regulate solar panel output

[justwantin]

I have two solatr panels 12V and 9V which I've picked up along the way and I'd like to put them to work chargeing batteries but since their output will vary with the brightness of the sun so using something like an lm317 is a lost cause.

Can anyone point me in the direction of what I should be googling for. I'm new to electronics as a hobby and don't mind doing the research but I still need to know what to be looking for.

 

[MikeMl]

First, your panel should put out a voltage at least 3V higher than the battery you are trying to charge, in which case you can use a linear regulator like a 317.

If the panel puts out only a slightly higher voltage than the battery charge voltage (14.5V for a "12V" battery), then you can use a special "low dropout" regulator.

If it puts out less than the battery charge voltage, then you will need a boost-converter SMPS to step-up the panel voltage.

 

[justwantin]

your panel should put out a voltage at least 3V higher than the battery you are trying to charge

It is my understanding that I can harm a battery by charging with too high a current. For example no more than 13.5V for a 12V SLA battery and 6.75 for a 6V SLA.

My 9V panel may put out 12V and my 12V panel may put out 20V when the sun is bright

What would a 317 be set for in such circumstances? For example if I set low to charge when it is bright but overcast as soon as the sun breaks through I've run up past what I read to be the safe charging limit.

 

[MikeMl]

It is my understanding that I can harm a battery by charging with too high a current.

Yes, too high a current will harm a battery. No panel you can afford will put enough current to harm your battery. You need to limit the maximum voltage reached by the battery after it is fully charged.

For example no more than 13.5V for a 12V SLA battery and 6.75 for a 6V SLA.

Actually, while charging a SLA, it is ok to allow the voltage to rise to 14.5V, but after the charging current into the battery drops to a low value, your charge controller must detect that and switch to a lower float voltage of 13.5 to 13.8V. This is called a 3 state charging algorithm. Read this.

My 9V panel may put out 12V and my 12V panel may put out 20V when the sun is bright

So you need to limit the voltage applied to the battery, not the current. The current will take care of itself.

What would a 317 be set for in such circumstances? For example if I set low to charge when it is bright but overcast as soon as the sun breaks through I've run up past what I read to be the safe charging limit.

If you build a one-state charger which simply limits the output (battery) voltage to 13.6V, you will not overcharge the battery, but it will take a lot longer to bring the battery to full charge compared to what a three-state charger could do. Look up a data sheet for a PB137 IC chip.

 

[justwantin]

Thanks that will keep me out of trouble for awhile

 

[justwantin]

If you build a one-state charger which simply limits the output (battery) voltage to 13.6V, you will not overcharge the battery, but it will take a lot longer to bring the battery to full charge compared to what a three-state charger could do. Look up a data sheet for a PB137 IC chip.

Am I correct in thinking that at >=15V in the PB137 will provide 13.6V out witrh just two additional capacitors as per the application diagram circuit? Probably need a decent heat sink though.

In the first instance I was interested in using the 9 volt panel to charge a 6V SLA. At present I am running an array of 15 LEDs off a microchip all night. This simulates a flickering flame and is a prototype for something I want to do with Japanese Lanterns on the verandah. I use an LM317 to drop the voltage to 4.5 to run the PIC circuit with LEDs

Running all night I drop maybe .2-.3 volts from the battery and I figure when I set up a lantern it won't be much more so there is no great daily requirement on charging, Just a top up so to speak.

I had a look at Welcome to STMicroelectronics and there does not seem to be equivalent to the PB137 for lower voltages. I'm thinking that would be all I'd need and I could make a proper charger for the 12V panel in another project down the line. Of course with the voltage fixed at 13.6 it'd be easy to drop it again with an LM317.

At present I am charging SLAs with a variable benchtop PS I built but it seems a bit of a waste with these panels laying around

 

[MikeMl]

The PB137 is just a convenience for 12V SLAs, and it has a lower drop-out than a 317. A LM317 with the appropriate two resistors and two capacitors will do the same thing, just program the output to 13.6V+-0.1V. The unregulated input can be supplied either by a solar panel or an unregulated DC wall-wart that puts out a minimum of ~15V

For a 6V SLA, use a LM317 programmed for 6.825V+-0.05V. If running a PIC off the battery, you might need a low-power, low-drop-out (LDO) regulator (see TI or National or Fairchild) to get 5V off the 6.8V.

 

[colin55]

The first thing I have to know is the wattage of each panel.

 

[justwantin]

I had a look at the LM317 with a solar panel last summer on a breadboard but couldn't get the Vout from the LM317 circuit right with the comings and goings of the sun behind clouds. I can't say I stuck with it but I'm now a little more interested in this sort of thing as a hobby

I'm just using the 317 with the PIC circuit because I have it and anything under 5V is safe. I only have i PIC16f675 ATM.

A few more questions if you have the time:

Does the Vout from the LM317 charger drop down to battery voltage then rise as battery charges?

I take what you say above to mean that if I just set the LM317 to, say, 6.825V then the battery will just charge up to 6.825 and float there as long as the panel puts out sufficiently? I know I'd need a diode to keep the battery from discharging back if it doesn't

Would there be any advantage to shut down the "charger" when battery reaches a full charge? This could be done with something like the L200, **broken link removed**

Thanks for your answers

 

[colin55]

Before any designs can be contemplated, I need to know the wattage of the panels and the Ahr of the batteries.

 

[justwantin]

I have two different SLAs, Diamec 6V 1.3 Ahr and Century 6V 4.0 Ahr.

I'm not too sure about the panel. It was salvaged from a unit used to electrify a paddock fence. The glass is 145X145mm and there is a bit of paper pasted onto the backside but the details were penned in and difficult to read:

PM - 1.3
VM - 6
VOC - 90 in a diamond shaped red stamp also containing a backwards C and an H
IM ? - 1.76 or 176 ( there's a bit missing between the one and seven)

I came up with the idea of using the L200 from reading thisSealed Lead-Acid Battery Charger Circuit

 

[colin55]

It looks like about a 1 watt module. You can run that directly into the 4AHr battery via a diode and connect the 1.3AHr battery in parallel. They will never overcharge if you use them a small amount each day.

 

[justwantin]

It looks like about a 1 watt module. You can run that directly into the 4AHr battery via a diode and connect the 1.3AHr battery in parallel. They will never overcharge if you use them a small amount each day.

Does that mean high Vout from panel is not so important? Right now in Melbourne (13:00) I can get 11.75V out of the panel with the sun half behind a cloud. I thought this would be too high go directly into a 6V battery.

How would I figure this out according to watt rating of a panel to battery requiring charge? Am I to think in terms of replacing via solar panel what has been discharged from the battery in terms of watts or miliamps per hour.

Can you give me some keywords to google in order to get some info relevant to sizing panels to batteries. I just happen to have these batteries now but I'm not sure if they or perhaps something else would be more suitable by the time I get to doing my latern project although the 4 Ahr battery is the goer for my prototype as it runs the LED's much brighter than the 1.3 Ahr.

Sorry, I'm trying to get a handle on all this so I can start sorting things out myself. An .avi of the LED flame is here if you want to have a squiz. **broken link removed**

 

[colin55]

I live in the next street to you and the panel automatically adjusts the battery voltage. This only happens because I live so close to you.

 

[justwantin]

North or South? ;^)

 

[colin55]

- relative to the size of the world - Dingley

 

[justwantin]

Langwarrin, big road.....!

the panel automatically adjusts the battery voltage

I note that when a battery reads, say 5.45 and I hook up a lm317 circuit set up for 6.7 Vout hooked up to a panel putting out 10Vout that the battery will still read 5.45V. Would a direct battery hook up to the solar panel behave same with battery voltage rising as it charges. What happens when the battery reaches full charge connected directly to a panel putting out a higher voltage. Is this automatic adjustment in the lower voltage at the battery?

 

[colin55]

If you add a regulator, all you are doing is wasting 10% of the output of the panel.
The panel automatically adjusts to the voltage of the battery AT ALL TIMES.

 

[MikeMl]

Simplistically, a solar panel is like a current source; if you connect a small panel directly to a battery, the terminal voltage of the battery will be determined by the state-of-charge of the battery. As the small current from the panel flows into the battery, the battery voltage will slowly rise.

To the extent that the daily Ah output from the panel exceeds the self-discharge and load discharge current of the battery, the battery will eventually reach full charge, as evidenced by the SLA battery voltage creeping up above 2.35V per cell. That is point at which you either have to disconnect the panel, or put a regulator set to 2.25V per cell between the panel and the battery. If the average external Ah load (plus the battery's self discharge rate) is equal to or greater than the Ah output of the panel, then the battery will just hold its own, or slowly loose charge, in which case the battery voltage will slowly get lower, and no regulator is needed.

 

[colin55]

In the discussion above, a crutial point is omitted. A 4Ahr battery will tolerate an overcharge at less than the 14 hour-rate and the energy will simply be converted to heat.
In our case the battery will take 60 hours to charge and this will take 12 days, so a regulator will never be needed.