# Control voltage level from a solar panel or not? And if so how?

Discussion in 'General Electronics Chat' started by spinnaker, Feb 6, 2010.

1. ### spinnaker Thread Starter AAC Fanatic!

Oct 29, 2009
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I have seen two opinions. One of which says you do not need to control the voltage output from the panel. While not directly agreeing with this, Battery University does say that you need to have between 2.3 and 2.45 volts per cell depending on your charging application.

I have also seen others say there is no need to control the voltage. You just need to monitor for overcharging.

So which is it? Do you need to control voltage level? And if so how? I have seen shunt regulators used and linear regulators used in charge controllers. From what little I know about linear regulators, they are very insufficient. What about shunt regulators?

Or would something like this. Be a better way to go?

2. ### GetDeviceInfo Senior Member

Jun 7, 2009
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It will depend on the current capacity of your panel. Typically, a panels output is small, so when you utilize it's full current capacity, the voltage falls to a level that is acceptable.

If however you have a large panel charging a comparetively small battery, you will have to control the applied voltage/current

3. ### hgmjr Moderator

Jan 28, 2005
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Also it has been my experience that as the voltage on a solar cell reaches its maximum, there is precious little current to be had. I would think that if the solar cells maximum voltage is slightly greater than the maximum charge voltage of the battery, then the battery is probably not in any great risk of damage.

hgmjr

4. ### MikeML AAC Fanatic!

Oct 2, 2009
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Which battery chemistry? SLA? If so, the correct charging algorithm is limited current (see the battery data sheet, the solar panel may put out less current than allowed by the battery) until battery terminal voltage reaches 14.7V; then hold 14.7V while watching current into the battery; when current drops to < few tens to few hundreds of mA (depending on battery AH), then drop voltage to 13.8V, and hold until the sun goes down.

5. ### spinnaker Thread Starter AAC Fanatic!

Oct 29, 2009
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Yes it is an SLA.

But from what I read from above if my panel buts out a low current (which it does). Then there is really no need to control the voltage level. The battery will do that for me.

Did I understand the opinions correctly?

Actually I am a bit worried that my panel won't put out enough current.

6. ### MikeML AAC Fanatic!

Oct 2, 2009
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The only way to tell if you need to regulate the panel is to connect it to the battery. After a day or two, while the sun is still shining brightly, get a DVM and measure the battery voltage. If you see more than 13.9V, you need a regulator...

The panel would have to be really wimpy (<50mA) not to need a regulator. Even the Harbor Freight Chinese Crap panels have a built-in regulator.

7. ### spinnaker Thread Starter AAC Fanatic!

Oct 29, 2009
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I tested the panel with 100 ohms and measured .12 amps, 50 ohms measured .21 amps, 25 ohms measured .32 amps, 3 ohms measured .37 amps, open circuit .6amps.

I did not know Harbor Freight panels had a regulator. Thanks. It mat be the way to go for now.

Last edited by a moderator: Feb 8, 2010
8. ### spinnaker Thread Starter AAC Fanatic!

Oct 29, 2009
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If I connected the panel to the battery and had nothing else, that means I would not be monitering for over voltage but in my final project I will be monitoring for over voltage. That is the easy part. The harder part (for me) os to build an efficient voltage regulator.

Last edited by a moderator: Feb 8, 2010
9. ### MikeML AAC Fanatic!

Oct 2, 2009
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Excuse me, but if the battery is fully charged, and the job of the regulator is to prevent overcharge, who cares how efficient the regulator is?

10. ### spinnaker Thread Starter AAC Fanatic!

Oct 29, 2009
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But the regulator needs to be running while it is charging the battery, If it is inefficient then that is valuable power going to the regulator just to run it, to what could be charging the battery.

Am I wrong?

11. ### MikeML AAC Fanatic!

Oct 2, 2009
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I have a regulator circuit which connects the panel to the battery with a FET which has an on resistance of ~20mΩ. My panel puts out 2.5A at 14.5V (36.25W), so I am loosing all of 20mV*2.5A = 0.05W for an efficiency of 99.9%.

After the battery is charged, the FET regulates the battery voltage, and at that point it dissipates some power, but at that point, like I said, who cares...

12. ### spinnaker Thread Starter AAC Fanatic!

Oct 29, 2009
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Are you willing to share? Is it a good fit for my wimpy panel? Could I turn the regulator on and off with a PIC? Or would I need an additional FET to shutdown the panel, when the battery is charged?

13. ### MikeML AAC Fanatic!

Oct 2, 2009
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Do you want a standalone regulator (PFET, an LM431, few resistors), or a FET switch (controlled by 5V? PIC) which just disconnects the panel from the battery if the PIC thinks the battery is charged?

14. ### spinnaker Thread Starter AAC Fanatic!

Oct 29, 2009
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I really don't have anything yet.

Sorry for being so dense (hey I am just a newb).

So I can use the LM431 with maybe a couple different resistors to get my different charging voltage levels? Or don't bother and set it for one?

Can you suggest a FET? I have been looking for something that is easy to work with. Most of the packing is so tiny.

I do have an SC70 on hand but I don't know if my hands are steady enough to solder that little thing.

What about using PWM to control the regulation? I would have no idea at all how to build the control circuit but seems to be the PIC could monitor the output of the regulator and then adjust the PWM accordingly. I did an extensive search for a sample curcuit but came up with nothing.

15. ### 3ldon Active Member

Jan 9, 2010
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contact the manufacturer for the proper float voltage, and equalizing voltage.

are you looking for directions on how to build a 3 stage charger or asking if you need one at all?

You need a charge controler unless the cells are designed to accept constant over charge. some are.

16. ### spinnaker Thread Starter AAC Fanatic!

Oct 29, 2009
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It is a UB1250 battery. It is a SLA 5ah battery. On the battery it says.

Standby Use 13.6-13.8
Cyclic Use 14.5 - 14.9.

But I am considering just using one of these Maybe a bit of overkill for my project?

I also would not be against buying another battery if it meant I did not need a regulator and just needed to switch it off and on.

But I still would need a suggestion on which LL FET to use. Hopefull something in a DIP, TO220 or similar easy to work with packaging.