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morphy_richards
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Solar Shed II - new charge controller.

Sun Feb 07, 2016 1:59 pm

At some point in the last two weeks my shed solar charge controller went wrong. At the moment I only go in there at about 5:45am to get food out to feed the rabbits in the adjoining run. I noticed my battery voltage had been steadily dropping but put it down to it being winter. The controller cuts power to load at about 11.5 v and for about a week I have been without shed lighting. As a precaution, a few days ago I disconnected the main load cable but still the voltage continued to drop.

Yesterday I went out to have a better look at what was going wrong. Battery voltage on my golf buggy battery was down to about 3.5 V. Some testing showed 17V coming from the solar panel itself so the culprit must be the controller. Some sort of short perhaps? ... anyway, technically speaking it's now an ineffective doorstop.

I'm hoping to resurrect the battery, currently plugged into a smaller solar cell and controller and its back up to 11V (but will it hold charge or is it terminally sulphated?) For Christmas I was bought a 120 Ah deep cycle leisure battery. I've not yet connected it up. (fortunately otherwise it would have been wrecked!) I want to make a really good shed power system based around it.

I'm now thinking about a long term project to keep me going with occasional tinkering until the summer. I want to make my own charge controller.
Until now I've been using PWM based controllers.

I would like to make a controller that can step up low voltage (from early morning and late evening light) to a useful voltage instead of just being wasted.

I would like my charge controller to have the capability of totally preventing power from leaving the battery if voltage drops below (say) 11.5 v thus preventing a repeat of my golf buggy battery catastrophe with my more valuable leisure battery.

At some point I am going to add other power sources such as a wind turbine, so it should be able to handle that as well as determine when to start "dumping" power.

It would be nice it it could be smart in other ways too, for example it could include a real time clock that can boot up and shut down shed-pi at certain times (including provide a safe shutdown for when battery power gets low). It could also use PIR proximity and remote radio activated switch to power up the main shed systems remotely from in the house.

My big solar panel can output 120 watts. Currently I am thinking along the lines of using something like an arduino and a few voltage regulators (but can I find any that can handle that sort of power)

A- A step up regulator to bring power up to 14.5V ish when light is low.
B - A step down regulator to bring power to 14.5V ish when light is high. *
When battery is charged redivert the output from A and B to a further regulator that will maintain a lower (~13.5v) float charge.

The (future) wind turbine output would enter the circuit in parallel with the solar power but when the battery is in float mode, the output from the turbine would simply be diverted elsewhere.

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morphy_richards
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Re: Solar Shed II - new charge controller.

Sun Feb 07, 2016 4:10 pm

Could I simplify this and just continuously provide a float charge voltage? I suppose this would slow down the recharge rate a bit but would it harm the battery?

rzusman
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Re: Solar Shed II - new charge controller.

Sun Feb 07, 2016 4:27 pm

You should investigate a "MPPT" charge controller, and not try to re-invent the wheel.

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morphy_richards
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Re: Solar Shed II - new charge controller.

Sun Feb 07, 2016 4:41 pm

rzusman wrote:You should investigate a "MPPT" charge controller, and not try to re-invent the wheel.
The popular DIY version cannot cope with my solar panel's output , commercial ones have about 3 figures.
Can't see any suitable regulators out there.
I am coming to terms with the fact I may need to wire my own buck converters.

rzusman
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Re: Solar Shed II - new charge controller.

Sun Feb 07, 2016 4:47 pm

Well, there are MPPT ICs available which you can use to design your own charger.
I think that a microcontroller is (as is so often the case here) a much better idea than using a Pi. The Pi consumes 5W or so, and that is a LOT of power to be wasting on a Solar charge controller. Better to use µW with a small microcontroller.

Buck regulator design isn't too difficult. Look up the UCN2906 Lead-Acid charge controller IC.

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morphy_richards
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Re: Solar Shed II - new charge controller.

Sun Feb 07, 2016 5:17 pm

Thank you :)
So the IC contains pretty much everything and controls pwm via a transistor to allow power from the solar cells to charge the battery?
http://uk.rs-online.com/web/p/battery-c ... s/6200773/
Image

rzusman
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Re: Solar Shed II - new charge controller.

Sun Feb 07, 2016 5:27 pm

Yes, it's a complete temperature-compensated Lead-Acid battery charge controller.
Note that there are app notes for both linear (like the schematic you posted) and switching chargers.

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Tage
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Re: Solar Shed II - new charge controller.

Mon Feb 08, 2016 4:05 pm

If you have a large solar panel and battery and want to maximize the amount of charge power then you need something like LT8490. It is a solar powered battery charger and can also be used with a voltage source input. There is a demo board on the Linear website that is set up to charge a 12V battery at up to 16.7A from a solar panel with voltage range from 36 cells (18V) to 72 cells (36V).
Most cheap solar powered charge controllers have very simple function, they turn a transistor on or off to control the battery voltage. And if you try to use a conventional switching regulator with a solar panel it will not work, as the panel voltage will collapse.

The charge controller must be able to control the solar panel voltage so it can keep the voltage at the point where you can get 100% of the available power out. The LT8490 can also detect partial shade which means it can continue to operate at maximum panel power when the panel is partly shaded and the panel voltage has dropped. It can operate with panel voltage below or above the batter voltage.

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morphy_richards
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Re: Solar Shed II - new charge controller.

Mon Feb 08, 2016 4:56 pm

That's really interesting. The IC data sheetshows a simplified schematic on page 1 which looks fairly easy to realise. The demo board price though is a bit eye watering. Why is that I wonder when the chip itself and handful of components mentioned in the applications information appears very affordable.

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morphy_richards
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Re: Solar Shed II - new charge controller.

Mon Feb 08, 2016 8:34 pm

This "mictronics" LT8490 project website is really handy.
Apart from other things it even contains a spreadsheet, you enter your parameters and it calculates the values needed for all the external components! (Although I'm struggling to find or work out a value for the inductor)

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Tage
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Re: Solar Shed II - new charge controller.

Tue Feb 09, 2016 1:59 am

the LT8490 contains an LT8705 controller, so you could go to the Linear website and type 8705 into the search and click on Design Tools and you can download LTpowerCAD which contains an excel spreadsheet that is really useful if you want to design your own charger. And you can simulate the power stage using LTspice. (You can of course just ask me and I give you the information that you need, such as example designs that are already tested).
If you only need one charger you definitely are better off just buying the demo board DC2069A instead of making your own board. You can also just download the board files and order your own board or tweak the layout as you wish.
The demo board has six layers and you should be able to modify it for up to 20A by changing some components. That would be if you add another 36 cell panel and series connect so you have a total of 72 cells in series. Then you may want to increase the charge current limit by changing a resistor value. Or buy a 250W 60 cell panel which is one of the cheapest per watt, and use the demo board as is.
morphy_richards wrote:This "mictronics" LT8490 project website is really handy.
Apart from other things it even contains a spreadsheet, you enter your parameters and it calculates the values needed for all the external components! (Although I'm struggling to find or work out a value for the inductor)

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morphy_richards
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Re: Solar Shed II - new charge controller.

Tue Feb 09, 2016 2:48 pm

Thanks for everything, especially putting me onto this. :) I dont get why I need to spend >£100 though.

See the attached schematic. Surely, all I need is the IC itself, a handful of resistors, diodes and capacitors, four suitable FETs (that are able to handle the load) and an (as yet undetermined) inductor... :? (And a suitable heat sink)
:)
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mppt.png (29.87 KiB) Viewed 7431 times

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Tage
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Re: Solar Shed II - new charge controller.

Tue Feb 09, 2016 3:38 pm

the key word here is "simplified" 8-)
a heatsink is most likely not needed if the panel is max 200W-250W
morphy_richards wrote:Thanks for everything, especially putting me onto this. :) I dont get why I need to spend >£100 though.

See the attached schematic. Surely, all I need is the IC itself, a handful of resistors, diodes and capacitors, four suitable FETs (that are able to handle the load) and an (as yet undetermined) inductor... :? (And a suitable heat sink)
:)

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morphy_richards
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Re: Solar Shed II - new charge controller.

Thu Feb 11, 2016 8:20 am

If its true (and I'm sure it is as I have no reason to doubt you) I may as well get one of these.
My shed has an ethos. (Heehee) it has to be off grid and as much as possible everything in it must be begged, borrowed, reclaimed, hand made or no more than about 20 quid.

Interestingly I've been monitoring the charge on a second battery these past few moments which is attached to a smaller 10 W solar panel and one of these
Image
It started at about 12.4 v after several months on a shelf but this morning was down to 12.01 v. There is no other load on this battery.
Was that because it was very cold this morning or does the controller leach battery power in weak winter sunshine?
Perhaps the UC3906 is a better option given my objectives. I will further investigate it's switched mode option.

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Tage
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Re: Solar Shed II - new charge controller.

Thu Feb 11, 2016 4:12 pm

the CMP12 controller draws about 0.020A if you believe its datasheet, that is about 6Wh a day, so I would myself just connect the small solar panel directly to the battery, perhaps with a diode in series to block reverse current. If you have only 10W panel and a big battery there is no need to control charge current as you can never get the battery to overcharge.
I have 60W panels on my motorhome and threw away the charger and put in a mechanical switch instead. But my batteries are bigger, about 440Ah.

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morphy_richards
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Re: Solar Shed II - new charge controller.

Thu Feb 11, 2016 5:28 pm

I have a 120 Watt panel on the roof of my shed. The 10W panel is just what Im tinkering with now seeing as the 20A charge controller the big panel was attached to died.

Would it be worth just connecting it ( the 120 W panel) directly to my 90 Ah battery via a voltage monitor and something like a latch relay?

An active Arduino draws something like 0.2 mA (according to their forums) which I would make a superbly simple monitoring circuit. I think I've got the bits knocking around to make one now.

Your mechanical switch, is that similar to what I'm describing or do you mean manually operated?

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Tage
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Re: Solar Shed II - new charge controller.

Fri Feb 12, 2016 2:13 am

I think that your battery needs some type of overcharge protection with a panel that is able to provide 10A current. It depends on if you have any load on the battery, how sophisticated the charge controller need to be. The one you have is quite well suited for that panel and battery. and it disconnects the load when battery voltage drops which is important. so it handles both overcharging and deep disharge protection.
A latching relay is a good idea as long as you make sure the arduino knows the position of the relay, or make sure that an off pulse actually will open the contacts.
but the sun sets every day this far, so overcharging is usually not such a danger with solar panels as with ac powered chargers.

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Re: Solar Shed II - new charge controller.

Fri Feb 12, 2016 7:18 am

Thanks again.
For overcharge and undercharge protection would this be a case of just monitoring the battery voltage and disconnect solar cells or load as necessary? (actually I didn't realise I can just use my cmp12)
I suppose using a latch relay would be a problem because I would need to continuously disconnect the solar cell to check the voltage across battery. Driving a relay to do that would be a big drain.
Some sort of transistor switch would ptobably be the way to go instead.

Ideally I'd like to add a second power source, I've got an old cordless drill and I'm wondering if I can repurpose the motor to make a small wind turbine.
I don't think I can just use the cmp12 with that and the solar panel by attaching them both to the controller because there is no obvious way to redirect the output to a dump load when the battery is charged.

Could I charge the battery from two power sources at the same time? Solar panel and cmp12 in parallel (with diodes to prevent current from one source going into the other) with hypothetical wind turbine if it had its own monitoring circuit that was set to cut off at the same overcharge point as the cmp12?

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Tage
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Re: Solar Shed II - new charge controller.

Fri Feb 12, 2016 3:33 pm

Using a latching relay is OK if you have some electronics to avoid that the relay has to click on and off continuously when the battery is completely full. you could turn off the relay at 14.5V and turn it back on when the battery voltage has dropped to 13.5V, and if it starts to click on and off too much you just lower the turn on threshold to 13V, for example. Battery charge voltage is very much depending on temperature, so if your shed is not heated you should adjust the on and off thresholds with temperature, which is easy with a microcontroller.
I have used latching relay in a big Litihium battery, as disconnect switch. One issue is of course the lifetime, so MOSFETs is usually the choice for any battery switches or charge controllers. npn, pnp or Darlington transistors are not good because they require a constant and relatively high base current. the MOSFET is driven by a voltage so it does not consume power..

it is possible to charge from several pwoer sources, but easiest way to do it is to have different switching charge controllers for each source, and just connect the outputs to same battery. Wind turbines can have very different characteristics so it is not possible to give any recommendations without detailed knowledge of the turbine generator.
One controller that is quite flexible oand often used in chargers with different power sources is LT8705. it has an input voltage regulator so it can work with solar panels. the input voltage regulator is set at the maximum power point voltage, for example 18V for a 36 cell panel, 30V for a 60 cell panel. you can control the input voltage set point with a microcontroller to adjust the panel voltage with temperature and you have an MPP charger. the 60 cell 250W panel is a popular choice as it is usually the cheapest per Watt, as it is used on rooftops in large quantities.

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