BATTERY RUNNING OUT VERY FAST DURING THE NIGHT

I have installed a 40watt panel with Solar Charge Controller PWM,12V/24V 10A,12V/24V 20A,12V/24V 30A and a 12v 75ah new Chloride exide free maintenance car battery. Despite limiting my usage to four 3w dc lamps for lighting between 6.30pm to 7.00 am the battery is low (red indicator in charge controller) in the morning. What could be the problem?

Ok, you have a 900 Wh battery, with 50% DOD leaves you with 450Wh.
Assuming your battery is fully charged to begin the night, 150Wh should not empty it... so, let’s see what a 40W panel can produce.
40watts in good sun will produce 200Wh a day average... so, if you started with a full charged battery on day one, day two should have been recharged the next day IF there was good sun. If not, let’s say 100Wh made it to the battery... so, by day 4 you are down to 50%DOD at the end of the night, and the next day cannot full that far, so, you are in the red.

bottom line. You need more panels.

I suggest to install a battery monitor shunt so that you can see what is coming into and going out of the battery.

I find it helpful to do the math, so people can plug in their own numbers to suit their needs. In general, for lead-acid batteries, my rule of thumb is that you have three choices for charging rates, low (0.05C), medium (0.1C), and high (0.15C) where C is the capacity in amphours (your's being 75AH). In comparison, at a low rate, it might take 2-3 days to fully charge a depleted battery. At medium, 1-2 days, and at high, less than 1 day.

You multiply the charging rate by your system voltage, in this case, 12V. I always include a fudge factor to compensate that panels almost never put out their rated current. I like to use a fudge factor of 1.2.

So for a minimal charge level the math works out to BAH x CR x V x FF =panel watts, or in your case 75AH x 0.05 x 12V x 1.2 =54watts. As you can see from the numbers, your 40W panel is below minimal in whats acceptable. Expect it to take more than 3 days to charge a depleted battery

For a medium charge the numbers are: 75AH x 0.1 x 12V x 1.2= 108Watts. For High they are: 75AH x 0.15 x 12V x 1.2 = 162Watts.

So, the numbers are saying to replace your 40W panel with one of 100-160watts. I've used this formula for years now, and I have never had a depleted battery.

MichaelK said:

I find it helpful to do the math, so people can plug in their own numbers to suit their needs. In general, for lead-acid batteries, my rule of thumb is that you have three choices for charging rates, low (0.05C), medium (0.1C), and high (0.15C) where C is the capacity in amphours (your's being 75AH). In comparison, at a low rate, it might take 2-3 days to fully charge a depleted battery. At medium, 1-2 days, and at high, less than 1 day.

You multiply the charging rate by your system voltage, in this case, 12V. I always include a fudge factor to compensate that panels almost never put out their rated current. I like to use a fudge factor of 1.2.

So for a minimal charge level the math works out to BAH x CR x V x FF =panel watts, or in your case 75AH x 0.05 x 12V x 1.2 =54watts. As you can see from the numbers, your 40W panel is below minimal in whats acceptable. Expect it to take more than 3 days to charge a depleted battery

For a medium charge the numbers are: 75AH x 0.1 x 12V x 1.2= 108Watts. For High they are: 75AH x 0.15 x 12V x 1.2 = 162Watts.

So, the numbers are saying to replace your 40W panel with one of 100-160watts. I've used this formula for years now, and I have never had a depleted battery.

Click to expand...

This is all correct, but he only has a 150Wh load overnight.
My thinking is he isnt getting 40W output from the panel enough of the day.
1.2 likely is still under ideal conditions... I'd lean more to 1.5... and still only during sunny days.
On cloudy or partly cloudy, he likely never approaches 100Wh from the panel...

MichaelK said:

I find it helpful to do the math, so people can plug in their own numbers to suit their needs. In general, for lead-acid batteries, my rule of thumb is that you have three choices for charging rates, low (0.05C), medium (0.1C), and high (0.15C) where C is the capacity in amphours (your's being 75AH). In comparison, at a low rate, it might take 2-3 days to fully charge a depleted battery. At medium, 1-2 days, and at high, less than 1 day.

You multiply the charging rate by your system voltage, in this case, 12V. I always include a fudge factor to compensate that panels almost never put out their rated current. I like to use a fudge factor of 1.2.

So for a minimal charge level the math works out to BAH x CR x V x FF =panel watts, or in your case 75AH x 0.05 x 12V x 1.2 =54watts. As you can see from the numbers, your 40W panel is below minimal in whats acceptable. Expect it to take more than 3 days to charge a depleted battery

For a medium charge the numbers are: 75AH x 0.1 x 12V x 1.2= 108Watts. For High they are: 75AH x 0.15 x 12V x 1.2 = 162Watts.

So, the numbers are saying to replace your 40W panel with one of 100-160watts. I've used this formula for years now, and I have never had a depleted battery.

Click to expand...

I think this is helpful.

MichaelK said:

I find it helpful to do the math, so people can plug in their own numbers to suit their needs. In general, for lead-acid batteries, my rule of thumb is that you have three choices for charging rates, low (0.05C), medium (0.1C), and high (0.15C) where C is the capacity in amphours (your's being 75AH). In comparison, at a low rate, it might take 2-3 days to fully charge a depleted battery. At medium, 1-2 days, and at high, less than 1 day.

You multiply the charging rate by your system voltage, in this case, 12V. I always include a fudge factor to compensate that panels almost never put out their rated current. I like to use a fudge factor of 1.2.

So for a minimal charge level the math works out to BAH x CR x V x FF =panel watts, or in your case 75AH x 0.05 x 12V x 1.2 =54watts. As you can see from the numbers, your 40W panel is below minimal in whats acceptable. Expect it to take more than 3 days to charge a depleted battery

For a medium charge the numbers are: 75AH x 0.1 x 12V x 1.2= 108Watts. For High they are: 75AH x 0.15 x 12V x 1.2 = 162Watts.

So, the numbers are saying to replace your 40W panel with one of 100-160watts. I've used this formula for years now, and I have never had a depleted battery.

Click to expand...

Great post. I don’t get the fudge factor though. I get what your saying in that panels never produce 100%. But then shouldn’t the ff be like.8? I.e. 80% of capacity. Whilst isn’t 1.2 effectively 120% (or am I completely misunderstanding).

Thanks

On cloudy days a 40w panel barely makes 3A a day. Youre using 12 amps. You need about 15A to recharge with inefficiencies.

Youre also destroying the battery everyday it goes without being fully recharged.

Ryang said:

Great post. I don’t get the fudge factor though. I get what your saying in that panels never produce 100%. But then shouldn’t the ff be like.8? I.e. 80% of capacity. Whilst isn’t 1.2 effectively 120% (or am I completely misunderstanding).

Thanks

Click to expand...

It depends on what you are calculating.
X1.2 to determine NEED x.8 to determine available...

Ryang said:

Great post. I don’t get the fudge factor though. I get what your saying in that panels never produce 100%. But then shouldn’t the ff be like.8? I.e. 80% of capacity. Whilst isn’t 1.2 effectively 120% (or am I completely misunderstanding).

Click to expand...

It's the reciprocal value for use in multiplication instead of division. I did that so a new person might not be overly confused by the math. You are correct in the use of your 0.8 value if your dividing the work instead of multiplying. I base my fudge factor on the real-world production of an array with a documented output.

Let's say I just installed a new array with four 245W panels (I did). That's supposed to be 980W. When I actually hook up those panels and angle them directly perpendicular to the sun, I see I'm getting 850W. So, the fudge factor is 850W/980W = 0.867, or 86.7%. The reciprocal of 0.867 is ~1.15. Using 1.20 instead of 1.15 just makes the assumption a little more conservative. I've see guys on other sites using numbers as high as 1.3, which works out to be 77%

The fudge factor is simply the assumption that you won't get the STC rating out of the panel, and you need the conversion factor to produce the amount of power you actually need. So many people don't realize this and get so disappointed when their newly designed system does not live up to expectations. BTW, what I've found in the real-world is that the expected wattage is about the average between the STC rating of a panel, and it's NOCT rating. The NOCT rating is based on testing at 800W/Msquared light instead of 1000W/Msquared light. It is also run at 43degrees C instead of 25C. So, split the difference between STC and NOCT and you don't have to do the math. But hey, I'm a math nerd, so why wouldn't everyone want to do the math?

One more thing I could add, which dovetails with Supervstech's post #5. My fudge factor is for a panel that exactly facing the sun, not at any kind of angle. Let's say you laid your panel flat on a van roof. The sun is at an acute angle to the panel. In that case my fudge factor might be 2.0 or even 4.0, instead of 1.2. On a rainy day, the fudge factor might be 7-10. One word of caution here. You don't want to go crazy and design your system with too high a FF, because on a clear sunny day under perfect conditions, your amperage will go sky high. I've got good controllers with amp limiting functions, but a more economical controller (cheap) might not.

MichaelK said:

It's the reciprocal value for use in multiplication instead of division.

Click to expand...

Thanks, I was wracking my brain for the name.
Consider the following formula for inverter dc watts.
1000 ac watts / .85 efficiency factor = 1176.470588235 ac watts.
1000 ac watts * 1.15 efficiency factor = 1150 watts.
Close enough and errs on the high side which doesn't hurt.
Plus figuring out the reciprocal is slightly more effort than shifting the decimal place.

All makes sense, cheers guys.

My ff is actually 2 as the panels lay flat on the van roof.

I hate to be the "Master of the Obvious" here. But did you check the voltage of the battery when the indicator told you that it was full? And did you check the voltage of the battery when it told you that it was low? I never trust charge controller voltage readings. I check them against a meter of known accuracy. And I especially don't trust the ones with green yellow and red LEDs on them. But that's just me

Ped said:

On cloudy days a 40w panel barely makes 3A a day. Youre using 12 amps. You need about 15A to recharge with inefficiencies.

Youre also destroying the battery everyday it goes without being fully recharged.

Click to expand...

Thank you for this reply. However, my charge controller blinks (green) during sunny days between 11 am upto 4pm when there is 0 load on the battery. This happens most of the days. My assumption is that when my PWM charge controller blinks, the battery is fully charged. Now, the battery flattens as a few minutes when i switch on my 300watt modified sinewave inverter of several hrs after switching on 4 lamps. Attached is the charge controller i am using.

The best way to check to is to grab a cheap multimeter and test the actual voltage of the battery.