Help a newbie make sense of Panel Specs

keyter

Hi guys

I'm trying to do some planning to get a simple solar setup. But I need some help with some questions please.

Let us use this as an example of the specs on a panel:

Construction - Monocrystalline
Peak Power - 150W
Nominal Cell Efficiency - 18.6%
Dimensions - 1210x808x35mm
Voltage @ Max. Power - 22.80V
Current @ Max. Power - 8.35A
Open Circuit Voltage - 18.80V
Short Circuit Current - 7.98A
Tolerance - ±3%
Max system voltage - DC1000V
Cable Length - 800mm
Operating Temp. - -40°C to +85°C
Std. test conditions - AM1.5 1000W/m² 25°C
Manufactured in China
Certification - CE
Net Weight - 12.0kg

1) Can this panel be connected directly to a 12V battery (without a charge controller) and will it charge the battery? Will it Damage the battery? (Obviously i"ll have to check for over-charge manually)
2) What will happen with the voltage once connected to a controller (22.8V)? Does the controller step the voltage down to around 12/13V?
3) The Current rating of 8A. Will the panel charge a 12V battery at this rate, or is it 8A at 22V, thus 4A at 12V?

I ultimately need to know at what rate this panel will charge a 12V battery, and HOW do I work it out?
If I get a 300W panel, with 37V and 8A, will it be better suited (more effecient) to have a 24V battery bank than a 12V bank?

Thanks
Jak

BB.

Re: Help a newbie make sense of Panel Specs

Welcome to the forum Jak!

The specifications on your panel appear to be incorrect.

keyter wrote: »

Construction - Monocrystalline
Peak Power - 150W
Nominal Cell Efficiency - 18.6%
Dimensions - 1210x808x35mm
Voltage @ Max. Power - 22.80V
Current @ Max. Power - 8.35A
Open Circuit Voltage - 18.80V
Short Circuit Current - 7.98A

First, the Max Power voltages and current is swapped with Open Circuit Voltage and Short Circuit Current. Voc and Isc should be larger values than Vmp and Imp. If it is a simple swap, it should read:

keyter wrote: »

Construction - Monocrystalline
Peak Power - 150W
Nominal Cell Efficiency - 18.6%
Dimensions - 1210x808x35mm
Voc - 22.80V
Isc - 8.35A
Vmp - 18.80V
Imp - 7.98A

And it works out correctly with 150 Watt panel:

Pmp=Vmp*Imp= 18.80v * 7.89a = 150 Watts

1) Can this panel be connected directly to a 12V battery (without a charge controller) and will it charge the battery? Will it Damage the battery? (Obviously i"ll have to check for over-charge manually)

Yes, you can connect this directly to a 12 volt lead acid battery and charge it. However, without a charge controller, it will over charge the battery (assuming the battery bank is smaller than ~798 AH @ 12 volts or a smaller than ~1% rate of charge).

Once the battery is fully charged--Without some way to control the current from the solar panel (or dump excess energy), it will eventually over charge the battery bank (unless the loads keep the battery from over charging--But that is a bit of a balancing act I would not trust).

2) What will happen with the voltage once connected to a controller (22.8V)? Does the controller step the voltage down to around 12/13V?

For optimum charging of a 12 volt battery bank, the Vmp of the panel (or array) should be around 17.5 to 19.0 volts when using a PWM charge controller (the less expensive type of solar charge controller). Vmp drops as the solar panel gets hot (as in full sun on a warm day). And you have voltage drop in the wiring/charge controller. Plus, you need ~14.4 to 15.5 volts to fully/quickly recharge a deep cycle lead acid battery. Your panel will be just fine for a typical 12 volt battery installation.

Solar panels are (more or less) a current source. I.e., in full sun, your panel will output ~7.98 amps if the battery is dead (10.5 volts) or charging (~14.5 volts). It is how a current source works when charging a voltage source.

There are other types of solar charge controllers. MPPT (Maximum power point tracking) can use the higher voltage of the solar panel and give you more power to the battery bank:

PWM Controller Power:
7.98 amps * 10.5 volts = 83.79 watts into battery bank
7.98 amps * 14.5 volts = 114.405 watts into battery bank

MPPT Controller (ideal, no losses):
7.98 amps * 18.80 volts = 150 Watts
150 Watts / 10.5 volts battery battery = 14.3 amps into battery (charging "dead" battery)
150 Watts / 14.5 volts battery battery = 10.3 amps into battery (charging "near full" battery)

But MPPT controllers are not ideal--plus solar panels are not ideal either. In real life, the MPPT advantage with a "12 volt solar panel" are not really that great. And you should never take a deep cycle battery to 10.5 volts--It will ruin the battery bank.

MPPT controllers are more expensive than PWM, and where MPPT controllers are really useful is for larger solar array and higher voltage panels/arrays (Vmp>>18 volts). If done correctly, you can use less expensive large format solar panels (>200 watts) and use a higher voltage array (upwards of 100 Volts Vmp-array) and use much smaller diameter copper wire for long wire runs from array to charge controller/battery bank.

3) The Current rating of 8A. Will the panel charge a 12V battery at this rate, or is it 8A at 22V, thus 4A at 12V?

More or less, in full sun, the solar panel will output ~8 amps from 0 volts to ~15 volts (technically 18.80 volts under standard test conditions--which is rarely actual operating conditions).

I ultimately need to know at what rate this panel will charge a 12V battery, and HOW do I work it out?

Yes, it will work fine for a ~80 AH to 160 AH @ 12 volt battery bank with either a PWM or MPPT type solar charge controller. If you want to charge a 24 volt battery bank, you will need to put two of your 150 Watt panels in series for Vmp-array~37.6 volts.

If I get a 300W panel, with 37V and 8A, will it be better suited (more efficient) to have a 24V battery bank than a 12V bank?

Are you sure of your panel's Vmp and Imp? There are few (if any) 300 Watt 37 Vmp / 8 Imp panels. Now--If you have two of your 150 Watt panels in series--Yes, that would be fine.

Depending on the exact brand/model of MPPT charge controller--You can put more of your 150 Watt panels in series for higher Vmp-array voltage with an MPPT charge controller--Which will efficiently take high voltage/low current array and "down convert" to low voltage / high current needed to charge the battery bank. You would put upwards of 5 panels in series for:

Vmp-array = 5 * 18.8 volts = 94 Vmp-array

Note--We have a lot more to discuss regarding Vmp-array/Imp-array and the type/brand/model of solar charge controller--It is a bit complex the first few times you go through the design exercise.

In general, you need to know your loads (amps/watts, hours per day you run the loads, amount of sun you get, voltage needed for battery bank, type of loads/load ratings, etc.) first. Then design the battery bank. And lastly the solar array. After you have the paper requirements, you can look for charge controllers, AC inverters, etc. needed to support your loads.

And for solar, conservation is key. Solar power is relatively expensive (panels, charge controller, AC inverter, wiring, battery bank) and you need to replace the batteries every ~3 to 8 years or so--And new electronics every ~10+ years. Over-sizing the system is expensive to build and maintain. Under-size a system will not do what you need.

-Bill

keyter

Re: Help a newbie make sense of Panel Specs

Thank you very much for your effort Bill.

I now have a much better understanding of how the panels work.

Just to confirm, in your opinion, the PWM controller on 12v systems is not really worth the extra cost?

Thanks again,
Jak

Photowhit

Re: Help a newbie make sense of Panel Specs

keyter wrote: »

Just to confirm, in your opinion, the PWM controller on 12v systems is not really worth the extra cost?

Backwards again! A MPPT charge controller for a small 12 volt system, with a single panel, would not be worth the extra cost, if you have true 12 volt nominal panels available, such as the panel described.

Instead of trying to get an answer for a question that is poorly defined, why don't you tell us what your needs are? Like;

My Needs Are wrote:

I want to charge a single 12 volt battery, a Trojan T-1275 (note many new people try to use the wrong types of batteries for deep cycle use, car batteries and even 'so call' deep cycle marine, are a waste of money!)

I will use it to power an 300 watt inverter, that will run a single light (wattage) and a laptop computer(wattage) for 3 hours a night.

It will be used in coastal Washington mostly in the winter. (a lot of difference between Washington in the winter and Arizona in the summer!)

I want to use a 150 watt solar panel with these specs;
Voltage @ Max. Power (VMP)- 18.80V
Current @ Max. Power (IMP) - 7.98A
Open Circuit Voltage (VOC)- 22.80V
Short Circuit Current (ISC) - 8.35A

Would this work for me and what additional equipment would I need?

additional information like;
Distance from panel to battery bank, Bank to load
Where your battery will be stored
Time the system will not be attended, like a winter vacation cabin

...and the might's and want's...

I might add loads, panels, batteries
I'd like to power my cabin, house, goldfish pond, fountain...