Series or parallel

[doorgunner]

I've been experimenting with a dedicated third battery this summer. This one is only hooked up to the fridge, because the beer cooler is #1 priority. This batt is rather small a 31 AH (20 hr) 12 volt gel cell. It is mounted in the rear ot the fridge access area.I've got a switch on the solar panel to flip flop between this batt and the main array of 2 - 12V parallels. Then the Honda 1000 for the hair drier and compressor.We use mood candles at nite for a romantic setting while pounding down the cold Duffs.
Why all this need for power?
Where's all the romance gone to anyway?
I'll soon by tryiny to power the fridge with a 4 amp lipo 14.8V, just for kicks.

[benwd]

With regard to bringing it over from the escapees forum. That is what it is and the OP gave the link to the other forum. No secrets about where the information comes from.

The subject was series or parallel, and the author of the airstream owners guide came it with claims, you read it and saw the post, and the subject went from series or parallel to the claims in the airstream owners guide. The thread needs to be here so airstreamers can discuss what is in the guide.

The quick answer in the guide says to consider several factors when selecting battery configuration and that makes sense, but, if everything is a wash and the claim that there is a capacity advantage to parallel, all else equal, then some may choose to go that way when series might have been a better choice.

The term in the guide (C/I^n)-(C/(2I)^n) is the term to determine discharge time of a battery or battery set for a given current drain and twice that current drain. It has been mis-applied to the series vs the parallel configuration to argue that the parallel configuration has more capacity because each parallel battery sees half the current. Read the discussions in the other thread.

The parallel batteries have twice the number of plates that the series batteries have for the same total capacity therefore the plates are half the mass of the series configuration and see half the current for the same current density in the plates. The current density is what is overlooked in the equation. If it were taken in to consideration there would be no capacity advantage shown for parallel.

It is happenstance that the author of the owners guide is an a/s forum person but that isn't the issue. The only real issue is that the guide mis-applies the equation.

The guide is confusing in the area of batteries and several things can be read from it, but the claim of parallel capacity advantage is clear.

What kind of an a/s am I looking at? Maybe some help here. Our best friends have one and I've helped with some of the restoration and rewiring. I would dearly love to have one as I'm a nostalgic type. Our rv is for sale but there isn't much market. If someone with an airstream wants a coach we might talk.

[uwe]

Originally Posted by richinny

did any of the early streams have 6v systems like cars did?

Ricky,

Early Airstreams, as in 63 and earlier, had very little in the 12V category. No converter, no charger, just a battery that would charge from the tow vehicle's charging system while driving. Most of the interior features were dual voltage, 110V, and 12V. Light fixtures were dual voltage, so were ceiling fans etc.
I am not aware of there ever being a 6V system for Airstreams.

[uwe]

Here is a case for discussion. This member could have gone out with one 12V battery just fine......
http://www.airforums.com/forums/f449...oke-36635.html

[bryanl]

"the author of the airstream owners guide came it with claims"

This is an error of jumping to conclusions. It is an example of the kind of irrational logic being used in much of this discussion.

"The term in the guide (C/I^n)-(C/(2I)^n) is the term to determine discharge time of a battery or battery set for a given current drain and twice that current drain. It has been mis-applied to the series vs the parallel configuration"

This is a specious claim. The terms are simply the statement of Peukert's relationship applied to the current of a single battery compared to the current from a bank made of two batteries just like the single battery, which is what the website says it is.

"Read the discussions in the other thread."

Like many, it is an ad hominem attack that is trying to deny such things like 'sky is blue' and 'grass is green' - even the title of that thread is contradicting what you can find on any website that credibly discusses the behavior of lead acid batteries.

"The parallel batteries have twice the number of plates that the series batteries have"

Another interesting example of the confusion that exists in trying to refute what is well established. Depending upon the batteries used, the number of plates may be equal or less or greater. But that begs the question about what the point of this assertion may be. The number of plates may influence total capacity (easiest determining by battery weight) and the insternal resistance. This could be an interesting discussion because it gets into the difference between RV and starting batteries.

"but the claim of parallel capacity advantage is clear. "

That perception is interesting because it is so far off the mark and so definite. It is so definite that anyone who might possibly have a different perception is maligned and impugned. That is not an indication that profitable discussion is being sought.

"The thread needs to be here so airstreamers can discuss what is in the guide. "

Hubris, maybe?

Why should anyone here have a 'need to discuss' what is on some RV oriented web page? Is there something else going on, here, to make it a need?

If a discussion was desired, it would be asking for clarification of what was meant and how things fit together. The fact is that the effort started with, and continues to be, an absurd fault finding expedition aimed at a particular individual. Bashing and trashing is no way to promote a productive conversation, especially when it so off base in methods of inference, matters of basic fact, and civility.

[2airishuman]

Originally Posted by benwd

With regard to bringing it over from the escapees forum. That is what it is....

as suspected.

the sierra nevada unit website is chock full of great info...

and like all written communication shouldn't be taken as gospel.

it's unlikely that very many (if any) here choose to spend 100s of bucks ...

on the 6v conversion based on the p equation, properly applied or not.

so the post has been made,
the public service announcement has been served
it is archived here for all eternity...

and so on.

i almost joined the escapee forums JUST to support the one member beat on unnecessarily in that thread.

i will defend those who have worn the rasta beret!

and bryan has

ok mon?

now let's get back to making sparks in d'moonlight!

candles, romance and cold beer...

doorgunner has spoken,

and clearly has a soft side.

cheers
2air'

[caseyd45]

You guy's have way too much time on your hands. The question has more than one right answer, depending on need and space available. I was a master mechanic for 20 years and I can tell you that both 6v and 12v batts. have there place depending on what result you are looking for. Just to ad a note, the battery technology has come so far in recent years that anyone who thinks 6v batts. are a thing of the past, is living in the past! All that said I'm sure after reading this someone will disagree.

[68 Overlander]

That was a very fair post. The very last person I would listen to is one that doesn't listen to his audience.

[bryanl]

Originally Posted by caseyd45

The question has more than one right answer, depending on need and space available. ... both 6v and 12v batts. have there place depending on what result you are looking for. ... the battery technology has come so far in recent years ...

That is exactly what I read from the owner's guide page and what I think is good advice.

[benwd]

The term (C / In) - (C / (2I)n) in the owners guide is not intended for a single battery or a battery bank that looks like a single battery. While that is what it is suggested to be it is mis-used as indicated by the previous term in the equation. Here are both terms used Tp - Ts = (C / In) - (C / (2I)n) . The subscrpts in the first term are p and s which is parallel and series. The intent of the second term is to show a longer discharge time for a parallel arrangement over series by stating the series batteries see twice the current as each parallel which doesn't hold up because each parallel battery is half the ah capacity of series string.

Here's an example:

There are batteries available that have very nearly the same ah capacity with very nearly the same peukerts exponent.

For the example the batteries will have identical ah capacities and identical peukerts.

Since in both series and parallel arrangements the voltage is 12 volts the power isn't an issue because it washes out. But, we'll do a power calculation at the end for demonstration.

We have two 6 volt batteries each rated at 240 ah. We discharge each at the 20 hour rate which is 12 amps and we get 20 hours of current.
If the two 6 volt batteries are in series and we discharge them at the 20 hour rate the current is still 12 amps and we still get 20 hours of current.

We have two 12 volt batteries each rated at 120 ah [the same Vah as the sixes] and we discharge each at the 20 hour rate which is 6 amps and we get 20 hours of current. [note the 6 amps is one half the 20 hour current of the sixes]
If we connect the two 12 volt batteries in parallel and discharge the pair at the 20 hour rate the current is 12 amps [same as the current for the sixes] and each 12 discharges at 6 amps, it's 20 hour rate.

What's the result? The sixes in series lasted 20 hours. The twelves in parallel lasted 20 hours. Both configurations drawing the same total current.

With the same peukerts, change the current and the time is the same for either arrangement.

Power [watts] The sixes provided 12 amps at 12 volts for 20 hours. 12X12X20=2.88 kwh.
The twelves provided 12 amps [6 amps each] at 12 volts for 20 hours. 12X12X20=2.88 kwh.

Plugged in to the first term of the owners guide equation Tp - Ts = 20-20=0, ie, no difference.

-------------

The owners guide says---

"The capacity gained by reducing energy draw or current drain in a parallel configuration versus a series configuration can be determined by evaluating Peukert's Formula T = C / In where T is how long you can drain current I from a battery that has a capacity C and an internal resistance characteristic n. For the case of two six volt batteries in series versus two 12 volt batteries in parallel, the comparison is when the current changes by a factor of two (when voltage doubles, current halves and vice versa for the same amount of power). The formula would be
Tp - Ts = (C / In) - (C / (2I)n) = C(2n - 1) / (2n In) = Tp ( (2n - 1) / 2n )"

Which is a clear statement that a parallel battery arrangement will provide more energy than a series arrangement of batteries of equal energy and the same peukerts. It doesn't work.

The owners guide says---"{subscript p for parallel and s for serial, serial has twice the current of parallel, the LCD (2I)n which is where the 2n comes from in the Tp term to be able to subtract the fractions, percent change divides both sides by Tp and then multiplies by 100 - check the algebra yourself and let me know if you think you see an error!}"

The error is the substitution of (C / In) - (C / (2I)n), which is the term for calculating the difference of time usage for a battery or battery arrangement that looks like one battery when the current drain is doubled,ie, (2I), for Tp - Ts ,the example shows the difference to be zero, whether in time, total amp hours, or watts.

I understand that there are other considerations for selecting battery arrangements, such as cost, ruggedness, cable connections, and on and on, but a capacity advantage is not one of them.

[richinny]

theory is important but i also require real tests. will someone please do the energizer bunny test for me! :-)

[bryanl]

We have two 6 volt batteries each rated at 240 ah. .. If the two 6 volt batteries are in series and we discharge them at the 20 hour rate the current is still 12 amps and we still get 20 hours of current.

We have two 12 volt batteries .. [the same Vah as the sixes] ...

If we connect the two 12 volt batteries in parallel and discharge the pair at the 20 hour rate the current is 12 amps [same as the current for the sixes] and each 12 discharges at 6 amps, it's 20 hour rate.
...
The error is the substitution of (C / In) - (C / (2I)n),

The problem here is the key to this whole issue.

When you connect 2 6v in series, All of the current in the circuit goes through both batteries. There is no adjustment to be made in available capacity due to a Peukert effect as a result.

When you connect 2 12v in parallel, the current is shared between them. Each battery in the bank has only half the current of the circuit. Since the current for each battery changes, there is a need to adjust the available energy capacity of each battery.

In other words, from the example above, if we take 12 amps out of a twelve volt bank we will be pulling 12 amps from each battery in a 6v series bank but only 6 amps from each battery in a 12v bank. This is the fundamental nature of series versus parallel circuit configurations.

When current draw changes, the available energy changes. For a serial configuration where the current in each battery is the current of the bank, we can easily combine specs from an individual battery to create a bank spec. For a parallel configuration, we need to adjust for the current sharing between batteries before we can combine specs for a bank spec.

On the SKP thread, I provided an example using the Trojan T125 and the SCS 200 batteries (both mid line batteries in the 6v and 12v series) to show how this adjustment could be made with references to the Trojan spec sheet and the Smartgauge website calculator. Since the overall energy capacities of these batteries are within 5% of each other, the power draws are comparable when using standard time based capacity ratings (a reason why the specs are based in this manner, BTW). They provide an example of what a typical RVer might do for thinking about battery capacity if he wanted to expand from a one battery bank to a two battery bank.

The first level of comparison is just to add rated energy capacities and be happy. The second level would be to make adjustments for how the current draw changes between using 6v or 12v batteries in your bank. Figuring this out is what the calculation was all about.

The so called error in substitution is a definition of this change. To look at the change in capacity when the current changes by a factor of two, you are comparing I and 2I just as the expression states.

It turns out that cutting the current draw from a typical RV battery in half will increase its capacity by about 15%. (this is characteristic of logarithmic functions like Peukert's relationship). This is something to note because it is not only useful for a 2nd level of precision for comparing battery capacities in small battery banks but may also provide ideas about how even small differences in battery use can greatly extend its available energy even with just one battery.

The key to understanding this issue is in how current is distributed in a bank. In series, all of the current goes through all of the batteries and the voltage is distributed between them. In parallel, the current is distributed between the batteries and the voltage is common to all.

The owners guide page was an attempt to show how this could be calculated assuming some knowledge of basic algebra, Ohm, and Kirchoff (which is why it is in the 'understanding' section). It was one issue of several described that might be considered by someone thinking about small battery banks. It was assuming an intelligent and curious RVer who wanted to understand how to improve analytic skills. The same calculation could be done by plugging numbers into a calculator (see my last entry on the SKP thread) but my preference is to see the guy behind the curtain and how he does things.

From what I have seen, anyone who does what I have tried to do is only providing bait for folks who are more into fault finding than improving analytic skills. That, I think, is a terrible thing. People should not be subject to harassment and denigration just because of disagreements or misunderstandings. I have seen several professionals try to contribute here and in other venues leave rather than put up with it. We all loose when that happens. It may make some folks happy as they have managed to rid themselves of an irritant but just sometimes (I think often) there is something to learn by thinking about what irritates us.

If you don't want to think and your knee jerks faster than your brain works, please don't bother me. If you want to explore some nifty nerd stuff appropriate to how my Airstream RV works, then I am definitely along for the journey.

[benwd]

The reason your logic doesn't work is because each 12 volt battery is half the capacity of the series string [half the lead], and therefore the 20 hour rate is one half that of the series string. Add another 12 and the 20 hour rate is the same for both arrangemments. It's pounds of lead and amps through lead. Voltage doesn't matter other than for calculating what power you end up with. Since both arrangements are 12 volts the only comparison is amps. Go through the example again. You can choose whatever discharge rate you want to establish capacity and both arrangements will still deliver the same ahs or power, ie, choose your discharge rate for both arrangements and they will both die at the same time.

Reposted:

The term (C / In) - (C / (2I)n) in the owners guide is not intended for a single battery or a battery bank that looks like a single battery. While that is what it is suggested to be it is mis-used as indicated by the previous term in the equation. Here are both terms used Tp - Ts = (C / In) - (C / (2I)n) . The subscrpts in the first term are p and s which is parallel and series. The intent of the second term is to show a longer discharge time for a parallel arrangement over series by stating the series batteries see twice the current as each parallel which doesn't hold up because each parallel battery is half the ah capacity of series string.

Here's an example:

There are batteries available that have very nearly the same ah capacity with very nearly the same peukerts exponent.

For the example the batteries will have identical ah capacities and identical peukerts.

Since in both series and parallel arrangements the voltage is 12 volts the power isn't an issue because it washes out. But, we'll do a power calculation at the end for demonstration.

We have two 6 volt batteries each rated at 240 ah. We discharge each at the 20 hour rate which is 12 amps and we get 20 hours of current.

If the two 6 volt batteries are in series and we discharge them at the 20 hour rate the current is still 12 amps and we still get 20 hours of current.

We have two 12 volt batteries each rated at 120 ah [the same Vah as each of the sixes] and we discharge each at the 20 hour rate which is 6 amps and we get 20 hours of current. [note the 6 amps is one half the 20 hour current of the sixes]

If we connect the two 12 volt batteries in parallel and discharge the pair at the 20 hour rate the current is 12 amps [same as the current for the sixes] and each 12 discharges at 6 amps, it's 20 hour rate.

What's the result? The sixes in series lasted 20 hours. The twelves in parallel lasted 20 hours. Both configurations drawing the same total current.

With the same peukerts, whatever current both arrangements are discharged at will result in both arrangements delivering equal amp hours or power.

Power [watts] The sixes provided 12 amps at 12 volts for 20 hours. 12X12X20=2.88 kwh.

The twelves provided 12 amps [6 amps each] at 12 volts for 20 hours. 12X12X20=2.88 kwh.

Plugged in to the first term of the owners guide equation Tp - Ts = 20-20=0, ie, no difference.

-------------

The owners guide says---

"The capacity gained by reducing energy draw or current drain in a parallel configuration versus a series configuration can be determined by evaluating Peukert's Formula T = C / In where T is how long you can drain current I from a battery that has a capacity C and an internal resistance characteristic n. For the case of two six volt batteries in series versus two 12 volt batteries in parallel, the comparison is when the current changes by a factor of two (when voltage doubles, current halves and vice versa for the same amount of power). The formula would be
Tp - Ts = (C / In) - (C / (2I)n) = C(2n - 1) / (2n In) = Tp ( (2n - 1) / 2n )"

Which is a clear statement that a parallel battery arrangement will provide more energy than a series arrangement of batteries of equal energy and the same peukerts. It doesn't work.

The owners guide says---"{subscript p for parallel and s for serial, serial has twice the current of parallel, the LCD (2I)n which is where the 2n comes from in the Tp term to be able to subtract the fractions, percent change divides both sides by Tp and then multiplies by 100 - check the algebra yourself and let me know if you think you see an error!}"

The error is the substitution of (C / In) - (C / (2I)n), which is the term for calculating the difference of time usage for a battery or battery arrangement that looks like one battery when the current drain is doubled, ie, (2I), for Tp - Ts ,the example shows the difference to be zero, whether in time, total amp hours, or watts.

I understand that there are other considerations for selecting battery arrangements, such as cost, ruggedness, cable connections, and on and on, but a capacity advantage is not one of them.

----------------------

Your last two paragraphs is making this personal and so I have nothing to say about them. I'm addressing what the owners guide claim is.

[bryanl]

It's pounds of lead and amps through lead.

This is a real nice summary for any battery bank! Higher available energy capacity means more pounds and/or lower currents.

Posting the same thing over and over again really doesn't help anyone much, I think. We need a different viewpoint or way of looking at things. I'll give it some thought.

Your last two paragraphs is making this personal and so I have nothing to say about them.

since I did not address any particular person and simply stated my preference, I don't think I can agree with this assessment.

I understand that there are other considerations ... but a capacity advantage is not one of them.

I don't think I'd put it as firm as this but I think we agree in essence. Capacity differences aren't worth much worry compared to everything else.

[herrgirdner]

Gents, I believe it might be time to tie a knot in this thread.
I admit it, I earned that there is a Peukert effect, and I googled it, and read more than I ever want to know about it. This is one of the most valuable things provided by this forum: knowledge.
And humor, sometimes.
But, please, no Family Feuds.
regards

[benwd]

leipper---herricaner came in on our discussion and got it closed before we got things sorted out.
Your paper on understanding peukerts reveals some interesting points. We're still trying to sort out the names.

Who are Stan and Ken?

[bryanl]

Your paper on understanding peukerts reveals some interesting points.

This is where the discussion should focus.

The emotional issues are interesting and also worthy of study but probably not here. Airforums seems to be particularly tainted by agendas that are not fully understood, even by those who carry them. To some extent, that is to be expected of the 'gifted and talented' crowd but it becomes socially cancerous when it leads folks astray. We all need to be careful.

[HiHoAgRV]

Interesting discussion, I can add that using dual 12v's has always had one drawback for me. If one battery dies that charger has always killed the second by puekerting the acid dry . I averaged a new set every 2 years but with a single 12v I've yet to have a problem. If I ever need duals, I'll try 6's

[sander17]

Originally Posted by HiHoAgRV

Interesting discussion, I can add that using dual 12v's has always had one drawback for me. If one battery dies that charger has always killed the second by puekerting the acid dry . I averaged a new set every 2 years but with a single 12v I've yet to have a problem. If I ever need duals, I'll try 6's

In my setup, I have two 12V batteries, approx 3 years old, with individual switches. When boondocking, I run just one battery until the monitor panel shows its voltage dropping. Then I switch it out and switch in the other. I do this on the assumption that they are probably not matched to one another anymore and I do not want the fresher one charging the weaker one. Also, if the first one dies prematurely, I still have the second one. With lights and water pump use they typically last a couple of days each.

Is this a recommended procedure? Or would I get better results by running them both together all the time?

[HiHoAgRV]

Thast sounds like a great solution. Where I hit the wall is in a storage mode with the charger running. It seemed like any mis-match between parallel batteries resulted in the charger trying to top off the low battery and frying the already hot one. At that time I was using a 3 stage charger and no battery disconnect switches.