T-Pain Is On A Boat: Re: [Electric Boats] Re: Solar charging question: Panel voltage vs. battery bank voltage?

I would like to weigh in on this topic, but my take may raise some eyebrows.

solar panels are like batteries. The voltage per cell is based on the tec used
to produce it. Sticking with the standard configurations render some basic
voltage and power. 12 volt panels are not really 12 volts. 18 or 19 volts is
the true voltage open circuit. 24 volt panels have roughly 36 volts open
circuit. For my 24 volt battery bank the panel open circuit is 49.6 volts.

A lead acid battery of 12 volts can be charged continuously at 14.4 volts.
Modern cars electrical system charges and maintains the above 14.4 volts and
that is why I gave that figure. Reasoning that a 12 volt system is safe with
14.4 to obtain a safe charge for 24 volt systems we can double the 14.4 volts
and charge at 28.8 volts. as we increase the system voltage the same will
apply. We can call this the charge maintaining voltage for the system.

As far as panels go the number of cells on the panel equal the panel voltage.
The size of those cells determines the amperage at the panel voltage. Ideally
we could avoid a charge controller if we sized the panel VOC (Voltage open
circuit) to the desired charging voltage. It might be a good time to state
that unconnected panels (my panels) vary little on voltage when light is on
the panel no matter what time of day or cloud conditions. When connected in
circuit the voltage will fluctuate depending on battery bank state. This
condition has a lot to do with resistance and ohms law.

Believe it or not I can connect my 49.6 volt panels up to a 12 volt battery
that need a charge and it will charge that battery. You must watch the voltage
and the power is quartered in the process.

The point is this. If You built your own panels and matched the voltage to the
charge requirements of your lead acid batteries you could do away with charge
controllers. If you take different voc panels and connect in series the voltage
is equal to the two panel VOC added together. You must use panels that have
the same type and size of cells if you do this.

If you would rather use pre-fab systems and cells keep sizes the same. If you
don't your system will produce at the smallest panel capability. PWM
controllers will offer less than full wattage rating of the panel based on the
battery bank voltage. On the other hand MPPT controllers use buck boost
methods to change the voltage from the panel to match that needed to charge
the battery bank. This conversion is almost as efficient as a transformer.
Design limits of buck boost converters have design limits. For a 24 volt
system you should not consider panels with a VOC over about 55 volts. With
these specs you can consider losses around 15% of the panel wattage. This is
much better than the losses of 50% you can expect from a PWM controller, if
you can even use a PWM controller for such a system.

About MPPT controllers. Panels have a value called maximum power. If this
voltage level is maintained the panel is more capable than it is any other
voltage, fact is it is this point that wattage is rated at. MPPT systems are
designed to maintain this panel voltage as well as offer up best charge voltage
for the bank. The algorithm is more concerned about the voltage of the panel
than the charge of the battery. It is figured the best charge comes from
keeping the panel voltage at it's best design value.

One last point 12 volt inverters lose more power in conversion than larger
voltage systems. Resistance is our enemy in all things electrical. consider
larger voltages over 12 volts.

Kevin Pemberton

On Tuesday, May 19, 2015 09:25:10 AM you wrote:
> Capt Carter,
>
> This is a little off topic as it doesn't involve electric boats, but it
> is near the water! I've got some property on an island in St Thomas
> harbour. In the future we're going to put up a prefab round house on the
> property. In the meantime, we're camping there for a week or so at a time.
> We've been using a goal zero camping battery/solar panel setup. We need
> more power, so I figured I'd do a homebrew system to leave there. Shipping
> to there is expensive, so anything we take needs to fit checked baggage
> limits. So that limits me to at the most 100w panels, although 50w would be
> easier. I'd like to eventually have about 400w of panels total, with two
> Trojan golf cart batteries (we can buy those there at reasonable cost).
> The goal is to run a small chest freezer with a beer brewing thermostat to
> make it into a refrigerator. Let's say that I figure out how to get the
> 100w panels out there, would I wire two strings of two in series for 24v
> each string? The wiring runs would be minimal since I can put the panels
> almost right next to the controller and batteries, although from what I've
> been reading if you are forced to make runs of over 10 feet or so the
> voltage drop forces some fairly large wires, so I agree with you the higher
> voltage the better. Is it possible to wire them all in series for 48
> volts? I'm just wondering what the best way to do it is.
>
> Thanks,
>
> Jerry Barth
>
>
>
> _____
>
> From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com]
> Sent: Monday, May 18, 2015 8:32 PM
> To: electricboats@yahoogroups.com
> Subject: Re: [Electric Boats] Re: Solar charging question: Panel voltage vs.
> battery bank voltage?
>
>
>
>
>
> Kenneth,
>
>
>
> The mppt (maximum power point tracking) charge controller will provide the
> maximum possible output of the solar panel by regulating the power to
> batteries at the optimum voltage for charging, thus achieving the maximum
> amperage.
>
>
>
> So basically if you have a 100W, 24V panel it will regulate the output to
> 12V and still give you 100W, less a few percent conversion loss, or
> approximately 8 amps. Sounds a bit like magic but it really works.
>
>
>
> Also consider that your typical 12V panel puts out about 18Volts in the full
> sun. When you hook it up to a 12 volt panel with a pwm or other type of
> controller it will be forced to operate at 12V. If you look at the power
> curve for the panel, you will see that 18V is NOT the maximum power point
> and you are not getting all the energy that the panel can produce. So even
> using a nominal 12V panel on a 12V battery, an mppt charge controller will
> give you up to 30% more power from your solar collector.
>
>
>
> BTW, if you're feeding your electric motor directly with solar, the mppt
> charge controller will also dial in the optimum voltage that the motor needs
> and give you 30% more power too. It really does work, I've cruised almost
> 2000 miles on solar energy using it. MPPT controllers are well worth the
> extra expense.
>
>
>
> Capt. Carter
>
> www.archemedesproject.blogspot.com
>
>
>
>
>
> On Monday, May 18, 2015 7:45 PM, "Kenneth Reese kcr@kcrproducts.com
> [electricboats]" <electricboats@yahoogroups.com> wrote:
>
>
>
>
>
> Two 12v 50 watt solar panels... You are actually advising someone to
> charge a 12v battery with 24v x 4 amp (panels in series) vs using a 12v x 8
> amp (panel in parallel) ?
>
>
>
> On Mon, May 18, 2015 at 5:54 PM, Carter Quillen twowheelinguy@yahoo.com
> [electricboats] <electricboats@yahoogroups.com> wrote:
>
>
>
> Kenneth is right if you don't use an mppt charge controller but dead wrong
> if you do. There are several good reasons for going with a nominal 24V panel
> and an mppt charge controller.
>
>
>
> First, 24V panels are much cheaper. You can find them for as low as 50
> cents a Watt as compared to 12V panels that range from $1-$3 per Watt.
>
>
>
> Two, the higher voltage lets you use smaller wires.
>
>
>
> Three, an mppt charge controller will get you up to 30% more power from your
> panels than a pwm controller.
>
>
>
> The only downside to going with the standard 24V module is that they don't
> come in small sizes. They are all about 42"x56" or larger and can be a
> little too big to find a convenient mounting location on smaller boats. But
> if you've got the room to mount them, they are definitely the most
> economical way to go, even with the added cost of the mppt charge
> controller.
>
>
>
> Capt. Carter
>
> www.archemedesproject.blogspot.com
> <http://www.archemedesproject.blogspot.com/>
>
>
>
>
>
> On Monday, May 18, 2015 10:15 AM, fitloose <no_reply@yahoogroups.com> wrote:
>
>
>
>
>
> Well I'm afraid I must disagree with Kenneth. Using a 24V panel to charge
> 12v has benefits providing you are using a suitable MPPT solar charge
> controller. The easy way to see this in action is to experiment with this
> spreadsheet:
>
> http://www.victronenergy.com/support-and-downloads/software#mppt-calculator-> excel-sheet
>
>
>
> Note you can also add panels of your own choice in the custom field. Higher
> voltages will be more efficient in terms of losses, the higher voltage also
> means you will also start charging earlier in the morning and continue later
> in the evening. For more information about matching panels to MPPTs see:
>
> http://www.victronenergy.com/blog/2014/03/28/matching-victron-energy-solar-m
> odules-to-the-new-mppt-charge-regulators/
>
>
>
> If I had 12V batteries and say 2 x 12V panels of 50 Watts each I'd put them
> in series so 24V nominal @ 50 Watts instead of 12V nominal @ 100 Watts in
> parallel. Whilst the wattage for both (Volts x Amps = Watts) is the same for
> both the ones in series have less losses due higher voltage and less
> current. Try it in the sheet. Note most controllers will need to be +5V
> over battery voltage to start charging. That decides the number of cells
> you need in a panel to be sure you have a suitable voltage in the first
> place. Another reason to use higher voltages, plus the charge earlier,
> charge later i.e charge longer times overall as mentioned before.
>
>
>
> Personally I found the sheet a very useful tool, along with the graphs, to
> see what is happening.
>
>
>
>
>
> John