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Batteries in Series

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paulmillner

Joined: 09 May 2007
Posts: 21
Location: Berkeley, California

Posted: Sat Feb 23, 2008 3:42 pm Post subject: Batteries in Series

Hi Bob,



I enjoyed reading your recent discussion of paralleling batteries.



Instead, I'm wondering about converting one of my two alternators to 24 

volt (28 volt, whatever nominal voltage you prefer!) to charge two 12 

volt batteries in series.  That will allow me to install the air 

conditioner (!) I want, which only comes in a 24 volt version.  I'd 

leave the ship at 12 volts, and power those loads from the inter-battery 

connection.



I'd connect the remaining 12 volt alternator to that "lower" battery as 

well.



Let's say the 12 volt battery is offline and the 28 volt alternator is 

cranking away... the "upper" battery will eventually get fully 

charged... the upper batter will then just be a wet piece of wire (kind 

of) to the 12 volt load?  Or would there be a problem with the lower 

battery not getting charged enough, or the upper batter overcharged?



Paul, making my head hurt mode

_________________
Paul Millner, Berkeley CA [OAK]

tigerrick(at)mindspring.c
Guest

Posted: Sun Feb 24, 2008 9:00 am Post subject: Batteries in Series

Air conditioning!



Wuss.



Rick Lindstrom



--

nuckolls.bob(at)cox.net
Guest

Posted: Sun Feb 24, 2008 10:40 am Post subject: Batteries in Series

At 03:38 PM 2/23/2008 -0800, you wrote:



 	  Quote: 	
		  

<paulmillner(at)compuserve.com>



Hi Bob,



I enjoyed reading your recent discussion of paralleling batteries.



Instead, I'm wondering about converting one of my two alternators to 24 

volt (28 volt, whatever nominal voltage you prefer!) to charge two 12 volt 

batteries in series.  That will allow me to install the air conditioner 

(!) I want, which only comes in a 24 volt version.  I'd leave the ship at 

12 volts, and power those loads from the inter-battery connection.



I'd connect the remaining 12 volt alternator to that "lower" battery as well.



Let's say the 12 volt battery is offline and the 28 volt alternator is 

cranking away... the "upper" battery will eventually get fully charged... 

the upper batter will then just be a wet piece of wire (kind of) to the 12 

volt load?  Or would there be a problem with the lower battery not getting 

charged enough, or the upper batter overcharged?



Paul, making my head hurt mode


	


    This is an intuitively attractive idea but

    fraught with many potholes in the physics

    of implementation.



    The only time that a battery benefits the

    operation of the 28v system is to support

    inrush currents for getting the compressor

    motor going. If the 28v alternator only

    runs the AC compressor motor, then I'm thinking

    you could craft a controller/regulator that would

    let the alternator drive the motor directly without

    a battery. This pre-supposes that the compressor

    motor will soft-start at torque levels that

    do not exceed the upper limits of the alternator's

    magnetics.



    I think this is the case. It would be relatively

    easy to demonstrate on a drive stand before

    going after the controller/regulator design.



    Given that you have a mechanical connection

    to the engine to drive a dedicated alternator,

    the obvious elegant solution is to drive the

    compressor from this location and eliminate

    an alternator, a regulator/controller, a

    compressor drive motor and possibly a second

    battery.



    Believe me, we pondered this question multiple

    times at Beech for about 40 years . . . and

    the slickest way to do it is put the compressor

    right on the engine.



    Bob . . .



        ----------------------------------------)

        ( . . .  a long habit of not thinking   )

        ( a thing wrong, gives it a superficial )

        ( appearance of being right . . .       )

        (                                       )

        (                  -Thomas Paine 1776-  )

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

jmcburney(at)pobox.com
Guest

Posted: Sun Feb 24, 2008 12:13 pm Post subject: Batteries in Series

Paul (and List),



Drawing 12v from the center of a 12+12v string won't work, at least not for

long.  We tried that on an MCI bus at our church -- a small 12v load

FLATTENED the battery it was connected across, even though the 28v regulator

was holding solid and the other battery was fully charged.  We finally found

a battery equalizer from MCI and it solved the problem.  See

http://www.powerdesigners.com/powercheq.htm for an equalizer suitable for a

plane.



Blue skies and tailwinds



Jim

CH-801

DeltaHawk diesel

Augusta GA

90% done, 90% left

paulmillner

Joined: 09 May 2007
Posts: 21
Location: Berkeley, California

Posted: Sun Feb 24, 2008 12:50 pm Post subject: Re: Batteries in Series

>> fraught with many potholes in the physics of implementation



Bob, 

I see Jim's note about the failure of a series battery loaded at the center tap.  What are the physics that allow the series to appear to have 28 volts while the lower battery is flat?



Paul

_________________
Paul Millner, Berkeley CA [OAK]

paulmillner

Joined: 09 May 2007
Posts: 21
Location: Berkeley, California

Posted: Sun Feb 24, 2008 12:53 pm Post subject: Re: Batteries in Series

>> We finally found a battery equalizer from MCI and it solved the problem. 



Not sure what an MCI bus is, Jim, but thanks for the data point and the point out of the equalizer site.  Looks like the equalizer can handle 2 amps, so if your tiny load is that or less, on average, the lower battery can stay charged... but I'm not sure I understand how the series battery could appear to be charged to 24 volts with the lower battery flat?



Paul

_________________
Paul Millner, Berkeley CA [OAK]

schaefer(at)rts-services.
Guest

Posted: Sun Feb 24, 2008 2:03 pm Post subject: Batteries in Series

As a battery get's old it's internal resistance increases.

The bad battery is just going to get warm when you apply the 28 volt across

both batteries. It will drop the voltage as an I*R loss. 



--

paulmillner

Joined: 09 May 2007
Posts: 21
Location: Berkeley, California

Posted: Sun Feb 24, 2008 4:47 pm Post subject: Re: Batteries in Series

>> As a battery gets old its internal resistance increases.

The bad battery is just going to get warm when you apply the 28 volt across both batteries. It will drop the voltage as an I*R loss. 



Isn't that EXACTLY what you want to see happen?  Drop 12 volts across the "upper" battery to impose 12 volts on the lower battery that's feeding a 12 volt load.



Is it your position, then, that this would work with new batteries but not with old batteries?  That doesn't seem intuitive.



Paul

_________________
Paul Millner, Berkeley CA [OAK]

rjquillin

Joined: 13 May 2007
Posts: 123
Location: KSEE

Posted: Sun Feb 24, 2008 6:29 pm Post subject: Batteries in Series

At 15:38 2/23/2008, you wrote:

 	  Quote: 	
		  Let's say the 12 volt battery is offline and the 28 volt alternator 

is cranking away... the "upper" battery will eventually get fully 

charged... the upper batter will then just be a wet piece of wire 

(kind of) to the 12 volt load?  Or would there be a problem with the 

lower battery not getting charged enough, or the upper batter overcharged?


	


I'm not sure I've seen mentioned just what type battery is in use.



But I suggest, at least for a flooded cell battery, that each cell is 

isolated from it's neighbors, that is it is a single ~1.5 volt 

unit.  If that premise is accurate, just how is a charging system to 

know the physical packaging of the 12 cells it is charging?  One 

package x 12 or two packages x 6 each...  Dead, weak or otherwise 

'different' cells are just that, no matter where in the series of 

cells they occur, and the charging system couldn't care less.



Ron Q.

paulmillner

Joined: 09 May 2007
Posts: 21
Location: Berkeley, California

Posted: Sun Feb 24, 2008 7:42 pm Post subject: Re: Batteries in Series

>> Dead, weak or otherwise 'different' cells are just that, no matter where in the series of cells they occur, and the charging system couldn't care less. 



The difference, Ron, is that we're serving some 12 volt load from the "middle" of the battery... which is a bit of a cell equalization challenge.



Paul

_________________
Paul Millner, Berkeley CA [OAK]

schaefer(at)rts-services.
Guest

Posted: Sun Feb 24, 2008 10:14 pm Post subject: Batteries in Series

When every you try to charge two batteries in series that are not similar

(Size, Capacity, Discharge State, Overall Condition ... all the same) you

often get improper charging. This is because the current needed to charge

each battery to the charged level is different, but since they are in series

they will be getting the same current. As you try to pump more current

through the good battery you get a full 12V from the good battery plus it's

IR loss which will cause a lower than needed voltage on the battery that

needs to be charged. Hence it will deteriorate over time. 



--

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