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# Thread: How long will it take to fully charge my Batteries?

1. ## How long will it take to fully charge my Batteries?

Hi Everyone!

I just need help in figuring out how long in "real time" will it take my batteries to be fully charge. Here's the deal:

4pcs. 100w PV Panels @ 12v each and 5.71a.
>I wired two of them together in series to make them generate 24v
>In the philippines, we get an average of 5 full sun hours per day all throughout the year, so I'd be generating around 2000wh or 2kwh per day in an MPPT controller.

I have 2pcs. 12v AGM batteries @100Ah each.
>I also connected them in series to make them 24v @ 100Ah.
>I know that with my batteries, I have 2400wh of stored energy (but I will only use 600wh out of it).

Question: How long will it take my Panels to top of my batteries if ever @ the following states?
70% SOC or 1680wh available=?
75% SOC or 1800wh available=?
80% SOC or 1920wh available=?

Please share the formula as well if possible so I can compute it on my own in the future.

Thanks!

2. Well let's tech you to fish rather than give you food.

If you are using a MPPT controller the maximum efficiency you can achieve is 66%. So with 200 watt of solar panels and 5 sun hours the most energy you can generate = Watts x Hours x .66 = 400 watts x 5 hours x .66 = 1320 watt hours so you were way off with 2000 watt hours because you did not account for efficiency.

For the batteries to get a rough idea take the battery Voltage x Amp Hours = Watt Hours. So 12 volts x 100 AH = 1200 watt hours, and you have 2 batteries so 2400 watt hours of reserve capacity.

By now you have all you need to know. All you have to do os factor out the watts to be left with time in hours.

So if your batteries have 50% charge you need to replace 1200 watt hours. Your panels can generate 1320 wh with a 5 sun hour day or 1320 wh / 5 hours = 264 watts. So factor out the Sun Hours 1200 wh / 264 watts = 4.55 hours. The formula is w = wh/h, which means h = wh/ w, and wh = w x h.

FWIW this tells me you do not have enough battery capacity for your system, you need another 8 batteries.

3. This is great sunking! Thanks for the lesson! I appreciate it.

I now understand it a lot better, two follow up questions though; how about a norman PWM charge controller? what is it's normal efficiency? it is close to 66% too?

and also, can you explain to me (please) why I would need another 8 batteries in my bank?

Thanks!

4. Originally Posted by raymondalmeda
how about a norman PWM charge controller? what is it's normal efficiency? it is close to 66% too?
At very best is 50%

Originally Posted by raymondalmeda
and also, can you explain to me (please) why I would need another 8 batteries in my bank?
Battery capacity are deigned to have a number of days reserve capacity with 5 being the lowest number which actually is only 2.5 days of usable as you never want to go below 50%. Si if your winter or minimum production is 1320 wh as you have, Then the proper battery capacity = 5 days x 1320 wh = 6600 watt hours. So with a 12 volt battery would be 6600 wh / 12 volts = 550 AH @ 12 volts.

Don't get the wrong idea that you have to buy more batteries, because you do not, it is just your system is a bit out of normal design parameters. Having a larger panel wattage than needed is not bad thing providing the charge current does not exceed C/8 for flooded lead acid batteries, or C/4 for AGM. Th ereal problem is having too small of a panel wattage whereby the charge current is to low to agitate the battery acid (bubble).

5. Understood! So with my current panel wattage; it's just fine to add 8 more batteries tops?

Thanks Sunking!

6. Originally Posted by raymondalmeda
Understood! So with my current panel wattage; it's just fine to add 8 more batteries tops?
Nah my bad. Currently you have 200 Ah 550AH is the max, or 3 more batteries.

7. ## Confused

Originally Posted by Sunking
Well let's tech you to fish rather than give you food.

If you are using a MPPT controller the maximum efficiency you can achieve is 66%. So with 200 watt of solar panels and 5 sun hours the most energy you can generate = Watts x Hours x .66 = 400 watts x 5 hours x .66 = 1320 watt hours so you were way off with 2000 watt hours because you did not account for efficiency.
Shouldn't this equation read 200 watts x 5 hours x .66 = 660 watt hours?

8. Oh okay, sorry about that. He has 4 PV panels at 100 watts each.

Okay thats why it's 400.

Got it.

9. ## Got confused again

I was re reading this question and got a little confused.

Given: 5 hours of insolation X 400 watts X .66 = 1,320 watt hours
1,320 watt hours X 5 days = 6,600 watt hours

Shouldn't the next equation to figure out the Ah capacity of the batteries be
6,600 watt hours / 24 volts = 275 Ah since he is working with a 24 volt system?

In which case the total battery capacity would be batteries in series 24 volts with a total Ah capacity of 275 Ah?

Thanks for any clarification.