Mixing batteries discussion

This is a rather common routine problem with the OEM isolater solenoids used by Winnebago.

In the absence of ignition being on, coach and house batteries are isolated from one another via the battery isolator solenoid. When the ignition key is on, the solenoid closes, connecting the two sets of batteries together, allowing the house batteries to be recharged. After a bit of use, the contacts on the isolater solenoid become charred, and fail to conduct current to the house batteries for the purpose of charging.

The replacement solenoid must be of continuous duty spec. And the good part, is that they only cost about $20 at almost any auto parts facility.

If they do not charge on shore power either perhaps the battery terminals need to be cleaned; or the house battery ground connection is defective.

As for the comment that "Batteries in parallel, like yours (and mine) should ALWAYS be replaced in complete sets with the exact kind/brand/capacity."; it's the other way around: Batteries connected in series must be matched. For batteries connected in parallel, it doesn't matter. 

 

Stan Birch said:

As for the comment that "Batteries in parallel, like yours (and mine) should ALWAYS be replaced in complete sets with the exact kind/brand/capacity."; it's the other way around: Batteries connected in series must be matched. For batteries connected in parallel, it doesn't matter. 

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That is incorrect.

I'm with John. It probably matters less with a parallel hook-up, but I would never suggest that it doesn't matter at all. A bad battery in parallel will still drag down system voltage and cause the charger to work overtime trying to bring it up to snuff, thus overcharging the other batteries in the bank.

Mixing batteries without an accurate assessment of their individual conditions is always likely to be problematic.

Gary RV Roamer said:

A bad battery in parallel will still drag down system voltage and cause the charger to work overtime trying to bring it up to snuff, thus overcharging the other batteries in the bank.

Mixing batteries without an accurate assessment of their individual conditions is always likely to be problematic.

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Nope! All wrong!

Mixing batteries works just fine, and they charge together just fine! There isn't a motorhome out there that doesn't employ a system of charging dissimilar batteries in parallel! Motorhomes have two entirely different battery systems, of different purpose, age and capacity. The house batteries are deep cycle, and the chassis battery is a standard automotive battery. Yet when you are going down the road, the alternator charges all of these batteries charge together uneventfully.

Contrary to what you have stated:

1. A bad battery in parallel NOT drag down system voltage; and

A "bad battery" by definition, is also commonly referenced as "a battery that will not take a charge"; i.e a battery that that is so old and badly sulfated that it will no longer draw a charging current. And if it is not drawing a any current, then it is obviously NOT going to drag down the system voltage.

2. Will NOT cause the charger to work overtime trying to bring it up to snuff; and

A battery that is not drawing current will not cause the charger to do anything. A charger is a dumb entity has no knowledge of what might be attached along the charging path. A charger only knows how to do one thing: provide a constant voltage; and it is up to the batteries, lights, electronic accessories to take what they want from the constant voltage source. And if one battery doesn't want to draw any current, the charger neither knows nor cares. It's job is just to supply a constant voltage. 

3. Will NOT cause overcharging the other batteries in the bank.

A weak battery will NOT cause overcharging of other batteries in the bank. Batteries will only draw current from the constant 13.8 voltage (or whatever) until they reach full charge, and then quit drawing further current. They don't know nor care if other batteries in the bank have yet reached full charge. They just quit when they are full.

Two dissimilar house batteries connected in parallel tend to work just fine in service. With a 100 AH battery yoked with an 80 AH battery, they will simply discharge at a proportionate rate; so that when the 100 AH battery gets down to 50 AH, the remaining capacity of the 80 AH battery will be about 40 AH. 

When it comes to series  batteries, they MUST be matched exactly, because they are all part of a battery string. The best way to ensure that series batteries are perfectly match, is to simply buy a 12 volt battery, which is merely a collection of six perfectly matched batteries connected in series. If you choose an awkward arrangement, like installing 2 golf cart batteries in series, then you have to be very careful that they are perfectly matched. If one battery in a series is weaker than another, then you get into all sorts of bad stuff, such as the weaker battery becoming reverse-charged if it is discharged beyond it's capacity.

Interesting point of view Stan.  Quite different from other opinions that I have read on various forums.  Where did you obtain your expertise?  You sound as though you have had a significant amount of practice in dealing with RV battery systems.

I could pick every one of his comments apart point by point to show where he's completely wrong (and each is), but think it would be a waste of time.

Just because manufactures charge dislike batteries (house and chassis) in parallel through the chassis alternator doesn't make it right. I've often considered installing a switch on the contactor to kill it since I really don't need the engine charge anyway with Solar.

Stan
I suggest you test your "theory" on parallel connected batteries by connecting a good battery and a defective one in parallel and observe the voltage. Be sure to keep the group informed of your findings. I suspect that could be a cause of Sak557's problem.

I' m with Gary, et al. and I taught this stuff for a couple of years. Note that  shorts are fairly common failure mode!

Ernie

SargeW said:

Interesting point of view Stan.  Quite different from other opinions that I have read on various forums.  Where did you obtain your expertise?  You sound as though you have had a significant amount of practice in dealing with RV battery systems.

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I am a licenced mechanic. When you are doing that kind of work, it helps to have a little bit of knowledge about how motor vehicle electrical systems work.  :

As for RV battery systems: While most of our RVing involves boondocking or camping without electrical hookups, my last set of batteries lasted 10 years. 

Stan Birch said:

I am a licenced mechanic. When you are doing that kind of work, it helps to have a little bit of knowledge about how motor vehicle electrical systems work.  :

As for RV battery systems: While most of our RVing involves boondocking or camping without electrical hookups, my last set of batteries lasted 10 years. 

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It also helps to have a basic understanding of electronics in general, the impedance properties of batteries, and an understanding that as they age that these things change, and the ramifications of using non-like batteries, even those of the same make and model battery, but completely different in both charge and discharge curves and the reasons why.

As mentioned above, just because the chassis manufacturers charge non-like batteries from the engine, does not make it OK or at all healthy on the batteries.

There is a reason that each battery manufacturer specs their own charge curves, and they are often completely different from the competition. It is very poor practice mixing batteries, either in series OR parallel (it doesn?t matter) that have different discharge rate times due to age, or simply a different brew in the plates; their charge curves are rarely the same.  Batteries that charge faster than others when in the same stack simply heat up, out-gas and boil over, while the other battery is trying to complete it?s charge.

Congratulations on your 10 years, I hope mine are right up there. But any boondocker with an electronic background will never mix non-like batteries for these, and many other reasons I don?t profess to understand when it gets into the chemistry.

^^Exactly^^

I just replaced our seven year-old Lifeline AGMs this summer more on a whim than with any issues with the batteries.  I had just installed the solar panels and sort of got on a roll  .

We 'boondocked' (swinging on the hook) on our sailboat for 15 months straight with about three-four weeks of that time spent at a marina on shore power.  We had a solar panel, wind generator, small diesel genny and a 150 amp alternator on the propulsion engine.  I made my own sophisticated <cough> charge controller (a rheostat that applied current to the alternator field winding) so I could baby the charge for our several gel batteries.  Learned quite a bit about batteries in the process of learning how to be a successful blue-water cruiser.

We're getting on a bit of a tangent here (and I'm contributing  ), so why don't we move on.  It would be really nice if the original poster would give us an update.

SCVJeff said:

their charge curves are rarely the same.  Batteries that charge faster than others when in the same stack simply heat up, out-gas and boil over, while the other battery is trying to complete it?s charge.

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You were doing reasonably well until you got to this point; and then you totally flubbed it!

Healthy batteries supplied with an appropriate charge voltage on an indefinite basis will NEVER overheat, out-gas nor boil over! They can be left on charge forever, without being damaged, provided the charge voltage is appropriate. And appropriate charge voltage is somewhat different, depending upon where you live.

The long term day-long vehicle alternator voltage is determined by ambient temperature. Alternators usually start up at about 14.3 - 14.5 volts; and then as the alternator heats up, largely dependent upon ambient temperatures, reduces it's charge voltage to compensate. On a cold winter day, the alternator may continue to maintain a charge voltage at around 14.0 VDC; whereas on a hot summer day, the alternator will drop the charge voltage to 13.8 volts for the remainder of the day on the road.

So . . while some batteries might charge a bit faster than others based upon chemistry and impedance, these considerations are miniscule compared to temperature considerations. Remember the stuff we learned way back in junior school? i.e. With each 20F increase in temp, chemical rates double? And most of us are familiar with the the well known fact that the capacity of batteries at 32F, are about 50% of those at 80F.

Most converters are designed to address the needs of customers in more northern climates, by setting the continuous charge voltage at 13.8 volts. While it works great for those of us north of the 49th, that voltage could boil batteries dry in Texas on a HOT 110 degree summer day.

Converters are set to provide best service somewhere around central US; too little for the north and too little for the extreme south.

While I dedicated a lot of time to learning battery science and technology; that's obviously not a popular pursuit. While the majority of responses on this forum demonstrate miniscule knowledge of battery chemistry/physics/technology; with replacement batteries going a mere hundred dollars crack it's really no big deal!   

i'm speculating that both views are correct.... sort of.

engine alt charges both house and chassis batteries through seperate feeds? so not really paralell?

house batteries are generally 2 6v volt in SERIES not paralell. a bad one will drag a good one down. paralell wil too.

for those that don't know, series adds up voltage. paralell adds up amp hours. paralell is what jumper cables do. they hook two electrical systems together for amps to start a car. some heavy equipment runs series and paralell in order to get 24v and a bazillion cranking amps.

Ok, part of the problem here is that in RV's we have, for long  years, used pairs of six volt batteries in series.. now, fact is that once installed there are no six volt batteries in RV's only 12 volt batteries that were delivered in 2 pieces for ease in  handling (And a few other reasons that have to do with cost)..  At least we should think of the six volt PAIR as a single 12 volt..  of the two batteries in the pair, IDENTICAL is the rule, I mean would you want a 12 volt battery were half of it was Group 31 and  half was Group 24, or half was new and half was old. (Single battery not a bank of several).. Right.


But when you put them in parallel that all changes.

"One bad one will take down the entire bank"

If the batteires fail "high resistance" or "Low capacity" this is simply not true, HOWEVER this bad battery WILL place additional load on the others so it shoudl be replaced soon as possible, but till it goes bad,  No need to replace.

The other failure mode is shorted cell.. That WILL take down the others so get it off line NOW and replace soon as you can

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The smaller battery will charge first and signal "Full" stoppign charging

NO, can not happen.. Let's say you have two batteries, size does not matter, when you first hook them together one is at 75% SOC and the other at 95%.. Ok, the lower SOC means a lower voltage so this battery will take a greater share of the charging current (Relative to it's size) till it comes up to 95%, at this point the two batteries will lock step all the way to full, both hitting full at EXACTLY the same instant.  And thus they will remain till they are seperated again.  The laws of physics say it has to be so, These laws, unlike the laws of man, can not be violated.

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One battery will provide more of the current/take more of the current.

TRUE, the larger battery will provide more current, in direct porportion to the difference in size,  That is if one battery is say (To make it easier) 2/3 the capacity of the other. Then we that battery will provide (or accept) 40% of the current, The other one will provide 60%, (totalling 100%) again they will stay in perfect lock step as they charge or discharge.

As the Commune says: Each according to their ability, Each according to their needs.

Batteries are socialists, No question about it.

The only danger is if you mix wildly different batteires, Example a garden tractor with an 8-D.    Should the Garden tractor battery short out, the 8D can push enough current through it to cause it to explode.    With batteries closer in size, that is not likely to happen.  Plus high resistance is the more common failure mode.

(Also a fuse can prevent it).

The one caravat in all this is don't mix different TYPES of batteires, ie; Flooded wet cell and AGM.. Though it can be done (Carefully) there are differences in charging voltage that need to be addressed.

John From Detroit said:

NO, can not happen.. Let's say you have two batteries, size does not matter, when you first hook them together one is at 75% SOC and the other at 95%.. Ok, the lower SOC means a lower voltage so this battery will take a greater share of the charging current (Relative to it's size) till it comes up to 95%, at this point the two batteries will lock step all the way to full, both hitting full at EXACTLY the same instant.  And thus they will remain till they are seperated again.  The laws of physics say it has to be so, These laws, unlike the laws of man, can not be violated.

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Explain this with a bigger crayon please.. 
How can a GP24 and a GP31 attain full charge at the same time? If one is fully charged, and the other one is ~80% or so, how does that not affect the GP24 battery that is already at full charge? Anything extra turns into heat. While they will end up at the same SOC, getting there is  another issue.

If I charge 2 non-like (capicaty wise) Lipos off my helicopter, I stand a really good chance of puffing the smaller capacity one up, or worse. Somethings gotta give.

Jeff, you've been JFDed.

The problem with putting batteries of different ages together in a parallel bank has to do with the batteries' charge vs. discharge hysteresis - how much voltage it takes to charge the battery to 100% compared to how much voltage it puts out during discharge.

As a wet cell battery ages, these points get further apart - it takes more voltage to get an old battery to 100% charge, and it puts out less voltage while you use it.

With one new and one old battery connected in parallel, the "new" battery winds up doing the lions share of the work while the "old" battery loafs.  The new battery charges to 100% first, then holds down the voltage bubbling it's electrolyte while the old battery tries to get to full charge.

Then when you discharge them, the new battery with it's higher output voltage provides more current than the older, partially charged one.

That's what I said, and Stan thinks I'm wrong..  So I'll also invite HIM to explain this with a big red crayon.

Any two batteries, even two new ones, will differ slightly in capability and to some degree in SOC as well. As the difference grows, the effects are exaggerated until the types of problems mentioned here become evident. If you are paying attention to them, you probably will take action before things go too sour, but many owners do not and get surprised when the whole battery bank goes bad.

Ok.  I'll say it first.  I think Stan's philosophy on batteries is way off!  Sorry.  Hadda do it!

It doesn't make sense with the current knowledge of battery maintenance.  Or in a bank configuration.  Must be my "miniscule knowledge" showing.  I hate that when that happens!