Voltage drop musings [RE-wrenches]

Fri May 4 20:49:20 PDT 2001

<x-flowed>
Hi William,

Thanks for your input.  It is fun to toss this stuff around.  It seems that 
musings lead to more musings.

Current is what charges batteries, not voltage or total power.  If you have 
maximum power point tracking,  then the voltage difference between the 
array voltage and battery voltage can be transformed into charging 
current.  Without it, the extra voltage represents power that will be 
dissipated as heat.

The charge from the solar array is reversing a chemical reaction that took 
place in the batteries during discharge.  It is moving electrons.

In a standard deep cycle battery, the chemical reaction involves a transfer 
of  valence electrons between plates of lead peroxide and sponge 
lead.  Instead of the electrons transferring directly in the solution, they 
are conducted through a wire to complete the reaction.  During charging, 
the electrons are forced back to their original position.

It is the electrons that are forced through the battery that results in the 
charge.  An ampere is a measure of the charge, also described as the number 
of electrons that pass a given point in a  second.  An "amp" is equal to 
one coulomb / second, or 6.242 X 10^18 electrons / second.  It is the 
forcing of these electrons back to their original plates that results in 
the batteries charging.

Voltage is essentially electrical pressure.  You need sufficient potential 
to force electrons through the battery, but an excess amount is not helpful 
with a current limited device such as a PV module.  You can charge a 6 volt 
battery with a 12 volt solar module.  It will charge at the module's 
amperage, not the module's wattage.

If you are using a standard charge controller, the voltage between battery 
voltage and array voltage represents power that is not utilized.

A little higher voltage drop will slow the current down but a very small 
amount.  The only time the  voltage drop will have a negative effect is 
under conditions when the array is capable of  putting out maximum 
amperage.  That will only happen when the batteries can accept it, and the 
sun is at the best charging angle.

A system doesn't lose a percent of its monetary worth for each percent of 
voltage drop in the feed from the array.  A nominal 24 volt system may have 
an array voltage of 34 volts (Siemens SP 75s).  A 3% voltage drop will take 
away 1.02 volts, leaving 32.98 volts to charge the battery bank.

32.98 volts is sufficient to push 1640 amps through 100 feet of #2 copper 
wire.  The line loss in the feed from the array won't slow down the 
charging much.  The current output of a PV module is limited by the 
module's current producing capability, and not directly subject to I = V/R.

To fill in the equation for the total resistance, the resistance of the 
battery bank and charge control need to be taken into account, as well as 
the drop across fuses and switches.

To figure what percent of loss to the system monetary value a given drop in 
charging represents, all inefficiencies must be taken into account, 
especially inverter losses.

I try to stay under 3% VD.  I saw one system operate satisfactorily with a 
10% VD.  I sure wouldn't have gone that high.  As I recall, it had high 
voltage modules, and coils of number 10 wire laying on the ground.

With the advent of MPPT technology, lower voltage drops may become more 
important.  I was very impressed with the output a Solar Boost unit we 
installed last year.   If the meters were correct, the batteries were, at 
times, getting close to the array's rated wattage.

Drake

At 10:03 AM 05/04/2001 -0700, you wrote:
>Drake:
>
>I must not have articulated very well the whole point of this dilemma.
>Plus, I have to respectfully disagree with some points you have made:
>
>Due to the cost of purchasing and installing PV equipment, I believe we
>need to build systems that are much more efficient than the NEC would
>require.  Consider that a standard panel, say a Siemens SR100, costs as
>much as $580, and a 12 panel tracking rack costs about $1,750.00.  Add in
>the cost of an excavation contractor, 6" steel pipe, concrete and labor.
>The cost of installing 2400 watts of PV could be as high as $22,000 (at
>least here in California).  If you design a system with a 5% voltage drop,
>you are wasting almost $1,100.00 in installed PV costs in heating your
>undersized wire.  In other words, 5% of your PV output never reaches the
>charge controller.
>
>If, however, your wiring is sized for 1% voltage drop, you are consuming
>only $216 in installed PV costs.  All of these figures are based on your
>system is operating at rated output.
>
>The point of this discussion is to attempt to determine how often the
>system is operating at rated output.  If the system never operates at rated
>output, then designing for 1% may well be over-kill.
>
>Others have spoken about winter charging as being no less robust than
>summer charging.  While we do not have snow in my region, the description
>of some of the scenarios lead me to imagine a situation many of my clients
>may experience:  Our winter is really a rainy season.  The storms come in
>waves from the southwest.  Between storms we can have one or more sunny
>days.  I can imagine that after the rain washes the air clear, and with the
>ambient temperature cooler, the panels may charge closer to rated output
>than in the summer.  And after days of rain, my clients my need all of the
>charge they can possible get in a short time period.  This is leading me to
>believe that a design with a 1% voltage drop may be justified.
>
>You also mention that, "Current charges batteries, not voltage."  I have to
>disagree.  Power charges batteries.  Power is the factor that does any type
>of work, be it create light, lift water or create a chemical change in a
>battery (charging).  Power has two equally important components: voltage
>and current.  If you are heating undersized wire, you are losing power.
>
>True, in a series circuit, current remains the same in all legs.  But if
>you drop voltage in your wire, you need more current to create the same
>power.  These losses are know as "I squared R losses."
>
>I thank all of those who commented on these questions.  I value my
>connection to those who do this same work and wrestle with these same
>dilemmas.
>
>
>William Miller
>
>
>At 05:15 PM 5/1/01 -0600, you wrote:
> >You don't need to limit voltage drop to 1%.   Various Code interpretations
> >would limit it to no more than 2 or 3%, or even 5%.y.
> >
> >The difference in current will not be
> >great.   Current charges batteries, not voltage.
> >
> >Drake
> >
>
>
>__________________________________________________________________
>William Miller
>SLO Communications: Communications and Power Systems Consulting
>PO Box 50, Santa Margarita, CA 93453
>Voice :805-438-5600             Fax: 805-438-4607       VMail: 805-546-4875
>email: wrmiller at slonet.org
>License No. C-10-773985
>_____________________________________________________________
>Compatibility:
>Word processor: WP7
>Spreadsheet: Quatro Pro 7
>CAD: Microstation 95, DXF, Visio 4.1T
>_____________________________________________________________
>"I stand by all the misstatements that I've made." Dan Quayle
>__________________________________________________________________
>
>- - - -
>To send a message: RE-wrenches at topica.com
>
>Archive of previous messages: http://www.topica.com/lists/RE-wrenches/
>
>List rules & etiquette: http://www.mrsharkey.com/wrenches/etiquete.htm
>
>Check out participant bios: www.mrsharkey.com/wrenches/index.html
>
>Hosted by Home Power magazine
>
>Moderator: michael.welch at homepower.com
>

- - - -
To send a message: RE-wrenches at topica.com

Archive of previous messages: http://www.topica.com/lists/RE-wrenches/

List rules & etiquette: http://www.mrsharkey.com/wrenches/etiquete.htm

Check out participant bios: www.mrsharkey.com/wrenches/index.html

Hosted by Home Power magazine

Moderator: michael.welch at homepower.com

==^================================================================
EASY UNSUBSCRIBE click here: http://topica.com/u/?bz8Qcs.bz9JC9
Or send an email To: RE-wrenches-unsubscribe at topica.com
This email was sent to: michael.welch at homepower.com

T O P I C A  -- Learn More. Surf Less.
Newsletters, Tips and Discussions on Topics You Choose.
http://www.topica.com/partner/tag01
==^================================================================

</x-flowed>