Best Watt per panel

[zed]

...

[equinox]

Zed - you are way ahead... currently i cannot even charge the battery... I want to use it to power up my 200W water pump which run about 1 to 2 hours a day
White_Wolf - so in my case 100AH/12 = 8.4A, 100AH/10 = 10A, 100AH/8 = 12.5A.
8.4A x 18V = 151.2 W..
Amphour capacity C12 --> does it means the capacity of the battery used in 12 hours?

[zed]

Equinox-- you can't ask these questions and get a sensible answer until you specify how many amps per day the battery needs to supply.

You then multiply the amps that the panel can supply by the hours of sunshine in a day.

The trick is to make sure that the battery gets fully charged each day...thats what dictates the number of watts of the panel.


So...what is it being used for?

[Wy_White_Wolf]

Sorry I missed your post... so what is the formula for battery AH and required charge current... btw my battery is 12V 100 AH

For FLA batteries your charge rate should be between C/12 and C/8 with C/10 usually being ideal. C = the AmpHour capacity of your battery. So for a 200AH battery:

200AH/12 = ~16.6A Minimum Charge rate
200AH/8 = 25A maximum Charge rate
with
200Ah/10 = 20A ideal rate

To get the array/panel size when using a PWM controller in watts multiply the above numbers by the VMP (Voltage Maximum Power) of the panel. So using standard 12V panels:

16.6A*18V = 298.8 watts minimum
25A*18V = 450 watts maximum
20A*18V = 360 watts ideal.

WWW

[Wy_White_Wolf]

20 cells, 4.6 Watts per cell
my problem is the my controller is not showing any panel.. insufficient voltage I assume

You need to get your terms straight.

Cells only produce about 0.5 volts per cell. So your 20 cell panel can only produce 10 volts. That's not high enough to charge a 12 volt battery. For a 12 volt battery you need to use 36 cells.

WWW

[equinox]

No. It all depends on the size (in AH) of the battery.
If you have a 50AH 12V FLA battery, then a 50A panel is fine.
For a 200AH 12V battery a 100W panel is too small.

Sorry I missed your post... so what is the formula for battery AH and required charge current... btw my battery is 12V 100 AH

[zed]

sensij : Got it... so what is the range of allowable/identifiable voltage to go through a 30A PWM controller?
zed : "Most charge controllers need at least 3 volts more going in than coming out...so your problem is you need more cells to get a bigger voltage into the charger." ... I got 10V and the solar panel icon does not lit.

Listen carefully....you cannot charge a 12v battery with a 10v solar panel.

You need at least a 15v solar panel.

[equinox]

Thank you Sensij, I believe newbie with limited budget like me should be aware of this and not building panels with less or too much voltage to produce.
Is there cells that produce more than 0.5 Vmp? if the answer is no, then 12V / 0.5 = 24 cells which is ~120 Watt panel is the minimum requirement... adding another 80 Watts to my existing 80 Watts would be a waste, but do I have an option??

[sensij]

If you were to buy a panel to charge a 12 V battery, it would have a Voc of 21-22 V, and a Vmp of 17-18 V. That would be a good target. If you go higher, the panel will get less efficient, so you won't get as much power out of it. If you go much lower, on hot days, you'll risk dropping the panel voltage below the charge voltage and you'll see the same thing you're seeing now.

[equinox]

sensij : Got it... so what is the range of allowable/identifiable voltage to go through a 30A PWM controller?
zed : "Most charge controllers need at least 3 volts more going in than coming out...so your problem is you need more cells to get a bigger voltage into the charger." ... I got 10V and the solar panel icon does not lit.

[zed]

Thank you guys...
So my conclusion is that one should use at least 100W panel to be able to charge a 12V battery.. am I correct?

Nope...you can charge a 12v battery with a one watt panel...it!s not the watts its the volts that are your problem.

Most charge controllers need at least 3 volts more going in than coming out...so your problem is you need more cells to get a bigger voltage into the charger.

[sensij]

Thank you guys...
So my conclusion is that one should use at least 100W panel to be able to charge a 12V battery.. am I correct?
@Sensij... just to my curiosity.. why is your 475W panel not working with a 30A PWM?

I think you are assuming all panels are constructed from cells that are identical to yours. For cells that produce 4.6 W at 0.5 Vmp, then you should be really looking at building panels of at least 150 W to charge a 12 V battery. Many panels available commercially are able to charge an appropriately sized 12 V battery with 100 W or less, because even though the cells in those panels might still be 0.5 Vmp (so 36 of them are required), they produce less wattage per cell.

The 475 W panel in my example has a much higher Vmp, 70 V or more, which is not appropriate for a 12 V battery (with a PWM charge controller).

[inetdog]

Thank you guys...
So my conclusion is that one should use at least 100W panel to be able to charge a 12V battery.. am I correct?
@Sensij... just to my curiosity.. why is your 475W panel not working with a 30A PWM?

No. It all depends on the size (in AH) of the battery.
If you have a 50AH 12V FLA battery, then a 50A panel is fine.
For a 200AH 12V battery a 100W panel is too small.

[equinox]

Thank you guys...
So my conclusion is that one should use at least 100W panel to be able to charge a 12V battery.. am I correct?
@Sensij... just to my curiosity.. why is your 475W panel not working with a 30A PWM?

[inetdog]

Yes. so what are you suggesting.. serial another 80 W panel or 20W is sufficient?
What would be the drawback of mixing panels of various watts.

When you put two panels in series their current ratings need to be similar.
When you put two panels in parallel their voltage ratings need to be similar.
Power = Volts times Amps, so just knowing the power of the panel does not give you enough information.

In your case if you use the exact same type of cell to make another panel, it would need to be at least 10 cells to bring the voltage up to 15V when you put the two panels in series. They will then both produce the same amount of current, since they use the same size cells.
Even better would be 16 more cells, and just making another identical 20 cell panel would not be much of a waste.