# Determining the Correct Panel/Solar Regulator/Battery Combination?

Discussion in 'The Projects Forum' started by mystic, Sep 6, 2016.

1. ### mystic Thread Starter New Member

Sep 6, 2016
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I got a few mixed responses, but basically I am trying to find out what is the formula to determine the correct solar panel, solar regulator and battery combination. Right now, I only know the the voltage of the regulator must equal the battery voltage. I am not sure about the wattage required for the panel and also for the required current for the regulator and battery.

For example, I was asking what was the combination for a circuit whose power supply was 5 V/1 A. I want to power this circuit for about 10-12 hours/day. The responses I got were:

1. 15W panel (voltage not specified)/12V+5A controller/12V+10A battery
2. 20W/12V panel/12V+5A controller/12V+10A battery
3. 10W panel (voltage not specified)/12V+5A controller/12V+10A battery
4. 5V+1A panel / 6V+2A controller/3.7V+4A battery (??? this one made the least sense to me)
Do any of these look like the correct one? I would like to also know that if I were given some other power supply voltage and current requirement, how would I determine the correct combination?

Another question I have is that if my circuit needs 5V, would a 12V battery fry it or will it compensate?

2. ### ronv AAC Fanatic!

Nov 12, 2008
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How many days of no sun do you want it to work?

3. ### ronv AAC Fanatic!

Nov 12, 2008
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Lets do a napkin calculation so you don't need to wait for a rough estimate. Your load is 5 watts. Lets say 10 hours -- 50 watt hours.
The battery @ 10 amp hours is 120 watt hours... But you should only discharge it 50% so 60 watt hours. One days worth.
So if there is 10 hours of light a day the I would use at least a 10 watt panel since they only put out rated power when in the Arizona sun at high noon.
Adjust the battery and solar panel for cloudy days.

4. ### mystic Thread Starter New Member

Sep 6, 2016
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I see ok! Now what about the voltages?

The regulator voltage must match the battery voltage, correct?

Also, is it ok if the battery voltage is 12V but the circuit needs 5V?

Lastly, I assume that the current rating of the battery (10 A for instance) must be equal to or higher than the load's current (5 A)?

5. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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You use a VOLTAGE REGULATOR to provide the voltage your circuit needs. This allows industry to make just a few types of batteries of varying capacity, that can supply power to a huge number of circuits all with different voltage and current requirements.
IN your case a standardized 12 volt battery would work. Being made by the 10s of millions, the cost is very affordable.
Combined with a panel designed to charge the very common. 12 volt battery, which is cheap because it too is made in large numbers.
You can make this as difficult as you desire, but it doesn't have to be.
One voltage regulator, a couple of resistors and capacitors and BINGO, you have a steady, regulated 5 volt supply from a cheap and common 12 volt battery/solar cell combo.
Google up the term "three leg voltage regulator circuit".

6. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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Here is one.

It will work with a 12 volt input just as well if the adjustment is set to your 5 volt output requirement.

7. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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For current it is also a simple calculation.

5 amps at 5 volts is 25 watts.

12 volts at 2 amps is 24 watts.

So you will be drawing about 2 amps from your battery to get what you require in your 5 volt circuit.

2 amps for 24 hours is 48 amp hours. To run your circuit for one full day without damaging your battery with excessive discharge you need a battery rated around 100 amp hours.

Use the amp rating of your solar cell to subtract from this.

A panel for charging 12 volt batteries that supplies 2 amps can provide full power output for, let's say, 6 hours. That's 12 amp hours. Not enough to fully charge the battery.

A panel giving 10 amps would provide 60 amp hours. When charging you need amperage in excess of your circuit needs. So 2 amps go to your circuit and 8 amps into the battery. That would be 48 amp hours of charge. Just about enough to barely keep up with the imagined usage I layed out.
One cloudy dark rainy day would put a kink in that plan.

Get the idea?

8. ### ronv AAC Fanatic!

Nov 12, 2008
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If you buy a panel designed for a 12 volt battery and not over powered you can just hook it up to the battery without anything else.

No, that will make smoke. Use something like this and adjust it for 5 volts.
no, not really, within reason. The battery rating is amp hours. Meaning you can draw 10 amps for 1 hour before it is dead. Or 20 amps for 1/2 hour.

9. ### mystic Thread Starter New Member

Sep 6, 2016
15
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Hi ronv, thanks for your help.

So do you recommend, to be on the safer side, that I get a solar regulator/battery that has a higher voltage and a lower current or vice-versa?

I can always step-down the voltage, but which would be safer?

10. ### ronv AAC Fanatic!

Nov 12, 2008
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To be safe you could get a charge controller like this:
http://www.ebay.com/itm/NEW-20A-12V...259184?hash=item25a8d40cf0:g:py8AAOSw3ydVjjU9
I would use 12 volt battery and panel just because they are more common (cheaper) then step it down with the little regulator I posted. That way if you decide you want 9 volts or 3.3 volts you can do it.
I would overpower it because of the solar charge unpredictability. Maybe a 15 or 20 watt panel and a 15 - 18AH battery.

11. ### mystic Thread Starter New Member

Sep 6, 2016
15
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Thanks for the help, that really cleared it up!

When I meant solar regulator in my previous posts, I was actually referring to charge controller.

20 Watt/12V panel with 12 V charge controller and 12 V/15-18AH battery sounds about right, then just step it down.

Based on your napkin calculations, if I had a 10 W load and assume there are 10 hours of daylight, then I would need a 20W panel at least correct? This is correcting for the 50% discharge rule that you mentioned.

10 W * 10 hrs = 100 WHr
20 W * 10 = 200 WHr

And voltage (12V) and the Ah of the battery (15-18) would still be the same, correct?

12. ### ronv AAC Fanatic!

Nov 12, 2008
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Well, lets look at it. Say the battery is fully charged it would have 180 watt hours divide by 1/2 is 90. So you could run your 10 watt load for 9 hours.
The next day is bright and sunny so you get 240 watt hours to charge it - plenty. If you live in Oregon and it was cloudy all day your battery would be fully drained and not real happy but it might run another 9 hours. If it was sunny the next day you could still charge it completely with 240 watt hours.
Maybe okay?

13. ### Bernard AAC Fanatic!

Aug 7, 2008
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Seems that we have a requirement of 240 Wh. or on 12 V system 20 Ah. Some rough figures on a 15 W SP
gives about 31 Ah. so maybe a 30 W SP would do ?

14. ### mystic Thread Starter New Member

Sep 6, 2016
15
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Thanks for your replies. I've been doing a lot of researching in the meantime. I was wondering if I were possible to run a 6V12Ah battery (cheaper) instead of a 12V12Ah battery with either a 18V or 12V panel. Would there be any downside to this?

The two loads I am looking at are 5V/1A (would need step-down converter) and 9V/600 mA (would need step-up converter).

15. ### mystic Thread Starter New Member

Sep 6, 2016
15
0
Add-on: I took a look at the actual power consumption of the 5V/1A load that I mentioned before and it specified "1.4 W (no load), 3.6 W (full load)".