What am I missing??

[Samantharae]

My numbers aren't making sense.
Here's my question.
Let's say I have two 12 volt deep cycle FLA batteries, hooked in parallel.
The combined batteries have an amp hour rating of 170 AH total.

I put those batteries (at 50% DOD) on a 30amp smart charger with my gennie and they charge to 100% in about 1 hour.

Now, I take those same batteries (at 50%DOD) and hook them to a 40A PWM with 400w of solar panels. How long will it take for the batteries to charge?, assuming the panels are in full sunlight.

I want to see if you all get the same answer I did.

 

[MisterSandals]

40A PWM with 400w of solar panels. How long will it take for the batteries to charge?, assuming the panels are in full sunlight.

What voltage are you feeding your PWM controller? What does your array look like?

Disregarding PWM inefficiencies with input voltage and assuming 100% efficiency:
400W / 14V charge voltage = 28.5A max

 

[Samantharae]

What voltage are you feeding your PWM controller? What does your array look like?

Disregarding PWM inefficiencies with input voltage and assuming 100% efficiency:
400W / 14V charge voltage = 28.5A max

Okay so at 28.5A, how long would it hypothetically take for the batteries to charge? Because I keep getting about 3 hours. However my 30amp charger can do it in 1 hour or less.
It doesn't make sense.

 

[jberger]

Have you actually checked the settled voltage of the pack, 12 hours after charging to see if they are really full?
I'll bet your charger is turning off prematurely since you can't put 85amps (170/2) into the pack with a 30amp charger in 1 hour.
Even if it was 100% efficient, which it's not, it would only add 30amps in an hour.

Lead acid takes hours to get that last 10% charged.

 

[12VoltInstalls]

Okay so at 28.5A, how long would it hypothetically take for the batteries to charge? Because I keep getting about 3 hours. However my 30amp charger can do it in 1 hour or less.
It doesn't make sense.

There’s a thing called ‘surface charge’ and the 30A 120VAC charger probably ’sees’ attained voltage and shuts off, or floats.

The SCC is more sophisticated and doesn’t shut off - it keeps charging until the voltage is ‘absorbed’ by the battery bank. And then “floats” slightly above static voltage (probs around 13.2V).

Three hours is roughly the calculations answer.

The other thing is solar varies output a bit depending on sun position, clouds, etc so you might not get that whole ~30A all the time.

 

[mikefitz]

To fully charge from 50% discharged will take a long time.
End of charge will be identified when the charge current falls to less than 1%C at the absorbtion voltage and stops falling, it will take a minimum of around 5 hours, irrespective of the charge current capability.

A 30 amp smart charger cannot replace 85 amp hours in one hour. Its not possible.

The solar question cannot be answered until the solar panel specification is known. A guess of 4 off 100 watt panels with PWM controller will give 20 amps. With a good solar day your battery pack may just get to full with 6 sun hours and ideal charge voltages set up in the controller. A more realistic period would be 2 days.

Whatever mechanism you are using to determine SOC is not correct.

Mike

 

[12VoltInstalls]

Whatever mechanism you are using to determine SOC is not correct.

Three hours is roughly the calculations answer.

^^^what he said above. I should have elaborated on the absorption element.

No matter, the 120VAC charger isn’t actually filling the battery bank in an hour is the takeaway

 

[Samantharae]

To fully charge from 50% discharged will take a long time.
End of charge will be identified when the charge current falls to less than 1%C at the absorbtion voltage and stops falling, it will take a minimum of around 5 hours, irrespective of the charge current capability.

A 30 amp smart charger cannot replace 85 amp hours in one hour. Its not possible.

The solar question cannot be answered until the solar panel specification is known. A guess of 4 off 100 watt panels with PWM controller will give 20 amps. With a good solar day your battery pack may just get to full with 6 sun hours and ideal charge voltages set up in the controller. A more realistic period would be 2 days.

Whatever mechanism you are using to determine SOC is not correct.

Mike

Thank you all, I knew my numbers were correct but my husband could not understand, so I posed the question to all of you to try and find someone who could explain it for me.
I have felt all along that the smart charger wasn't completely charging the batteries but I couldn't prove it because there was no way I could let them sit for 12+ hours before using them.

 

[12VoltInstalls]

knew my numbers were correct but my husband could not understand

Sounds like my daughter and son-in-law LOL.
She has mechanical genes and he doesn’t.