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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 16th Feb 2020, 12:44 pm   #1
Tractorfan
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Smile Solar panel charge control.

Hi,
I was given an ex-Maplin's photvoltaic panel which is rated at 21 watts and outputs about 22 volts off load in full sunlight. (Oddly, after being in the dark for several months, it output almost 24 volts, but quickly settled down to the aforementioned 22 volts.)
I'd like to knock up a simple circuit to trickle charge a 12 volt lead acid battery that may be standing idle for weeks in my old tractor.
I assume that something more than a diode is called for.
Thanks in advance,
Cheers, Pete.
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Old 16th Feb 2020, 1:00 pm   #2
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Default Re: Solar panel charge control.

Unless you need it in a hurry the usual auction sites have them at prices well below what you could build one for.

Our local airstrip uses them for their solar powered runway lights.
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Old 16th Feb 2020, 1:09 pm   #3
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Default Re: Solar panel charge control.

It is just a matter of stabilising the voltage from the solar panel to the correct charging voltage for long-term application. If you don't the battery will be destroyed by loss of electrolyte. The difficulty is getting a circuit that is OK with the input from the solar cell being less than the battery. I think you can just buy a special version of a voltage regulator chip that does it. I was about to look at this problem myself.
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Old 16th Feb 2020, 1:24 pm   #4
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Default Re: Solar panel charge control.

Agree, purchase a purpose made charge controller, these are inexpensive and sold by many vendors.

A series silicon diode will work in preventing the battery from discharging into the solar panel at night, but gives no control over charge rate.
A simple diode will work OK if the charge rate is very low in relation to the battery size.
A very slightly better idea is to connect a suitable small lamp in series with the diode. In dull weather when the output is low, the lamp filament will remain cold, drop very little voltage, and have little effect.
In bright sun the drop will be greater and the charge current thereby limited.

Remembering the low price of charge controllers, it is debateable if such improvisations are justified these days.
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Old 16th Feb 2020, 1:33 pm   #5
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Default Re: Solar panel charge control.

If the battery is an open-vented type (reasonable to suppose for an old tractor, unless it's been retrofitted with a fancy new one) then you don't need anything more sophisticated than a diode (you can obtain a more-than-suitable one from an old PC power pack). In the event of overcharging, the battery will just bubble a bit more vigorously. (And where there is enough sun for PVs, there's often air conditioning nearby, with an inexhaustible supply of battery top-up water .....) As long as the capacity of the battery is more than about ten hours' worth at maximum current (21W @ 12V; call it 2A, so 20Ah or more) and it is not left unattended long enough for a cell to dry out completely, this can work. It's still a gentle tickle for a battery, compared to a DC dynamo charging system!

For the first level of sophistication, you can use a simple shunt regulator to clamp down the voltage. The excess energy will end up being dissipated as heat; but at least it didn't cost you anything. Just be sure to use a big enough heat sink, especially bearing in mind it's already in bright sunshine when it needs to work the hardest.

For the ultimate in charging efficiency, you would need a combined buck-boost regulator, so as to supply the battery with a steady voltage more or less independently of whatever the panels were producing. There will be ICs available to do this with minimal support components -- even using ready-wound inductors. This will convert, for example, 1A @ 21V to (ideally; perhaps not quite as much in practice) 1.5A @ 14V, so making better use of the excess; but it would also be able to step up an input voltage below the terminal voltage.
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Old 16th Feb 2020, 1:35 pm   #6
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Default Re: Solar panel charge control.

You can just buy a little "brick" that fits on a small heat sink.
They contain a shunt regulator and a diode.
They are commonly used on boats that are only used at weekends in the summer to keep the bilge pump going over winter.
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Old 16th Feb 2020, 1:58 pm   #7
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Default Re: Solar panel charge control.

If you want to build your own, I have built this one.

http://www.mdpub.com/555Controller/

The site is interesting generally, he gets up to some interesting stuff.

Ken
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Old 16th Feb 2020, 2:09 pm   #8
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Default Re: Solar panel charge control.

These really are cheap as chips nowadays, I saw this one recommended by a youtuber that I follow and yes, they really do work well, and for the price of a pint

Solar charge controller
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Old 16th Feb 2020, 2:13 pm   #9
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Default Re: Solar panel charge control.

Oh, you can buy stuff ready-made, but where's the fun in that? It's much more satisfying to build something from junk, and you get to learn something into the bargain. OK, sometimes what you learn is that ready-made is better; but if something is just lying about and full of salvageable parts, well, why not? There's a certain satisfaction to be had in doing for a shilling, what anyone could do for a pound.

You could even use the main switching MOSFET from the aforementioned PSU as the shunt regulating device. They usually have a reasonably-defined turn-on voltage, so you can get away with just a simple potential divider on the gate.

Note that one thing that no improvement in construction will ever change is the laws of thermodynamics! If you find out the hard way that your heatsink is not good enough for a home-made shunt regulator, it will not be good enough for a ready-made one either -- there will be exactly the same number of watts for it to dissipate.
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Old 16th Feb 2020, 4:41 pm   #10
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Default Re: Solar panel charge control.

Whilst the laws of physics are indeed unalterable, ready made charge controllers often use very small heat sinks and work fine.
That is because most types are NOT shunt regulators. They use Pulse width modulation. The whole of the output is directed into the battery when it is low. As the battery approaches full charge, the controller connects the PV module to the battery only briefly. The module is open circuited or short circuited the rest of the time.
The PWM frequency varies but is often quite low.

I have a solar charge controller rated at 100 amps into a 24 volt battery. It does not have a heat sink able to dissipate 2.5Kw.
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Old 17th Feb 2020, 12:35 pm   #11
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Smile Re: Solar panel charge control.

Hi, and thanks for your helpful replies.
Ideally, I'd like to build a controller, but seeing as they're so cheap, and it could take me months to get around to it (the perils of retirement! ) I'll bang an order in to jolly old eBay.
There are lots of them listed, but they all look the same to me. Probably the same type sold by different sellers.
Cheers, Pete.
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Old 17th Feb 2020, 7:18 pm   #12
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Default Re: Solar panel charge control.

Hi I went through several types for a remote electric fence energiser.

Some are on off with a gap - I would not reccomend these as the battery has to discharge down to the lower switch threashold each cycle and if you get a period of low sun right at the low point you miss out on charge right when you might need it.

Some are proportional control and maintain a float point which maintains a good float voltage as they would get in an automotive environment.

Some are more sofisticated and use a switch mode circuit to maximise the power transfer giving a better charge from a given size solar panel.

Some combine any of the above with an auto disconnect to prevent against discharging the battery below a point where it is damaged. Important if you have a continuous drain load like a fence energiser. ( I know you can buy some of these energisers with auto disconnect built in)

I settled for a proportional control float charger with load disconnect as a compromise between sofistication and price with battery protection . LCD status graphics is also availiable on the devices. - it seems to have worked but has needed mains top up this winter with the long periods of no sun.

Also check the quiesent current drain specification as this becomes a factor over periods of low light.

Of course you might find that a large tractor battery may never be damaged by the current availiable from your solar unit particularily if you off angle it in the summer to reduce efficiency , so a simple diode might be all that is needed! But with a shunt that clamps around 14.5 volts you can be more confident of not generating too much hydrogen! - and dont forget some fusing and reverse conection protection.

p.s. A friend of mine exploded a battery testing it with a light bulb. So althought I have worked with batteries all my life including building an acid concentrator as a teenager to unsucesfully "rejuvinate a tractor battery" and not had a problem good close fitting safty specs are a minimum in my opinion.
Pete

Last edited by G4_Pete; 17th Feb 2020 at 7:26 pm. Reason: p.s for safety glases.
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Old 17th Feb 2020, 9:26 pm   #13
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Default Re: Solar panel charge control.

I'd go with Julie and the simple diode. A tractor battery will be at least 40AH for an old petrol/TVO one and more like 70-90 AH for a diesel. The overcharge from a 21W panel really wont matter.
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Old 18th Feb 2020, 1:46 am   #14
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Default Re: Solar panel charge control.

Quote:
Originally Posted by G4_Pete View Post
p.s. A friend of mine exploded a battery testing it with a light bulb. So althought I have worked with batteries all my life including building an acid concentrator as a teenager to unsucesfully "rejuvinate a tractor battery" and not had a problem good close fitting safty specs are a minimum in my opinion.
Pete
Testing a battery with a bulb can set off the gasses if it has recently been charging.
Disconnecting the charger to move the leads to another battery without switching off can also do it too.
I was shown one with the top blown off from moving charger leads like that by a garage owner. The mechanic that did it had already been told off.
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