More battery or more panels?

[georgia088]

Hello all. I have a 101 ah battery. I have 200 watts of 12v solar panels. I am running some small WiFi electronics/ cameras 24 hours a day. According to the charge controller they draw anywhere from .8 amps to 1 amp around the clock. I also have some 12v lighting and 12v water pump that I only use occasionally off of this system. There are some nights that my battery is dying before the sun comes up. I only use the water pump and lights for a few minutes a couple of times per month. To keep the battery from dying, would I be better off buying more battery or more solar panels? Thanks for any help!

[SunEagle]

Actually you calculate how much battery is needed to run your loads each day and only drain the battery by 25% maximum. Once you determine the Amphour rating of your battery system you install panel wattage that will convert to the number of amps that will be delivered to your batteries to charge them.

Depending on the chemistry of your batteries will determine the amount of amps needed to properly charge them.

So for a quick answer if you have FLA type batteries, they need a charge rate between C/8 and C/12 where C = the battery system Amphour rating. A quick answer would be to deliver 1/10th (C/10) the amphour rating in amps to charge them.

Based on your case IMO you need more battery amp hour and more panel wattage. Since you did not mention what type of CC you are using I would say once you go above 200watts you should be using an MPPT type CC.

[nwdiver]

To keep the battery from dying, would I be better off buying more battery or more solar panels? Thanks for any help!

If your batteries are getting fully charged the day before then you need more batteries.

If your batteries are not getting fully charged the day before then you need more solar.

[scrambler]

I assume you mean 18V solar panel not 12V, as charging a 12V battery with a 12V solar panel, would not be great.

You also need to tell what the whether is like in your area, is it mostly sunny all the time, or not?
And you want to factor in how many days of low solar you want to be able to withstand.

200W of solar could produce about a kilowatt on perfect days, but if you have 3 days of dark stormy clouds, that will drop significantly.

Depending if you want to last 3 days without sun or a week, will impact the size of the battery you want. Oversizing the battery for that reason does not necessarily mean you have to scale solar to match 100%. If you live in a place were dark days are rare, you could afford for the extra capacity to require a few days of surplus to fully charge.
But if the weather is not so good were you are, then scaling Solar proportionally will be necessary.

[georgia088]

Actually you calculate how much battery is needed to run your loads each day and only drain the battery by 25% maximum. Once you determine the Amphour rating of your battery system you install panel wattage that will convert to the number of amps that will be delivered to your batteries to charge them.

Depending on the chemistry of your batteries will determine the amount of amps needed to properly charge them.

So for a quick answer if you have FLA type batteries, they need a charge rate between C/8 and C/12 where C = the battery system Amphour rating. A quick answer would be to deliver 1/10th (C/10) the amphour rating in amps to charge them.

Based on your case IMO you need more battery amp hour and more panel wattage. Since you did not mention what type of CC you are using I would say once you go above 200watts you should be using an MPPT type CC.

I have a lead acid battery. I don’t not have a mppt charge controller. I thought I did, but I looked. It’s not. What are the advantages of the mppt chargers?
i assume since my charger is saying that I am drawing .8-1 amp when I look, and the same devices work around the clock; and shouldn’t be changing power consumption dramatically for any reason this means I am drawing about 1ah every hour everyday? Is this how I determine my amphour rating for my system?
Thanks!

[georgia088]

I assume you mean 18V solar panel not 12V, as charging a 12V battery with a 12V solar panel, would not be great.

You also need to tell what the whether is like in your area, is it mostly sunny all the time, or not?
And you want to factor in how many days of low solar you want to be able to withstand.

200W of solar could produce about a kilowatt on perfect days, but if you have 3 days of dark stormy clouds, that will drop significantly.

Depending if you want to last 3 days without sun or a week, will impact the size of the battery you want. Oversizing the battery for that reason does not necessarily mean you have to scale solar to match 100%. If you live in a place were dark days are rare, you could afford for the extra capacity to require a few days of surplus to fully charge.
But if the weather is not so good were you are, then scaling Solar proportionally will be necessary.

I live in Georgia about 45 miles south west of Atlanta. Yes, they are 18v panels connected to a 12v battery. I would like the devices to work year round even during low solar days. I do not love where the solar is located. I have the WiFi and cameras set up to view the location remotely when I’m not there. But, this should mean the amp draw from the devices should be the same each day. There are only a few days per month that I use the lights or water pump, I really don’t think they are much of a factor.

[Mike90250]

Cloudy days will be the limiting factor. How many cloudy days in a row, do you want the system to be able to run thru ??
Then once you size the batteries for that, you size the solar to complete a recharge in 1 day, of the battery bank.
If you need 24ah of power daily (1amp x 24 hours) 3 days would need 72ah consumed, and twice that for the battery bank, 12V 144ah.
A 6v golf cart battery is about 190ah, so two of those in series gives you a comfortable 12v, 190ah battery, which is great if your 1amp current measurement is correct

To recharge 12V 72ah, that converts to 864watt hours. If your panels are well aimed, and you got 4 realistic hours of harvest, you need 1040 watts of harvest, or 260w per hour.

Since a PV panel only makes 80% of it's nameplate, and your array will only have optimum aim for 20 minutes a day you need about 340 watts of panel, a single panels or 2 - 170w panels, or 3- 120W panels. 170 & 340w panels are likely to be 27- 60v and would use a MPPT Charge Controller. 120w panels are likely 18v and would use a PWM Charge Controller

That's how I figure it to run 3 days of clouds

[SunEagle]

I have a lead acid battery. I don’t not have a mppt charge controller. I thought I did, but I looked. It’s not. What are the advantages of the mppt chargers?
i assume since my charger is saying that I am drawing .8-1 amp when I look, and the same devices work around the clock; and shouldn’t be changing power consumption dramatically for any reason this means I am drawing about 1ah every hour everyday? Is this how I determine my amphour rating for my system?
Thanks!

Simple answer is with an MPPT you have watts in = watts out. A PWM is amps in = amps out.

So with 200watts of panels for an MPPT CC that would equal 200w / 12v = 16.6amps. With a PWM you can add up the Imp of both panels and my guess is a total of maybe 11amps (5.5 amps each). That is a about a 32% reduction in charging amps.

The better way of determining your battery size is to find out how many watt hours you use each day. Then you can multiply the Ah by the battery voltage x 25% to get how many watt hours your battery can safely give you each day.

So for your 101Ah that comes to (101ah x 12v x 25% = 303 watt hours each day without hurting the battery.

[georgia088]

So, what would an inexpensive mppt controller be that would work for my setup? I will more than likely at least get another 100w solar panel (300watts total).

[scrambler]

300W at 18V is 16.7Amp, so you would need at least a 20Amp MPPT charge controller.
If you want to have some margin or go to 400W solar, then 30Amp would be better.
Some will support a single battery type, some will support different ones, Consider that if you want to future proof your system.

[SunEagle]

So, what would an inexpensive mppt controller be that would work for my setup? I will more than likely at least get another 100w solar panel (300watts total).

I would get a 30amp unit. There are a number of good ones but none are really cheap. Midnite and Blue Sky make then. Possible the cheapest is made by Victron. Some people swear by Renogy but I think they are at the bottom of the quality scale.

[chrisski]

I am building my first real PV system now, and I have done a spreasheet based off of what the panel manufacturer says I will get for voltage and amperage, and balanced that against the battery manufacturer's recomendation of no more than 13% the C20 AH rating to charge the battery.

This showed me I needed more panels to reach that 13%. I have 600 watts of panels on my roof, and around 900 watts would push 13% back into the system.

So I had a couple of options for panels chosen and got ready to purchase these when I measured the real short circuit voltage and the real short circuit amps to my panels. I did this in the morning three hours after sunrise when the days were slightly longer than 12 hours and the panels were pushing about half the short circuit amps I was expecting based off their specs, but this could have been because the time of day and not getting enough direct sunlight.

Now I've decided to get the system built, use measurements off my battery monitoring system, and see just how much is actually going into the batteries before I make my next purchase. Whatever I decide for my next purchase, more panels will mean an additional charge controller since there's no way I can put more of same panels in the same configuration on the roof. I'm finding I've made some purchases a little too early that are adding to the difficulty of me working around them. An example is I got a good deal on a new unused portable panel, but I need two more of the same type to put any real power into the RV, but the deals on two of the same type are just not out there.

If you're batteries had a 13% rate like mine, my spreadsheet would show your system putting the proper amount of amperage available to the batteries. However, my spreadsheet does not take into account any loss, which is what I'm waiting to see what it actually is. I think this project will be finished in no more than two weeks.

[sdold]

300W at 18V is 16.7Amp, so you would need at least a 20Amp MPPT charge controller.
If you want to have some margin or go to 400W solar, then 30Amp would be better.
Some will support a single battery type, some will support different ones, Consider that if you want to future proof your system.

I think most, if not all, charge controllers are rated by battery charge current. 300W/12V = 25A. You probably wouldn't see more than 80% of that, but I wouldn't get the minimum size charge controller that will work.

I also wouldn't wire three of those panels in parallel unless there was a good reason (like partial shading) because they will need to be fused. Instead, I'd wire them in series with an MPPT charge controller. Three panels in series would have an open circuit voltage of about 60V, and with 25A being the max expected battery charge current under ideal conditions, I'd buy something like the Victron Smartsolar (not Bluesolar) 100/30. I just bought two and really like them. The Smartsolar version has built-in Bluetooth that works with a phone app, which I really find useful. They are around $230-$240. You could even add a fourth panel if you need a little more power.

[Mike90250]

This is the difference between design and real life. In real life, you have heat and less the optimum sun angles, so your panels are never fully powered. For 20 minutes at the best of the angle for the day, 80% of nameplate is good, 70% is more the case, and the rest of the day is 50-70% of nameplate.. . Add some clouds (or smoke) and you will find you need more panels.
Some controllers can be set to limit the output amps, so you can over panel and not worry.

[Sunking]

I also wouldn't wire three of those panels in series unless there was a good reason (like partial shading) because they will need to be fused. Instead, I'd wire them in series with an MPPT charge controller. .

Steve I think you have a typo and meant if you wire the panels in parallel would need fuses.