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# Solar Charge Controller

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#### Flavini

##### Newbie level 5
Hey all, I am working on a solar charge controller for 12v lead acid battery. This is what I planned:

I am setting up a small inverter whose battery (70Ah) is charged by a 100W panel all day long (as long as required).

If panel voltage is > 13v, output to battery = input voltage.
If panel voltage is > 15v and upwards, output to battery = 14v regulated by a buck converter controlled by an ATMEGA.
If panel voltage is < 13v, output is disconnected, ie, battery does not charge.
If battery voltage is > 13.7v, charging is stopped.
If battery voltage is < 13.3v, charging is restarted.
If battery voltage is < 11v, load is disconnected.

I already got the buck converter working. Now, my question is, is my algorithm alright? Can it be used for an effective PWM solar charge controller?

Thanks.

I used solar panels that produced 23V open circuit under bright sun. This is designed to charge a 12 V battery with no modification being necessary, except that charging should be stopped when the battery reaches 14.5 V or so.

My charge controller was a simple homebrew circuit. It switched a mosfet off when battery V went over 14.5 V, and turned on the mosfet when battery V was under 14V. It had a hysteresis effect so as to resemble a taper charge.

The battery did not send current backwards into the panel at night. Therefore there's no need for you to add a blocking diode. Or else it's built into the panel. I don't know about all solar panels so you should test your panel and battery setup. In other words you probably do not need to disconnect the panel from battery at night.

What I have in mind is that, since, theoretically power in = power out. If I have a 100W panel that gives 20V output, I get 5A current for charging, but if I convert to about 14v, I get around 7A current. Losses are there obviously, but isn't this theory alright? Or am I missing something here?

Thanks.

Any help? ???

What I have in mind is that, since, theoretically power in = power out. If I have a 100W panel that gives 20V output, I get 5A current for charging, but if I convert to about 14v, I get around 7A current.

Keep in mind the panel has an internal resistance. You may get 5A into an ammeter. But then you won't have 20V across the ammeter.

About stepping down 20V to 14V... You'll find that 14V will not push 7A into a 12V battery.

The solar panel manufacturer tested performance for different amounts of photovoltaic cells.

They found a minimum number would charge a 12V battery most effectively. That's the number of cells in your solar panel. So you want all 23V to overcome the 12V of the battery and push several amps into it.

Flavini

### Flavini

Points: 2
Ok, thanks for that. I will do that.

I have a question about the battery voltage. In some places, I read that battery is fully charged when it reaches 14v, in other places I read that it's fully charged at 12.7v. Which is correct? I'm going to have a battery level indicator with 4 LEDs to indicate battery state. Can you give me some ideas at which battery voltages should I turn the LEDs on.

Have a look here a very good article
Silicon Chip Online - Build A 12/24V 3-Stage Solar Charge Controller

TheBackShed.com - PicAxe Charger

If you have lead acid then fully charged open circuit voltage will be 12.7V, Just a quick run through on charging batteries
The charger starts of in constant current the once 2.4V per cell is reach (known has the gassing/absorption phase) EG. a 12 battery 2.4*6=14.4V then the charging current is reduced normal run of the mill the absorption/gassing time is set to 3 - 4 hours timed. then stage 3 becomes constant voltage and holds the batteries at 13.8V, If you do a search on google there are more indpeth readings

Flavini

### Flavini

Points: 2
it is work fine ?
I have not built it myself but it should work has it described I think you can download the hex and PCB I think I've got a copy of the schematic some where if you want it

I have a question about the battery voltage. In some places, I read that battery is fully charged when it reaches 14v, in other places I read that it's fully charged at 12.7v. Which is correct?

12.7 or 12.8 V is typical of a healthy lead-acid battery at rest. 2.12 V per cell.

After charging for several hours the battery might soar as high as 14 or 15 volts.

Once charging stops the battery will spend a few hours settling down to normal.

I'm going to have a battery level indicator with 4 LEDs to indicate battery state. Can you give me some ideas at which battery voltages should I turn the LEDs on.

One of the most useful indicators I made was based on a 3914 bargraph IC, with 10 LED's showing my car battery voltage.

For a solar installation I suggest these 10 levels, the lowest being the customary figure where an inverter typical shuts off as it detects that the battery is discharged.

10.5
11
11.5
12
12.5 (settled state / light load)
13
13.5
14
14.5
15

If you just use 4 levels then I recommend 11.2, 12.4, 13.6, 14.8.
This covers a normal range of charge/discharge.
A battery at rest (or light load) will light the bottom two LED's.
One lit means it's close to discharged.
3 lit means it's getting a healthy charge.
4 lit means it's charging quickly and should be watched for bubbling.
Just my arbitrary line of thinking.

Flavini and Tahmid

Points: 2

Points: 2