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Old 10-07-2007, 11:05 AM   #29
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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.
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Old 10-08-2007, 10:51 AM   #30
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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.
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Old 10-08-2007, 01:07 PM   #31
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theory is important but i also require real tests. will someone please do the energizer bunny test for me! :-)
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Old 10-08-2007, 02:31 PM   #32
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Quote:
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.
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Old 10-08-2007, 03:41 PM   #33
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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.
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Old 10-08-2007, 07:16 PM   #34
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Quote:
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.

Quote:
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.

Quote:
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.
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Old 10-08-2007, 08:39 PM   #35
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Thanks

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
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Old 11-02-2007, 10:41 AM   #36
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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?
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Old 11-02-2007, 10:58 AM   #37
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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.
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Old 11-02-2007, 11:02 AM   #38
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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
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Old 11-02-2007, 11:45 AM   #39
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Question Running two 12V batteries with switches

Quote:
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?
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Old 11-02-2007, 12:21 PM   #40
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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.
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Old 11-02-2007, 01:12 PM   #41
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Quote:
charger has always killed the second by puekerting the acid dry
Quote:
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.
Quote:
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.
Q 1: if you are loosing electrolyte, your battery is being overcharged. This has nothing to do with how current draw can influence battery capacity.

Q 2: switching batteries is like running 12v in series in time. For typical 12v wet cell lead acid batteries, that technique will get you about 15% less available energy than if you just ran them in parallel, the way Airstream provides them.

Q 3: a mismatch in parallel batteries will not cause topping off one battery to fry another. That could only occur if there was a shorted cell and in that case it is the battery with the bad cell that would overheat and be destroyed. The good battery might suffer a bit of discharge until you discovered and removed the bad one, but that is all. Shorted cells are fairly rare these days as a cause of battery failure.

There are reasons why Airstream provides 2 12v batteries in parallel in its trailers.

Most RV's charge their house batteries in parallel with their starting batteries even those batteries are of different type (deep cycle and SLI), and often of different age and capacity. Both get charged. Neither suffer as a result.
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Old 11-02-2007, 01:36 PM   #42
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Quote:
Originally Posted by Leipper
Q 1: if you are loosing electrolyte, your battery is being overcharged. This has nothing to do with how current draw can influence battery capacity.

Q 3: a mismatch in parallel batteries will not cause topping off one battery to fry another. That could only occur if there was a shorted cell and in that case it is the battery with the bad cell that would overheat and be destroyed. The good battery might suffer a bit of discharge until you discovered and removed the bad one, but that is all. Shorted cells are fairly rare these days as a cause of battery failure.

There are reasons why Airstream provides 2 12v batteries in parallel in its trailers.

Most RV's charge their house batteries in parallel with their starting batteries even those batteries are of different type (deep cycle and SLI), and often of different age and capacity. Both get charged. Neither suffer as a result.
I'll inform my pile of dead batteries to quit faking and get back to work
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