Question regarding coach batteries tied in series/parallel

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hugh37

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Need help, please.  Have 2 sets of 6-volt Trojan T-105s tied in series/parallel.  One very definitely is "gone", but believe perhaps the other 3 are good. I'm told, however, that I should replace ALL 4, not just 1 or any less than ALL 4!  Is that the concensus, i.e., forget replacing any less than all???  Would greatly appreciate your responses this evening. Many thanks!
 

hugh37

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Thanks to those who responded to my recent dilemma.  I've replaced my 2-yr. old T-105 Trojans with four (4) brand new ones...and boy, is the price going up.  Understand there's another one to come in December.  Unquestionably, I should be getting maybe five (5) years on these batteries.  From the start I've sensed a problem, even with the original equipment Interstate Workaholics.  As you recall, I have two (2) sets tied in series/parallel.  I cannot understand why always the front of the left set reads considerably higher voltage than its partner or either of the other set.  The voltage of the left front is followed by the voltage of the rear of the right set. (Don't mean to over-complicate matters - It's just that this particular issue has always been hard for me to grasp).  I might add that the com-bined voltage of each set is always matches.  Except when the left front one re-cently "expired".  Seems that I've always had to add distilled water that one, as it appears to "boil out".  Understand that I try hard to be a "hands-on" type, always endeavering to maintain every aspect of my coach.  By the way...my coach is a 2002 Holiday Rambler Scepter, 40-ft. with a 350 Cummins, and mileage at almost 51,000.  Thanks, in advance, to all who feel they can help in this instance.  I might mention that I have never "equalized" my batteries, including the originals.
 

Tom

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Hugh,

All the battery manufacturers and suppliers claim the price hikes are due to the price of lead.
 

Karl

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Hugh,

I may be in the minority, but see no reason to replace 3 perfectly good batteries when only one goes bad. I recently added a second set of T-125's to the existing (3 1/2 yr old) T-125's, after checking their condition both by load test and specific gravity at full charge. I will add that you certainly don't want to mix different battery types (AGM's with flooded cell) or different capacities (exam: 180AH with 240AH). I'll even go so far as to say don't mix different mfg's batteries, but that's where I draw the line. If you have a 3-cell flashlight and replacing one of them makes it work again, you don't necessarily replace the other two, do you? 
 

John From Detroit

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Tom said:
Hugh,

All the battery manufacturers and suppliers claim the price hikes are due to the price of lead.


Yup, the lead in their..... You know what :)

The price of everything goes up, Manafacturers and suppliers tack on some additional price increase in the name of obscene profit
 

John From Detroit

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Karl said:
Hugh,

I may be in the minority, but see no reason to replace 3 perfectly good batteries when only one goes bad.

I"m inclined to agree with you Karl.  However in the case of six volt pairs I am inclined to treat each pair as one 12volt battery

Note: Inclined, not convinced
 

Gary RV_Wizard

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Like Karl, I have successfully mixed old and new batteries and have even mixed AGM and flooded cells, though in separate banks  rather than direct wired to each other.  The relative condition is the key factor and sometimes it can be difficult to tell.  It's always the safest approach to replace the entire set (even in the flashlight example) because a "fair" battery will quickly suck the power (and maybe some of the lifespan) from a "good" new battery. 

The "replace 'em all" rule is aimed at the average consumer who isn't into battery testing/monitoring and just wants things to work reliably when he turns the switch on. It's "good practice" but not absolutely necessary. As with most technical things, the general rules can be violated if you know what you are doing, know the signs of a potential problem and keep an eye out for those signs. For wet cell batteries, you can check the specific gravity of each cell to determine to the battery is in sound operating condition. If it has a weak cell, I would not couple it with a new battery, but if the cells all test decently I would not worry about combining them.

I might add that the com-bined voltage of each set is always matches.

It has to if they are wired in series - the good battery is dragged down by the poorer one, reducing the overall voltage in that set. It functions as a single battery when wired that way.

Are you saying that the batteries test out at different voltages (when disconnected from each other, of course)?  That's not a good sign. Surely the new ones were all the same when you installed them. How long ago was that?  You cannot reliably measure the voltage in one battery if it is wired to another, whether series or parallel. To get an accurate reading on a single battery, you must disconnect it from the others.

Corrosion at the posts or bad wiring between batteries (e.g. corrosion under the terminals where they clamp to the wire) can cause strange things to happen in both charging and discharging. Wonder if you have a bad wire somewhere?
 

John Canfield

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It is good practice to replace all batteries in your situation (series/parallel bank) unless you are willing to invest a lot of energy into monitoring the older batteries - I would not be in the least interested in doing this. 

On the boat we had, I specifically avoided paralleling 12V batteries due to difficulties in monitoring individual battery performance.  I used two battery switches so I could switch the load around every day.  Another formula for success was using gels on the boat and now AGMs in the coach for house batteries.  Gels/AGM don't have plate degeneration problems like flooded cell batteries do.
 

Karl

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John,

I agree that it might take a little more time to measure s.g. rather than just looking at electrolyte level in individual cells, but it's not that much longer. It's also good practice to do so, even with new batteries.

While you are correct that 'starved cell' batteries, Gel/AGM's, are less likely to have a certain type of plate degeneration problems (flaking off of Pb/PbO from the plates), they still can suffer from the formation of PbSO4 in certain situations and may require equalization also. Because they are sealed, a load test is the only accurate way to test battery condition; a long, drawn out process when done right. While the s.g. reading of flooded cell batteries helps in determining health (it doesn't replace the load test), it must be done properly to be of any benefit. You should wait at least 3 hours after charging before taking a reading, or place a small load across it to eliminate stratification of the electrolyte.

Back in the bad old days, cells were connected together externally by large lead straps, so it was possible to measure individual cell voltages and determine if and when a cell was going bad. You can't do that anymore, but that's not all bad - internal straps are shorter, so the internal resistance of the battery is considerably less.
 

John From Detroit

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=Back in the bad old days, cells were connected together externally by large lead straps,=

You have a good memory Karl.. They stopped making them that way right about the time I got my Driver's license
 

Just Lou

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I have followed this thread with great interest. ?It took a too familiar path. ?I felt there wasn't enough info (in the initial post) for me to respond and (given my slow and limited? typing skills) assumed that someone else would ask the right questions and come up with the best answer.? As usual, they eventually did.

I found it interesting, but all too typical, that the first responders made a flat recommendation to "replace them all " without asking the age of the current batteries or inquiring as to exactly how the one battery failed. ?Shorted cell? ?Open cell? Weak? etc?

I understand that none of us learns anything from (maybe YES - maybe NO) answers, but often we are to eager to quote the Party Line or cookie cutter answer with minimum ?knowledge of the facts. ?Often, (it depends....) is the best approach to an answer.

Hugh did state, in his second post, that the batteries were only two years old and that certain anomalies had existed for some time. ?That additional information inspired some well thought-out, logical and somefrom experience answers to quickly follow. ?However, Hugh had already replaced three expensive, two year old, batteries that likely had several years of life left in them. ?Maybe yes, maybe no - LOL.

I do understand the logic of replacing all old tires and allold batteries, but I also know that (unlike tires) it makes only good business sense to squeeze as much useful life from a battery as possible.? Batteries are seldom a life or death or safety issue.

Enough philosophy, but I was just wondering......
I have a burned out overhead light bulb... ?should I replace them all? ?How about that one foggy window... do I replace them all?? LOL

FWIW my 2 cents....

BTW - my first (and only) factory job was assembling batteries in Chicago Heights, Ill, at a Gould National Battery plant.? My job was to put the three bakelite cell covers (all automobile batteries were 6volt back then) in place and seal them with melted rubber so the next station on the line could put those two lead straps in place.? The job payed $1.25 an hour.? I quit and joined the US Navy.?  They paid $.14 an hour.
 

DonTom

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If you have a 3-cell flashlight and replacing one of them makes it work again, you don't necessarily replace the other two, do you?

It's not all that important to replace all the batteries at the same time when they are all in series, such as with a flashlight. It's different when they are in parallel or in a combination of series/parallel as then one battery will be a load on the other. When in parallel, they should all be the same  as much as possible. Same age, same capacity, same brand, same charge,  same everything.  If in series, such as two 6 volt to make 12, it's not a big deal to only replace one battery with a different age and brand, etc.

                                                      -Don-

 

Just Lou

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DonTom said:
  When in parallel, they should all be the same? as much as possible. Same age, same capacity, same brand, same charge,? same everything.? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? -Don-
And the beat goes on..........

Generally speaking, in a perfect world, the quoted statement is true...  In the real world it's very subjective.

BUT, with the supporting info (read ...  4-t105S, 2 years old, 1-bad, 3-good, expected battery life 5-7 years) do I need to replace them all?  The answer is a resounding NO.  Immediate disclaimer;  Cause of failure must be diagnosed and remaining batteries must be tested.  Whew!!just got that in under the wire.

In the above scenario, there is no earthly reason for me to replace all the batteries. 

Now if you will excuse me, I must go replace all my bulbs and windows........


 

John From Detroit

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Uh, Don, I think you have it backwards... But then, perhaps you don't

First I do agree with whomever said you should keep all batteries the same type.. Example, Do not mix AGM with Flooded wet cells (different charge settings for one thing)

But I consider a pair of six volts to be one 12 volt for all things.  If I have an older six volt with reduced capacity in series with a newer one with higher capacity two things, both bad, are going to happen.

1: When charging the older one will go into OVER CHARGE before the new one is fully charged and thus boil dry

2: When discharging the new one (Which never reached full charge) will go into deep discharge firest (I should say MAY go into)

However if I put a newer, or bigger 12 volt battery in parrallel with an older, or smaller 12 volt battery then the charges will equlize along percentage lines.  That is if one is 100% and one is 98 the one with the lower charge will gobble the higher current and likewise if discharging and one battery drops oh, say .0001 volt below the other, the other is going to supply most all the current till they are again in sync. (A millisecond or so later)  And the voltages should NEVER be different in fact.  Thus the batteries will form a perfect commune where each takes according to it's need and each gives according to it's ability

Note: in practice a smaller battery may have slightly higher internal resistance which can be different enough that under truly big loads (Microwave brewing coffee) will cause one or the other to change resting voltages slightly faster.  However again, this will level out soon as the microwave goes BEEP
 

DonTom

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Don, I think you have it backwards... But then, perhaps you don't

Nope. I am fairly sure. In parallel, if not exactly equal,  one battery will draw current and the other will have to supply it by draining, even if not used.  In series, such as your flashlight, if it's off, it's off and there is nothing to drain or discharge other than the normal shelf life. When on it still isn't much of a problem either, to have different batteries in series.

"But I consider a pair of six volts to be one 12 volt for all things."

Nope, ten six volt batteries in parallel is still six volts with about ten times the amperage capacity of a single battery, if all equal.  The same ten six volt batteries in series will make 60 volts and the current capacity will be the same as the single lowest battery.

1: When charging the older one will go into OVER CHARGE before the new one is fully charged and thus boil dry

At least the charging current will be the same in each battery, even if the batteries are different, when batteries are in series. But when in parallel, the one with the lowest internal resistance will charge at a higher current. No matter how you look at it, IMO,  a difference is less of a difference with the batteries in series than in parallel.


                                                                -Don-
 

DonTom

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" in a perfect world, the quoted statement is true...  In the real world it's very subjective."

Some people say you should NEVER leave batteries in parallel, period. Not even when all are as equal as reasonably possible. It's best to get a battery of twice the capacity. Since it's not a "perfect world" you can never match them perfect!

That doesn't mean it won't work. It will. But your batteries will fail at a faster rate. The more unequal, the faster they will fail.

                                                      -Don-
 

DonTom

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The "replace 'em all" rule is aimed at the average consumer who isn't into battery testing/monitoring and just wants things to work reliably when he turns the switch on. It's "good practice" but not absolutely necessary.

I agree.  However, those who know what they are doing, would not be asking about such.  ;D

However, others like me, might just be too lazy to test them often, so it's best for me to avoid batteries in parallel too  ;D

                                                                -Don-
 

Gary RV_Wizard

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I have to disagree with your advice about wiring batteries in parallel, Don. When in parallel, each battery acts pretty much independently, supplying power according to its capability and drawing charge amps according to its need and internal resistance. Parallel wiring is true socialism at work.  ;) The only time one battery in parallel draws from another is if a battery in a discharged state is coupled to a 100% charged one, in which case the charged battery becomes a charger for the weak one until they equalize.  Total available current remains the same, but now it is spread over two batteries instead of one.  This is pretty much a non-issue if there is a charger in the circuit as well.  Of course, a truly "bad" battery, e.g. one with an internal short, will continue to suck current and never charge up, dragging the whole system down. But that's true in series too. Bad is just plain bad, no matter how it is wired.

And I agree with John (Wow, John - it happened again!!) that two six volts in series are for all intents and purposes a 12V battery. Heck, a 12V battery is constructed by wiring cells in series, so it cannot be any different.  A bad cell anywhere among the 6 cells that make up the 12 volts has the same effect, whether in one battery case or two cases with external wires.
 
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