# Reduce volts/Charge controller

#### nicko17

Aug 17, 2010
9
I'm making a small solar panel to experiment with it. It will put out around 6volts and 20 watts. I dont know much about electronics.

Now i need a way to store the power generated , i will use a 6volt rc rechargeable battery. Is there an easy way i can control the charging of the battery so it does not over charge? I know there are 12/24 volt charge controllers out there but no 6volt systems. I have to go with 6volts because thats the most my panel will put out , unless im wrong about how this works.

Also i want to convert that 6volts from the battery to 5volts for a usb. I can just use a diode or a resistor for that? If so which one.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
You should look up shunt regulators for your solar panel. Check the specs of your battery to determine the voltage you should float charge it at.

Then look up LDO voltage regulators. You probably want a 5V LDO regulator for this application.

#### nicko17

Aug 17, 2010
9
You should look up shunt regulators for your solar panel. Check the specs of your battery to determine the voltage you should float charge it at.

Then look up LDO voltage regulators. You probably want a 5V LDO regulator for this application.

Shunt regulators control how much voltage goes through the wires. But i dont understand how i can use that to control the charge. Lets say my float charge is 6volts , so i would just set the shunt to 6volts? I dont know much about the battery other than its 6v , i'm charging it now using the regular charger to see how much power it produces.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
The shunt should limit the voltage from the solar panel to the float charge voltage recommended by the battery supplier.

It may well be 6.95V (I pulled that out of my... ear)

A rough approximation is: Per cell: 2.27 to 2.35 volts @ 25°C (that's 6.81 to 7.05 volts)

#### nicko17

Aug 17, 2010
9
I charged the battery , it now reads 6.88Volts.

The charger says on the plug it output 9v at 200mah
I put the tester to it and i get 10.6V approx.

What if my panel only put out 6 volts. And i need to use resistors with the shunt for it to get the right voltage?

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
That isn't a float charger, but let's assume that 6.88V is appropriate.

Now you need to measure to voltage you get from the solar panel in full sunlight. If it's less than 6.88V then it won't be able to fully charge your battery (without some electronic magic).

No, you don't need resistors, you need a shunt regulator (which may have resistors in it, but it not made up of resistors)

Here is a very simple example. Note that the values are for 12V, you would want the voltage set for (say) 6.9 volts -- half of what they suggest.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
No, in both cases their power ratings are insufficient.

I suggest you should be looking for an LDO regulator rated for 5V 1A, and a shunt regulator rated at 20W (or more -- essentially the max power output of your solar panel)

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
I wouldn't use ebay. Go to a store that ships to your area or (better) exists locally) and look up what they have.

Although the seller says they're LDO, they're not. Look at the specs he gives. Minimum input voltage 7V.

Go to something like mouser or digikey and look up low dropout regulators. A to-220 package is probably what you should be looking at.

Then get the spec sheet and find out what other components you need (a couple of capacitors) and the specs on them.

For example, on http://digikey.com you first search for "regulator", then select "...voltage regulators (32123)" on the next screen.

From the parametric search, select Positive fixed topology, 5V output, 1 regulator, all the dropout voltages up to 0.7V, 1 regulator, output currents 1 to 1.8A, through hole, and all the TO-220 packages with 3 leads (to-220, to-220-3 etc), then click on "Apply Filters"

Oh, you probably also want to select "in stock" and apply that too.

You'll be left with a short list (I get 6) regulators. In this case they're all different, but you might see the same regulator listed multiple times in different packaging, or in different quantities.

We can immediately eliminate the ones which have a max input voltage of 6V

It turns out that the top one is the cheapest for me, here it is: http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=497-6695-5-ND

Now click on the link for the datasheet and read it to see if it's suitable, and what else you'll need. This one doesn't tell you a great deal, but it looks like a 0.1uF input capacitor and a 22uF output capacitor are what you'll need. I's recommend a low ESR tantalum cap for the 22uF, and pretty much anything for the 0.1uF capacitor.

Next you can see what retailers have this chip. go to www.findchips.com and enter the part number (L4941BV). Looks like they're pretty common, mouser has them for under a dollar.

As for the shunt regulator, I suggested a couple of circuits earlier.

#### nicko17

Aug 17, 2010
9
The reason i was using ebay cause it easy , and most of them have free shipping. Local stores ... i have no idea who sells this stuff. The sites you showed are very helpful. I may buy from digikey or element14 because of the cheaper shipping to Canada.

I understand the LDO , what i need / how to hook it up . But i dont understand the shunt regulator. If the voltage is set at my float charge voltage for the battery cant it still overcharge it cause all its doing is regulating the voltage..

As for finding the right shunt regulator on digikey , all i see are ones that have max 100mA output...i search shunt regulator. http://search.digikey.com/scripts/DkSearch/dksus.dll?Selection

Im trying to understand all this , but i have no background what so ever with this type of electronics.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
The reason i was using ebay cause it easy , and most of them have free shipping.

But you may be buying stuff from someone who has no idea what they're selling. Sure, you may be able to get some bargains, but you have to know what you're buying.

I understand the LDO , what i need / how to hook it up . But i dont understand the shunt regulator. If the voltage is set at my float charge voltage for the battery cant it still overcharge it cause all its doing is regulating the voltage..

In essence, a Lead Acid battery cannot be overcharged if you charge it up to a certain voltage (it gets a little more complex if you consider temperature and/or charge current, but the general principle applies). A voltage regulator ensures that the battery isn't overcharged. For solar panels, a shunt regulator ensures that the maximum voltage is obtained from the panel (so it works in the lowest amount of light) but still regulates the voltage..

As for finding the right shunt regulator on digikey , all i see are ones that have max 100mA output...i search shunt regulator. http://search.digikey.com/scripts/DkSearch/dksus.dll?Selection

Im trying to understand all this , but i have no background what so ever with this type of electronics.

You'll have to either make or buy a shunt regulator. When I say buy, I mean a boxed product. To make one you'll have to create one from parts.

Here are some I've googled. (first one is better than a simple shunt regulator)

http://www.siliconsolar.com/6v-charge-controller-p-17850.html
http://www.idealhere.com/1A-6V-Solar-Deck-Light-Panel-Regulator-Controller_p1561.html

#### nicko17

Aug 17, 2010
9
But you may be buying stuff from someone who has no idea what they're selling. Sure, you may be able to get some bargains, but you have to know what you're buying.

In essence, a Lead Acid battery cannot be overcharged if you charge it up to a certain voltage (it gets a little more complex if you consider temperature and/or charge current, but the general principle applies). A voltage regulator ensures that the battery isn't overcharged. For solar panels, a shunt regulator ensures that the maximum voltage is obtained from the panel (so it works in the lowest amount of light) but still regulates the voltage..

You'll have to either make or buy a shunt regulator. When I say buy, I mean a boxed product. To make one you'll have to create one from parts.

Here are some I've googled. (first one is better than a simple shunt regulator)

http://www.siliconsolar.com/6v-charge-controller-p-17850.html
http://www.idealhere.com/1A-6V-Solar-Deck-Light-Panel-Regulator-Controller_p1561.html

Alright that helps. The battery is NiMH high capacity. That will make a difference wont it for charging.

I have car cell phone chargers , they say 12/24v input , 5v something output.. too bad i wont have 12volt input.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
The battery is NiMH high capacity. That will make a difference wont it for charging.

Oh yeah. You need a proper charger for them. and that's not a trivial exercise.

I have car cell phone chargers , they say 12/24v input , 5v something output.. too bad i wont have 12volt input.

They're probably just sources of 5V, not chargers. Do they plug into a phone, or do the batteries plug into them?

#### nicko17

Aug 17, 2010
9
The charger plugs into the phone not the battery it self.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
Yeah, then it's not a charger, it's just a power supply. The charger is in the phone.

So I guess I should (perhaps belatedly) ask: What are you actually trying to achieve?

edit: and have you measured the open circuit voltage of the solar panel? (what is it?)

#### nicko17

Aug 17, 2010
9
I wanted to charge my phone with the solar panel. The phone needs 5v and it can be on a usb cable.

But this is such a pain to work with 6volts. I'm thinking i should just get more panels to work with 12volts.

I dont have the solar panels yet so i dont know exactly what they will put out , but i know the specs on them are 0.5V , 1.8watts each x 12 cells.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
The problem with solar panels is that they are not a constant source of power.

As a cloud moves over, the power output can fall significantly.

Depending on how your phone reacts to lower voltages, it may just be better to shunt regulate the solar panel and plug it straight into the phone.

#### Resqueline

Jul 31, 2009
2,848
The spec's says the panel is exactly 6V. There's no headroom to charge a 6V battery, for that it'd need to have 16-18 cells. It could charge a lithium cell though.
There are several very cheap solar cellphone chargers with a built-in battery & a 5.5V usb output on ebay. Get one of those and replace its pathetic solar panel with yours.

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