Need advice on Solar Shunt Regulator
doggiedoc
Posts: 2,240
So I've finished building both of my solar panels but now I need to control the current for charging the battery for my automatic chicken coop door project. These panels will be in series for 12 volts and 4+ Amps. There will be many times where excess current will need to be shunted instead of to the SLA battery. I realize these panels will be overkill for this project, but it's what I've got.
I found a circuit that appears to be suitable for my needs. I am currently working to understand it completely. Any help would be appreciated.
Thanks,
Paul
I found a circuit that appears to be suitable for my needs. I am currently working to understand it completely. Any help would be appreciated.
Thanks,
Paul
Comments
Just out of curiosity, which circuit are you looking at? I've got no experience with solar panels or battery charging but am anxious to learn more.
Thanks,
-Phil
Solar Shunt regulators seem to be a blast from the past, from before MOSFETS...
I've never really 'got' the idea of dumping power into a Ballast load, when a MPPT regulator is that much smarter.
Perhaps for smaller systems, where simplicity helps and the WATTS dumped are not large, Shunt Regulators could appeal.
If you have Hot water storage, then the excess power can do some use, but even there, a MPPT with load-steering would seem the smartest choice ?
@Phil - I wanted something I could build myself so I started searching the web for DIY solar chargers. I came across this that seemed like what I might be needing: http://ludens.cl/Electron/solarreg/Solarr~1.htm
I may try to breadboard that circuit today. Does it display correctly now?
The one of the left controls the two active shunts, adjusting them to maintain battery voltage at the set point (if too high the shunts are turned on more, too low and they are turned off.
I'm not convinced shunt regulator is the best way to go - a series regulator would need less heatsinking due to lower voltage.
The assumption with both is that the battery can safely by charged at constant voltage with current limit of full panel output - probably reasonable for some circumstances, but you really need to consider the manufacturers recommendations for charging to maintain good battery life.
Actually there is one interesting property of a shunt regulator - the heat dissipated in the shunt would otherwise have to be dissipated in the panel itself, so in theory a shunt will mean the panel runs slightly cooler. Cooler means slightly higher output voltage...
[edit: actually I'd add a 47pF or similar between output of LHS opamp and its inverting input (-) just to ensure stability given the convoluted feedback path via the shunt and battery]
Solar Charge and Diversion Controllers
Shunt2 is the interesting version. It is based on a micro processor under-voltage reset circuit.
It works quite well.
Note! Today I would choose a better MOSFET with a lower gate threshold voltage.
I would suggest using an IRF3708.
Since the MOSFET is either full ON or OFF you probably don't need a heat-sink. Quite true with a properly chosen diversion shunt resistor. With a PV panel running at 15% efficiency the panel will run a bit cooler dissipating only 85% of the captured sunlight as heat, always a good thing. Although directly shorting out the panel is acceptable, dissipating only 100% of the captured sunlight as heat.
Duane J
Here are a couple of circuits I've used for charging SLA batteries from a solar panel, albeit a smaller panel (5W) than you are talking about here. Both of these circuits use a microprocessor reset chip, one of those 3-pin jobs that has a well-defined threshold, a bit of hysteresis, and low quiescent current. The first one operates as a shunt regulator to short circuit the panel when the battery reaches 6.9V and reapplies the current when the battery voltage drops below the hysteresis point. The circuit adds a bit more hysteresis above that provided by the chip. I adapted that circuit from a Motorola application note.
Note total short circuit when the mosfet is on. All the current is shunted, and the voltage across both the panel and the mosfet are near zero. This is operation very very far from the maximum power point, and the power dissipated in either is minimal. There is power dissipated in the series resistance of the cells and the wiring, but I believe it is quite safe for the panels themselves.
The next circuit is a battery cutoff. Shuts off current to the load to protect the battery, if the battery voltage drops below the threshold.
The idea of a shunt regulator presumes to have a lot of extra capacity from the solar panels that have to be dumped. Most people do NOT have the money to provide extra capacity and start out with too little. And to make matters worse, they select a battery chemistry that is more finicky than the Lead Acid about the charging cycle.
One has to protect these chemistrys both by limiting the charge rate and the peak voltage. When you start to be restricted to a C/10 charge rate with a battery that is supposed to be charged 150% at that rate for a full charge, you are never going to get a full charge in one day unless you live in Nome, Alaska. And then of course, that is only for part of the year. Both NiCad and NiMh are made with these goals in mind, though they will tolerate some abuse of faster charges if you can limit these to only when needed.
This chicken coop is a year round operation. Lead Acid perform the worse in cold weather and may deliver only a fraction of their capacity in the winter. Having the excess solar capacity combined with the glare from bright snow will make up for season problems. You could use Lithium batteries that are the least sensitive to cold weather, but I suspect that the charging and discharging control would be a much bigger technical challenge.
I see 6volt cells being charged from 12 volt panels and that seems about right because there are significant voltage drops involved in the addition of a blocking diode, voltage regulation, and current regulation. I think it is better to take the wattage rating of the solar panel as an indicator of overall power and presume that the voltage delivered under load with by at 75% of the so-called open voltage. In other words, a 12 volt 34 watt solar panel is really providing a useful 9 volts and still 34 watts. (Somebody may jump in with a better explanation).
So the 9 volts gets another 1.1 volts dropped if you use a rectifier diode like a 1n4004 (use the PDF for the real voltage drop, not a guess of .7v). Your voltage needs are getting squeezed from both ends as a 6 volt lead acid actually requires significantly higher voltage to charge properly, closer to 7 volts. That doesn't leave much headroom for voltage and current regulation. So a good solution actually matches the solar panel specs to the particular battery.
Beware, too many of these schematics just show 'a solar panel' and 'a battery'. Way too vague, both solar panel watts and actual output voltage are important, and the battery's real charge voltage, real charge rate, and the amount it requires to overcharge at that rate for a full charge must be considered.
Still with all that, you might be able to get some stats on the number of sunny days at your locality by month and the outside temperature for each month. This will help you to figure out if the solar panel will actual work through the winter with your choice of batteries.
Not quite. 12V panels do not output 12V, but are designed to start rather higher, see the load line curves :
http://www.altestore.com/howto/images/article/IV_curve.jpg
and for a single cell, with MPP
http://kamperbob.com/blog/wp-content/uploads/2012/05/Solar-Cell-IV-curve-with-MPP.png
So at colder temperatures, your MPPT will move higher in voltage (and power) - this means you waste more % of your power, when it is probably most precious.
(assuming colder generally means lower sun angles, and less total peak irradiation)
Also, the load lines show they work essentially constant current when simply pulled down (not switch-mode lowered), so in any linear/shunt circuit a 9V load on a 12V panel, will not be same watts, it will be ~the same current, but 3/4 the voltage, which is 75% of the power.
These days, if you are voltage drop challenged, you should use a P-FET as an ideal diode, in place of the standard series +ve diode, ( or a N-FET-ideal-diode in the -ve Lead).
Paul
I was getting nearly 14 volts before I built the frame and mounted everything on the coop. I suppose just a few to many cracked cells.
Back to the drawing boards.
Duane J
Are those figures Open Circuit voltage / short circuit current ?
Can you probe individual cells ?
It does sound like some 'degrade' occurred. Std cell is < 0.5V, so ~ 36 in series are used.
They provide a data sheet for the individual cells, reproduced below. Open circuit with full sun, you could see over 14V with 24 cells in series, but once you put the cover in place, that probably cut it down a bit. Probably not a cracked cell. The maximum power point stays at around 0.5V per cell, so 24 cells are not sufficient for charging a 12 SLA battery at >14V. Panels for charging 12V batteries do usually have 36 cells, and an open circuit voltage of near 22V and a maximum power point of around 17V.
The ones you made have 12 cells or 4V each.
But, my word, that kit is really expensive at $140/34W = $4.11/W.
So using 3 kits it would cost $420/102W and no warranty.
You can buy new panels for around $1/W with a 20 year warranty.
Duane J
@Duane - good to know. I started to just "go buy" panels for this project but in the spirit of building it from parallax parts I chose the kits. The value of the "learning experience" might not be there for a larger project but these kits have given me a new understanding of how solar panels are constructed. However, I'm a little bummed I didn't buy 3 while they where on sale.
Here's a pic from in progress:
1. The builder tends to buy less cells that he really needs.
2. The documentation tends to obscure the real output of the product.
Pre-fabricated, ready to use solar panels may actually be using special thin glass to cover them as a means to reduce losses. Such glass is not generally available at your local outlets.
My own feeling is that buying 150%-200% of what you think you need might be the best way to start unless you have very good information and help. Or, it might be easier to just buy a pre-made proven 12volt solar panel that is intended to work in lieu of an automotive battery as a power source.
I suppose Google or Wikipedia might tell you. It is not really important. AKA Bill W.
Drunks don't have to go to those damn meetings.