# Thread: How long to charge a battery?

1. Frequent Poster
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Jan 2010
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## How long to charge a battery?

I have 12 60W panels in parallel. They have a total current of 10.8A for the panels (.9A each). 300Ah 12V battery bank. If i discharge the bank to 50% capacity (150Ah), what other information do I need to figure out how long it would take to fully recharge this bank? I see the specs of the bank have a reserve capacity of 140min 25A, whatever that is. I have a FM-80 MPPT charge controller. Can anyone help?

2. Solar Panda
Join Date
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## Re: How long to charge a battery?

I have 12 60W panels in parallel. They have a total current of 10.8A for the panels (.9A each).
12 panels X 60w / 12v = 60 amps ?

Tim

3. ## Re: How long to charge a battery?

I have 12 60W panels in parallel. They have a total current of 10.8A for the panels (.9A each). 300Ah 12V battery bank. If i discharge the bank to 50% capacity (150Ah), what other information do I need to figure out how long it would take to fully recharge this bank? I see the specs of the bank have a reserve capacity of 140min 25A, whatever that is. I have a FM-80 MPPT charge controller. Can anyone help?
A 60 watt panel should be rated at about 17 volts Pmax and 3.5 amps Imax, check the back of your panels for exact values. 12 panels *3.5 amps = 42 amps. You will probably on average on see about 80 % of the rated power depending on your setup. .80 X 42amps = 33 amps. A battery requires about 120% of the power used to replace it. So if my calculations are correct 150 * 1.20 = 180 amp hours to charge the battery. 180 amp hours /33 amps = approximately 5 1/2 hours charge time. In reality you also need to extend that time for absorb time to approximately 8 hours.
IHTH Every body check my math, this is how I learn :)
Last edited by BB.; February 12th, 2010 at 10:15 PST. Reason: Use [quote] ... [/quote] tags... [code] shows text exactly as typed

4. Frequent Poster
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## Re: How long to charge a battery?

The panels are 48V panels. they have a Vmp of around 67volts, and an Imp of around .9. They are all connected in parallel so the amperage of the array is 10.8Amps. The bank is 12V but since its an MPPT charge controller the 12v bank can handle the 48v panels. Knowing this an my first post, what info do I now need to figure out how long to charge the battery bank.

5. ## Re: How long to charge a battery?

With an MPPT controller, the array voltage/current specs don't mean much specifically.

You have 600 watts/~13, so you are getting roughly 45 amps. Knock off around 15-20% for battery losses, voltage variation as the battery charges, etc etc and you end up with around 3.5 hours.

6. BB.
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## Re: How long to charge a battery?

And--in the end--both answers are correct... The devil is in the details.

The batteries will charge at approximately full available current from the charge controller "current regulation" / bulk charging until the batteries hit somewhere between 80-90% charge (bank voltage is below set charging voltage of the controller -- call it 14.5 volts for this example)...

At that point, the charge controller goes into "voltage regulation"/ absorb state (say 14.5 volts) and will hold that voltage. The current into the battery will gradually ramp down to a few percent of full charge current. At that point, the batteries can be said to be full, and the charge controller (if no other loads) will drop to "float" mode regulation (call it 13.6 volts and very little current flow).

The "absorb" phase is probably somewhere around 2-6 hours (real rough approximation) to reach full charge.

In terms of good battery operation--there is no need to go over ~90% state of charge every day (limited amount of sunlight, limited genrator fuel). Of course, it is nice to have 100% state of charge before you have 3 days of stormy weather coming up. But, from a system/generator fuel point of view, that last 10% charging is sort of inefficient.

People with off-grid systems will start running their loads in the afternoon to use the excess available solar panel current--since the battery bank does not need it all.

Dave Sparks (another poster here) like tracking solar arrays because it does give a longer time to collect power--which helps extend the amount of time that solar power available to charge the bank--If I understand his reasoning correctly).

When you need to use the genset--the most efficient time to charge is (usually) early in the morning--Use the genset at the time the batteries will accept maximum current/power (greatest fuel efficiency/shortest runtime) to "bulk up" the batteries. Then let the solar panels finish off the charging the rest of the day.

-Bill

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## Re: How long to charge a battery?

This makes sense, thanks all. There is just so many variables with so many solar questions, it is so complicated. I think it was BB who said "The more you learn about solar, the more you realize you don't know"... not the exact quote, more of paraphrasing, and man is that right.

8. Solar Panda
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## Re: How long to charge a battery?

The panels are 48V panels. they have a Vmp of around 67volts, and an Imp of around .9. They are all connected in parallel so the amperage of the array is 10.8Amps.
Yes 10.8 amps at 67 volts, you are stepping down to around 14 volts for charging, Using an MPPT charger you will get a down conversion loss of around 10%, So 10.8a x 67v = 723.6 watts, - 10% = 651 watts , divide that by your normal charging voltage of 14 = 46.52 amps at 14 volts, that would theoretically be your battery charge current on an "STC" day

What type of batteries ?

Tim

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## Re: How long to charge a battery?

USpower golf cart batteries. 450Ah 12v for the whole bank. 2 strings of 2 hooked in parallel, so 6v 225ah individually.

10. Solar Panda
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## Re: How long to charge a battery?

What are you trying to figure ? your absorb time ?

Tim

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