Re: [pvusers] Dual voltage systems mind teaser

["David & Sheila Knapp" <solar@aeroinc.net>]

That's 20 amps per radio, 3 radios, and 50 amps per amplifier, two
amplifiers.  It's not always that they are all transmitting at the same
time, but with three ham radio nerds in the family, it is certainly
happening some of the time from our personal experience.  Most of the time
it will be just twenty amps peak, but bouncing reliable signals off ionized
meteor trails requires quite a few solar electrons. ;-)

I still think it is far better to let the battery be a buffer and let a 20
amp regulator with slightly smaller wire catch up over a few minutes than to
try to provide two pairs of 4/0 cable over 25 feet. Lots of EMI in the
larger regulators too.  It's kind of how HP added their extra 12 volt
battery as a buffer down the line although they weren't changing voltages,
just trying to prevent one from sagging too much.

;-)

Dave

----- Original Message -----
From: "Cor van de Water" <cor_van_de_water@dolfijn.nl>
To: <solar@aeroinc.net>
Cc: <pvusers@crest.org>
Sent: Saturday, January 19, 2002 5:01 PM
Subject: Re: [pvusers] Dual voltage systems mind teaser


Dave,

The basics of your problem is not different from an EV (Electric Vehicle)
Which has one main battery bank of around 100V and needs the 12V too.

Usually the smaller 12V battery can be omitted completely and all
12V equipment runs off a DC/DC converter, drawing from the main battery
bank and delivering 12V.

You say, you need 20 Amp+ so this is 240W+ and not a big deal for a
good DC/DC converter.

I suggest you forget the 12V battery and all the headaches that come
with installing a dual voltage battery system; Wire your house and
shack for 24V and have a good DC/DC converter (or two) supplying the
12V when you need it. When your leave the shack; simply turn off the
converter if the 12V is not needed any more (unless for a phone, thats
why you may go for 2 converters)

Success designing,

Cor van de Water
the Netherlands
http://start.at/solar

> Originally we had specified two separate systems for our new solar home.
The
> main house system would be 24 volts nominal and the office/ham radio
system
> would be 12 volts nominal.
>
> I specified the main house system to be 24 volt because the daily load is
> about 3,000 W-H.  The separate office/ham radio system came about because
we
> have quite a few devices that consume 12 volt nominal power much more
> efficiently that using an AC power supply to convert back to low voltage
DC.
>
> Although the ham radios run off of what is called 12 volts, their best
> efficiency and ultimate happiness rests with running them at 13.5 - 13.8
> volts (a vehicles charging voltage).  Several other appliances including
our
> cell phone charger, answering machine, DC charger (AAA, AA, C, D cells,
> etc.), and a citizen's band radio for the local emergency network runs
well
> from it too.
>
> Initially I specified 300 watts of PV and 415 A-H of sealed batteries at
12
> volts for the office system.  The main house system would use ~1500 watts
of
> PV and a large bank of Rolls batteries located in the utility area of our
> solar earthship. The smaller 12 volt nominal office system with the sealed
> batteries would be located close to the ham radios in a Rubbermaid
container
> for a minimum voltage drop.
>
> It seemed like a workable solution until I examined the charging cycle
over
> a 24 hour period.  The 12 volt nominal system would use MPPT charging to
> bring the sealed batteries up to 13.8 volts during the day when the sun
was
> shinning and allow it to drop to resting voltage at night.  Most of the
> constant loads were small, but did add up over 24 hours.  The larger loads
> from the ham radios (up to three hundred watts on transmit) could
potential
> bring the battery voltage below 12.0 volts at night when the radios would
be
> used the most when the charging would already be finished for the day.
> Although the battery voltage would come back up after each transmit cycle,
> the radios are not very happy working down there.  They much prefer the
> automotive standard of 13.5 volts.  I have heard many backwoods radios
emit
> distortion when running from an RV battery during extended operation as
the
> battery voltage sagged.  Even during the day, the peak charging is only
~13
> amps (plus any MPPT boost). The ham radios draw 20 amps on transmit plus
the
> smaller constant loads.  I briefly considered using AC power supplies for
> the larger radios, but did not want to add an extra inverter and was
> bothered by the efficiency loss of so many power conversions.
>
> I then considered using AC from the main house inverter, but then I knew I
> was stealing power away from the house system and letting it go to 'waste'
> in the office system.  Then an idea was born, why not combine the two
> systems into one bigger system.
>
> The new design called for 1800 watts of PV wired up for the main 24 volt
> nominal system in two array's feeding a couple of MPPT charge controllers
to
> a Roll's 1,900 A-H battery bank.  There are a several 24 volt nominal DC
> loads (booster pump, emergency DC lighting, refrigerator, etc.), but the
> majority of the DC loads are for the office and require 12 volts nominal.
>
> I had the idea of using another MPPT charge controller to convert the 24
> volt nominal to 12 volt nominal and to use a small 210 A-H 12 volt battery
> bank as a buffer.  It would do this at 97% efficiency and I could
precisely
> control the output voltage 24 hours a day as long as I stayed under the 50
> amp limit of the charge controller (which was no problem).
>
> The big problem I have run into so far is have both systems grounded to
the
> same point.  This arises from the negative wires on both systems being
> grounded as required by code.  This effectively messes up the function of
> the MPPT controller in the office system which requires an isolated input.
> The ham radio system has to be negative grounded as all of their chassis
> connections are connected to the negative wire as also are all antenna
> shield connections and all antennas.  Even using a Trimetric meter with an
> external shunt to monitor the charge status of the office battery system
> won't work if I understand its operation properly.
>
> The NEC as adopted in Colorado requires a ground on all current carrying
> wires that aren't fused, yet one side must be grounded if I understand the
> NEC code properly.  Further, by having a common negative ground as I
> described it, I have set up the potential where the ground could carry the
> current intended for the negative wire because of the loop I had now
formed.
> Changing the MPPT controller to a voltage regulator (24 to 12 volts) seems
> to not change anything unless I go to a regulator that has a common
negative
> for both input and output.  At that point, using a shunt on the buffer
> battery appears to suddenly seem like that it will work okay.  It is only
in
> the controllers that require an isolated input that dual voltage systems
get
> into trouble.
>
> I am only giving up some efficiency in the charge regulator of the
> unisolated input negative type unless I can find a higher quality one that
> is using high frequency DC switching. It looks like the mechanics of it
will
> work, now I just have to convince the CO state code inspector that the
> smaller ham radio system subset is a legal system with regards to being
able
> to shutdown the main system and the second smaller system having it's own
> battery disconnects (as well as the 24 volt breaker feed at the main
> system).  It is impossible and impractical to install it all right at the
> main system within six feet of each other as the office system is some 25
> feet away located right at the point of use.
>
> Once I resolve all of these issues, I'll have to document it.  I cannot
> believe there already has not been an article in HP about similar
> circumstances such as these.  As more people mainstream into RE, it is
> likely that this will be encountered again and again.  A non-gadget guru
> would not understand the problems they would face in this scenario.  Even
> several dealers called it unusual as well as the MPPT charge controller
> manufacturer who suggested we simply leave the 24 volt system ungrounded,
> which I am not about to violate code or go to the expense of adding dual
> breakers for all 24 volt negative and positive wires in a system that
daily
> consumes 3+ KWh.  I'm sure there are some wrenches out there that might
have
> solved a problem or two like this and still kept the code inspector happy.
>
> ;-)
>
> Dave Knapp
> http://www.esolarliving.com
>
>
>
>
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