Battery and panel math.

[Smithalpha]

Just doing some math.

So I have 2 8d lead acid batteries.
I believe they are supposed to put out 600ah in total. This give me 4 hours of run time. So that means I’m burning up 150ah a hour?
I have 4-230 watt panels at best I’ve only been able to pull 17 amps out of them. But usually closer to 12. Best case scenario with the duration of sun exposure living in Washington state I get maybe 60 amps a day in total.
If all that’s accurate it would take me 10 days to refill what I used up in 4 hours.
Does that math out to everybody?

The thing that seems messed up is if that math is right I would need 12 8ds to support for 24 hours. 120 sollar panels. To charge it in the sun I have available every day. Am I missing something?

 

[littleharbor2]

150 amps is a big load. If you have 600 Ah of battery you really only want to draw them down to 50% soc. so you only have 2 hours of available power.

Coastal Pacific Northwest is a rough place to depend on PV power. Go East young man. I have a friend who lives in Pasco and it seems quite sunny there.

Are the batteries possibly on their way out? Have they been fully charged before your load test? Guessing your current draw the way you are doesn't sound too accurate. Get an AC/DC clamp meter to know the true amp draw on your batteries.

 

[Mattb4]

Watts =Volts X Amps

Batteries are rated in amp hours based on either 20 hours or 10 hour draw. Example a 100ah battery will be loaded at 5a for 20 hours. When the load goes higher than the rated draw rate than due to battery chemistry you get less total amp-hours from it. Example a 100ah battery at 50a load will only give 90ah for 1.8 hours run time.

Your panels tend to average in full sun about 75% or less depending on location of rated capacity. Since you have 920w of panels that would be 690w. If you are charging a 12v battery that would be 57.5a Your batteries you rate at 600ah would take ~11hours to charge based on that. All of that assumes no loading during charging.

 

[Smithalpha]

Mattb4
The panels are putting out at best 17 amps real world, at 24volts. Everybody says it should be higher but that’s what they are putting out.
And I have tested each panel .
There all contributing .
Even by your numbers that 11 hours would at best be broken into 2 days .
That’s rough. Need more power.

Littleharbor
I appreciate your input .
I’ll have to get a clamp on CT so I can verify the amp draw. I’ll try and update the amp draw figure later.

 

[Mattb4]

Where are you measuring amps and voltage at? 17a at 24v = 408w This would be low for 920w of panels in full sun. That would indicate you might have a problem with a connection or perhaps your wire run to the SCC. I do agree that you are under paneled for your loading needs.

BTW, what SCC are you using?

 

[littleharbor2]

Littleharbor
I appreciate your input .
I’ll have to get a clamp on CT so I can verify the amp draw. I’ll try and update the amp draw figure later.

When you say they're putting out 24 volts, how are you measuring this? 230 watt panels should be putting out right about 30 volts vmp. and about 36 volts voc. when measured with a dmm. Your charge controllers voltage reading is likely the battery voltage which could be 24 volts when your batteries are very low and on a 24 volt system.
You haven't you haven't told us whether you are running a 12 or 24 volt system. If 24 volts, your battery bank is 300, not 600 amp hours

 

[Smithalpha]

It’s a 24 volt system . Input voltage in to the controller is roughly 56 volts from the panel.
Amperage from panel to controller is 8.2-8.6.
The battery bank Is 2-300 ah 8d battery’s that I have wired in series.

I have 4 panels 230w .
They are set up as 2 parallel and series banks.
To get the voltage up.
Somebody on here suggested. Sounded reasonable to me.

 

[littleharbor2]

So 28 volts input. warm conditions will cause some voltage drop in output. Thats 28 volts per panel. Sounds ok there.
You have 300 amp hours of battery capacity. Based on your description your draw would be 75 amps, not 150. Now how did you determine the batteries were fully depleted? What is the load, a single high amp item?

 

[schmism]

I recommend you stop talking about amps on both ends (solar and battery) and talk about watts on both ends as the voltages are not the same

 

[Smithalpha]

So 28 volts input. warm conditions will cause some voltage drop in output. Thats 28 volts per panel. Sounds ok there.
You have 300 amp hours of battery capacity. Based on your description your draw would be 75 amps, not 150. Now how did you determine the batteries were fully depleted? What is the load, a single high amp item?

Fully depleted was a resting voltage on the batteries of 22 volts .
I’ve yet to figure out where it’s all going.
I do have a chest freezer and a large twin door fridge. I fear that’s where the draw is.

 

[littleharbor2]

Fully depleted was a resting voltage on the batteries of 22 volts .
I’ve yet to figure out where it’s all going.
I do have a chest freezer and a large twin door fridge. I fear that’s where the draw is.

That's a good start. You're talking a likely 3.5 kw. a day I doubt your small array can keep up with that. It's trying to recharge and run them during the day and is being overwhelmed. You really need more battery and more panels.

 

[Smithalpha]

I recommend you stop talking about amps on both ends (solar and battery) and talk about watts on both ends as the voltages are not the same

Wattage as in the the list rate of the panels or as in ohms law . Volts x amps= watts.
So in this case the total watts I have available appears to be 408 watts.
As figured by the amperage draw.

Just as a note, the reason I was using the before and after was to give as much clarity as possible.

And to use it more easily available for doing the math of capacity of batteries versus supply of current.