Off-grid solar power supply - 10W solar panel - regulate or not? - Page 1
Forum section: electronics / beginners

author: CyclicUK
author_rank: Contributor
timestamp: February 15, 2015, 03:33:37 pm
content: Dear all,So as part of my electronics adventure I decided to build a small off grid power supply in my “lab” (bedroom) for testing and build purposes. I picked up a tiny 10W solar panel and a 12v 60ah car battery. After reading about the dangers of car batteries I put in an automotive fuse box on the output side with fused outputs for 3A, 5A and 15A.Having read about battery conditioning I went and purchased a cheap PWM solar charge controller from ebay which I have established is complete rubbish and doesn’t work - probably because my solar panel is so small but possibly because it’s junk - pending investigation.I want to build my own charge controller but in the interim my battery is going flat and I want a quick and dirty solution for hooking up the panel but being middle aged I’m a scaredy cat so I thought I’d ask an expert.Now I realise that at 10W I can probably just hook that panel directly to the battery without a charge controller, it came with a diode soldered across the plus and minus terminal (1N4004) which I think is for polarity protection so I went ahead and soldered on a blocking diode (1N4004) on the positive terminal of the panel for reverse current protection.The panel came with standard battery connectors so I’ve set up a couple of connecting posts on my windowsill to connect those to with wire running directly to the battery.My concern is that if I was to accidentally short those 2 uninsulated connecting posts then I would be shorting my battery out which would be very bad - so I want to put some protection in as close to my battery as possible to prevent that happening.I’ve read that I can safely give my 12V (6 cell) car battery 14.1V even if it is fully charged (http://www.powerstream.com/SLA.htm) so I was wondering if a simple short term solution would be to hook up a voltage regulator in the battery box at 14.1V - I’ve got some of those LM2596 regulator modules (example:http://www.ebay.co.uk/itm/DC-DC-Buck-Converter-Step-Down-Module-LM2596-Power-Supply-Output-1-23V-30V-2014-/171369032731?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item27e663ec1b) sitting around which I thought might be fit for this purpose. I’ve examined the board and it has an SS34 surface mount diode which I’m sort of assuming is for reverse current protection but it seems to be mounted on the input side so I’m concerned it isn’t there to protect the regulator? I’m not very good at reading data sheets yet.So my question is: Can I use this regulator and if I can then do I need to add an extra protection diode on the output side (and have to suck up the extra voltage drop) OR can I get away with just putting a blocking diode in close to the battery and forget about the regulator altogether? - this is all for a short-term solution.My solar panel is: 12V, 10W with a nominal voltage of 17.5V - at 12V the current is P/V = 833ma (it’s just over 0.5A at 17.5 volts)The non-heatsinked version of the LM2596 which I have is rated at 2A.Sincerely,Middle aged scaredy cat

author: Seekonk
author_rank: Super Contributor
timestamp: February 15, 2015, 03:42:33 pm
content: That 2596 is a good match for that panel. I have used a number of them in solar applications. Search 2596 here for recent discussions. Don't take them too seriously.

author: SeanB
author_rank: Super Contributor
timestamp: February 15, 2015, 03:58:47 pm
content: You can just use a 5A fuse on the charge wire as well, though with a 10W panel in the UK the battery will have more self discharge than the panel can put into it. A 50W to 100W panel will be a better match, I am using a 30W panel, though here at 30S I do have the advantage of actually getting sun pretty much all day every day to charge the battery. In the UK you need a bigger panel just because of the shorter days in winter and the lower solar irradiance.

author: CyclicUK
author_rank: Contributor
timestamp: February 15, 2015, 05:34:35 pm
content: Thanks for your quick responses. Good idea on the 5A fuse on the charge wire. You’re right I realise about the panel size - I think 80W is about right for a 60ah battery isn’t it? (C/10) - with a charge controller of course. About that diode on the LM2596 module - is it there for protection? Is there a way of testing it or should I just put another diode on the output to be sure?Many thanks again,Old git

author: Seekonk
author_rank: Super Contributor
timestamp: February 15, 2015, 06:16:00 pm
content: The diode should be on the output of the module, not the input.

author: CyclicUK
author_rank: Contributor
timestamp: February 15, 2015, 06:28:58 pm
content: Roger that. Thank you - I'm building it now...

author: Seekonk
author_rank: Super Contributor
timestamp: February 15, 2015, 06:46:43 pm
content: Solar is a long learning curve. In less than optimum sun, the value of operating a panel at its power point can not be overstated. Look at the voltage, amp, power curveshttp://www.egr.unlv.edu/~eebag/Solar%20Cell%20Shading.pdf

author: CyclicUK
author_rank: Contributor
timestamp: February 15, 2015, 07:02:36 pm
content: Quote from: Seekonk on February 15, 2015, 06:46:43 pmI've been looking at that and I was wondering - can I use something like a boost converter or a Joule thief circuit to safely increase the voltage from the panel into the regulator to keep that input voltage around 16.5V (regulated down to 14V ish)? That could also be very useful for my arduino circuits which operate from a 6V solar panel?

author: IanB
author_rank: Super Contributor
timestamp: February 15, 2015, 07:30:19 pm
content: One thing to watch out for is that car batteries should be used in a well ventilated place (shed, garage, outdoor workshop, etc.). Using a car battery inside the house is not recommended. When charging it can give off flammable gases which makes a battery an explosion hazard if there are any sources of ignition nearby. The ventilation is needed to make sure these explosive gases do not accumulate near the battery. Even closed and "maintenance free" car batteries still have vents to let the gases out.

author: CyclicUK
author_rank: Contributor
timestamp: February 15, 2015, 09:01:29 pm
content: Yep - I'm aware of that. One thing I should've mentioned in my original post is that I've got a 12V PC fan on the outside of the box (which I run at 5V) driven by an astable multivibrator circuit which turns on for about 10 seconds every minute or so to suck out any hydrogen that might be lurking around - not sure I really need it though as from what I've read a car battery only gases when it's almost charged and being fed quite an over-voltage. I always blow on the terminals as well before I connect or disconnect anything directly just to make sure but thanks for pointing it out.

author: Seekonk
author_rank: Super Contributor
timestamp: February 15, 2015, 10:02:44 pm
content: The 2596 regulator you have can be modified to keep the panel at the power point voltage. Pin #5 can be lifted from the board and used to turn the converter on and off very fast. I explained it in the following thread.https://www.eevblog.com/forum/projects/solar-phone-charger-42391/msg601175/#msg601175

author: CyclicUK
author_rank: Contributor
timestamp: February 16, 2015, 12:04:19 am
content: Quote from: Seekonk on February 15, 2015, 10:02:44 pmRight now you might regret mentioning that because now I’m going to have noob questions about it which you might find irritating so I hope you don’t mind?So if I understand correctly, pin 5 on the LM2596 is the on/off pin and so the circuit you’ve posted here:https://www.eevblog.com/forum/projects/solar-phone-charger-42391/?action=dlattach;attach=134092;image is switching the regulator on and off depending on the conditions of the input voltage - in my case, I’d be opting for the NPN with zener solution because that’s just what I happen to have lying about.So let’s say my target input voltage is 16V and so I’ve chosen a 16V zener for this purpose (which is to be regulated down to 14.1V). When the input voltage is below 16V then the 1000uf capacitor is charging until its potential reaches the zener breakdown voltage which then switches the NPN on at the base which switches on the regulator to soak up the voltage - is that correct?Forgive me because I’m still a learner driver but I’m just a bit confused because the output to pin 5 comes from the collector which I’m used to seeing when the NPN transistor is initially in the on state and the base is being used to switch it off - but I’m thinking the capacitor is altering that relationship here by blocking that current until it is at least equal to the zener breakdown voltage (in my case 16V).Would that be a fair analysis? If you don’t mind me saying so, and if that circuit does what it looks like it does - that’s F’ing brilliant! That would be my joule thief idea / inductor woes completely solved!

author: Seekonk
author_rank: Super Contributor
timestamp: February 16, 2015, 09:26:16 am
content: If the zener is 16V, the target voltage will be 16V +.6V for transistor + about 1V for resistor loss = 17.5V which is a typical power point. It is a simple comparator. Pin 5 transition voltage is about 1.3V. In fact A comparator chip like a LM393 or LM339 would work even better. The reference voltage would come from two diodes glued to the back of the solar panel ( a bridge rectifier would be a convenient package). Then the panels power point would follow the temperature of the panel. Low temperatures produce higher panel voltage. A large capacitor connected to the panel is the storage during short periods when converter is off.

author: jamesd168
author_rank: Contributor
timestamp: February 16, 2015, 09:25:56 pm
content: Quote from: CyclicUK on February 15, 2015, 03:33:37 pmI am a little confused, if you solar panel is 12V output, why do you worry about regulating it so it does not exceed the safe charging voltage of the battery @14.1V?

author: Seekonk
author_rank: Super Contributor
timestamp: February 16, 2015, 09:34:24 pm
content: What are you confused about? A 12V solar panel open circuit can be 21-23V, maximum power V*A is at about 17.5V. If you don't want to waste power a switching regulator is used to boost current output.

author: CyclicUK
author_rank: Contributor
timestamp: February 17, 2015, 10:13:49 am
content: Hope you don’t mind but I’ve got a couple of other questions if I may?1. So when you say 2 diodes on the solar panel do you mean something like an 11V and a 5V in series? In that case would I be right in thinking that the 11V diode would experience an avalanche breakdown whilst the 5V would experience a zener breakdown and that to some degree the different temperature coefficients would cancel themselves out? Presumably giving a more accurate reference voltage than a single diode on its own?2. If the objective of the circuit is to keep the input voltage to the regulator above say 16V for example then what is the purpose of switching the regulator on and off? Could the same effect not also be achieved by simply connecting the positive input of the regulator to the emitter of the NPN transistor and leaving the regulator in the on state?Sorry for all the questions and I know these probably have obvious answers to you and may seem a bit basic and naive but I’m here to learn and ultimately I’d like to be able to design and build my own circuits so I really want to understand exactly what’s going on.

author: Seekonk
author_rank: Super Contributor
timestamp: February 17, 2015, 02:48:21 pm
content: Solar panels are basically exposed diodes. The forward voltage of a standard diode changes with temperature. If you mount a standard diode (preferably two or more) on the backside of a solar panel, that voltage it will track the power point. A large number could be used to replace the zener. I suggest you actually put the circuit together and see how it operates.I would like to know why you think your PWM regulator doesn't work. I have a Morningstar microprocessor based regulator, that actually won't even start operating till it has been connected to a battery for 5 minutes. It takes that long before it can determine the state of the world.If I have not answered your question it is because I do not understand what you have asked. Diodes do not act like zeners. If you want basics I suggest the many online sources like wiki. I will be happy to answer any questions directly related to the circuits provided.

author: knks
author_rank: Regular Contributor
timestamp: June 02, 2015, 10:41:45 pm
content: What are you going to do with the battery after you charge it? If it is a starting battery, it is not designed to stay discharged for more than few hours. It's designed to start the motor and then be fully charged immediately after this. Google about sulfation