer common lead and therefore a higher resistance for all three batteries which could well result in a lower CCA than if they were not diagonally-connected.

Like a lot of things to do with RV electrics, a theoretical "problem" rather than one that matters much in the real world.

[boogiethecat]

Well all I can say is that there are "good" practices and there are poor ones.  The diagonal setup is the "good" one and if you have the opportunity to connect things, why not just get in the habit of doing it correctly.  For instance if you're talking about house batteries and an inverter, the diagonal approach is the only way to do it properly or you will kill batteries just as Sean predicts.  
For start batteries, it's probably ok to hook them any way you wish, but "probably" is the best you can ask for.  As in many things on this board, "your way" may just work fine for you but it won't work for everyone.  To do it "right" do it diagonally.  That'll work perfectly for everyone, every situation, every time!!

[Sean]

For start applications where heavy draw is only going to last a couple of seconds, I can't see that not diagonally-connecting them is going to do much more than slightly reduce the total CCA because the second and third batteries are delivering through slightly more resistance.

This is not correct.  For one thing, "CCA" is a specification of the capacity of the battery, and has nothing to do with length of leads, etc., so a battery with 1100 CCA has 1100 CCA no matter how it is connected.  But more importantly, whether you connect on the diagonal or on the end, there are still the same number of interconnecting links and thus the amount of deliverable power is exactly the same either way.  That's not where the problem lies.

As for leaving the three batteries unbalanced - yes, obviously a theoretical consideration but aren't they going to equalise themselves within minutes as the third and then second batteries - being more fully charged - deliver charge to the least charged batteries.

Yes, they will equalize over time.  Again, that's not at all the point.  The point is that in between the periods of equal-charge state, the "close" battery will experience both more discharge as well as greater charge.  This is known as cycle depth -- the close battery will have a greater cycle depth than the far battery.  The more batteries you have, the worse this becomes, to the point where the farthest battery may experience almost no cycle depth at all.  Eventually this will have the effect of reducing the capacity of the entire bank; there are many resources on the 'net where you can read the technical details of why this is the case.

Assisting this will be the fact that the alternator will be delivering charge to the battery with the least resistance - the first battery. As charging rates drop towards zero, lead resistance becomes a no event so batteries end up near enough to equally charged.

"Assisting" is one word to use.  "Exacerbating" is another -- the greater charge rate at the close battery (and correspondingly the lesser rate at the far one) is exactly part of the problem.

Diagonal connection may actually involve a longer common lead and therefore a higher resistance for all three batteries which could well result in a lower CCA than if they were not diagonally-connected.

Here again, this has nothing to do with CCA.  Perhaps you meant to write that total deliverable power might be lower if one of the main leads had to be slightly longer.  This would be true, however in practice longer leads are seldom needed, and if they are, simply using a larger wire size can negate this entirely, whereas the resistance of the connection points can not be practically reduced.

Like a lot of things to do with RV electrics, a theoretical "problem" rather than one that matters much in the real world.

I beg to differ, there is nothing "theoretical" about this at all -- I can hook a meter up and show you the difference in real-time, which makes it experimentally verifiable.  Now whether or not it makes a great deal of difference to your wallet in the "real world" depends on many factors, such as how often you use your coach, how many start cycles per day, etc. etc..   It may very well be the case that for many people, their start batteries will die from neglect, lack of use, abuse, or simply expired shelf life long before the differential usage profile can make a difference.  But for those of us who full-time, start and stop our coaches daily or more, and nurse half a decade or more from a set of start batteries, this issue can make a real difference in bank longevity.

As Gary wrote, there is a good practice and a poor practice here, and since neither I nor you can know the details of how anyone reading this thread later will manage their batteries, we need to steer the path of recommending the good practice.  Dismissing it out of hand as having no practical value is, IMO, irresponsible.

FWIW.

-Sean
http://OurOdyssey.BlogSpot.com

[gus]

Most of this stuff is fine under lab conditions but in real life it is just theory.

The uncontrollable variable here is that no two batteries are the same. It is common to buy two or more identical batteries at the same time and have one last one year and another last five years no matter how they are hooked up.

I own 18 vehicles with batteries, five of them with multiple batteries, and it happens all the time.

The bottom line to me is it makes no difference which terminal pos and neg posts are used. I use whatever is easiest mechanically to connect and don't worry about the nits.