will any solar panel will do & how to know which is a good one?

[pibby]

current setup :

got a 100 amp hr battery bought a few months ago.

have got 2x32w unisolar which i used to have on my boat when i lived on it nearly 20 years ago.

currently this is all i've got solar wise and it does me just fine to put couple lights on and run a stereo.

do find in winter there simply isn't enough daylight hours to get enough charge in the batteries (mainly due to the amps that run the stereo) with the small panels and i think the panels being so old have lost a bit of efficiency.

looking at buying a 12v fridge and would like to add another battery (prob just get exaclty the same to keep things simple) and get a 200w-250w panel. current regulator is a BP solar 8 amp but i've got a 20amp something or other one sitting here i can use.

so can anyone point me in the right direction to what panel to get, would prefer good quality stuff to cheap junk. would it work with my existing panels?

do i keep it all 12v or wire the panels and the batteries as 24v (believe this is more efficient? plus the fridge can take 24v) then use something to step it down to 12v to run the lights and stereo?

thanks,
brett.

[loanrangie]

200w plus panel sizes are pretty big and getting heaving, quite a few here have 120-160w folding panels with 1 or 2 100/120AH batteries which seems to be enough for a few days or more if there is ample sunlight.

[pibby]

This for home not a vehicle.

[DiscoMick]

It really depends on what you are going to run off the battery. For example, when camping we find an 80 watt folding solar is usually just enough to recharge a 100 amp AGM to run an Engel 40 litre fridge and some LED lights, but its marginal in wet weather. Add demand and you need more solar and storage. First you need a total for the current draw, then size to match it.

Sent from my GT-P5210 using AULRO mobile app

[pibby]

not too worried about calculations or to be that precise about it. i know it's going to be enough for what i want to do with it. it's the other points in my question i'm asking about.

this will just get me through into a holding pattern - i can get 240v put on but i'm just waiting for the advances in battery storage and the big drop in $ that i keep reading and hearing about to actually make it into the real world. then i'll probably go standalone but that's a different topic altogether.

brett.

[Warb]

Not bothering to do the calculations is a quick way to waste money.....

Batteries die very quickly if they are discharged too much, and "too much" is actually surprisingly little. Even with a genuine deep cycle battery, discharging to 60% capacity (that's 60% remaining, not 60% gone), will reduce it's life - and the deeper you discharge it the fewer times it will fully recover. A deep cycle battery may be rated to thousands of cycles to 80% charged (i.e. 20% used), but that might drop to a few hundred cycles to 60% charged (40% used) and a few tens of cycles to 20% charged. A car type "cranking" battery can be seriously impaired by one deep discharge - I used to use special lightweight batteries in racing cars, and they could be permanently killed by a single deep discharge (cranking until flat = throw the battery away)

So the 100amp hour deep cycle battery has a capacity of about 1.2kwh, but using more than about 0.3kwh will reduce its life expectancy..... 40W of lighting will use 0.3kwh in 7.5hours.

The best approach is to work out your worst case power usage overnight, and size your batteries to suit. THEN you know how much charging you need to do each day, and can therefore work out what panels and charger you require.

OK, having said that:

24V is not inherently better than 12V. The efficiency gains revolve around cabling losses (lower voltage = higher current = more loss). Use heavy cable to reduce the losses!

A solar panel is (for example) rated at 200W, at 25C with 1000W/m2 sunlight hitting it. As the temperature of the panel increases, the output decreases. As the amount of light hitting it decreases, so does the output. That means that most of the time it will produce significantly less than 200W because the panels temperature will likely be much hotter than 25C, and apart from a couple of hours around midday in summer there is rarely 1000w/m2 sunlight.

Solar panels have a maximum current. That is an absolute maximum, which is important to understand. If a panel is rated "200W at 24V", it's maximum current is 8.3amp. The voltage of that panel is dependent on the load it is connected to. Connect it directly to a 12V battery, and it will produce 12V (actually a bit more than that, probably around 14V depending on the state of charge of the battery), but its maximum current will still be 8.3amp, which at 14V gives 116W. So now, even under ideal 25C and 1000w/m2 conditions, the 200W panel cannot produce more than 116W.

A cheap charge regulator will work exactly as described above, it will drag the panel voltage down to that of the battery, and effectively halve the maximum output of your 200W 24V panel.

Connecting dissimilar panels will do the same thing - all the panels will be limited by the "worst" one, and the actual output will be far lower than you read on the labels.

So, my advice:

1/ work out how much power you need in a worst case scenario (say 48hours without charge to 60% remaining capacity) and size the batteries to suit.

2/ work out realistically what panels you require to recharge your batteries. Again, worst case scenario is mid-winter where you will see perhaps 80% of the rated output of you panels for an hour or two at midday, far less for the rest of the (short) day, assuming you're not battling with cheap chargers etc.

3/ buy a decent "MPPT" charger of suitable capacity, which will provide the panels with the load they need to make most power and as a separate process will feed the battery with the power it needs to charge effectively. Note the word "decent". A cheap eBay "MPPT" charger does not actually do this, it simply fakes the load to the panels, but does not actually adjust ("track") to get the best from the panels. It's a tiny bit better, but it's not actually MPPT. Note that "domestic" chargers for home power systems often cater for higher voltage batteries and panels, whilst the low end and "camping" regulators use 12V or 24V panels and batteries.

4/ buy a matched set of panels, of a voltage that is suitable for your charge regulator, from a panel retailer. On eBay there are many companies that sell panels, many also sell golf clubs, umbrellas, barbecues and cuddly toys. Find one that only sells solar stuff. I have some panels from "Low Energy Developments" (no affiliation) that have been good. Mixing panels is fine, as long as you completely understand the specifications of those panels and know how they will behave. Otherwise you may well be crippling your new panels output.

5/ work out how to mount the panels at the best angle, and run big fat cables to minimise losses.

6/ buy a battery monitor with a "shunt". The shunt sits in the battery cable, and the monitor reads the voltage and current flowing in or out of the battery. It will therefore tell you not only whether your system is charging or discharging, but also how much charge is left in the battery. Armed with that information you can see whether you need to increase your battery capacity, amount of panels, or simply switch off the lights before you damage the battery!

Too much information? Then just buy a 200W panel and hope for the best. Don't expect to get anything like 200W from the panel, and don't expect your battery to last long.

The "big price drop" will never happen, so don't hold your breath!

[Background: I have nearly 20kW of solar power, comprising grid-interactive, hybrid grid/battery and straight battery charging, plus solar water pumps and camping systems. I have also spent "many happy hours" fixing poorly designed solar power systems in other peoples camper trailers, farm sheds etc.]

[pibby]

warb - thanks heaps. i'll need to read what you wrote a few times to take it in.

probably sounds like i'm being cavalier in my approach but it is my home i am talking about. come 1st jan 2016 i would have been home for 2 weeks since march 2015. i am away so often as i simply refuse to go on the dole and always travel to where there is work.

like your comment about not waiting for prices to drop - a fellow realist!

again - thanks for taking the time to respond in detail.

cheers,
brett.

[loanrangie]

This for home not a vehicle.

Nothing like leaving out important details ;).

[carlschmid2002]

Thanks Warbs that was very informative. I have recently purchased a Power Top from Bain tech power products and I am now looking for a suitable solar supply. This is for car/ trailer.

[pibby]

Nothing like leaving out important details ;).

Yeah, fair point. :D

[Warb]

Thanks Warbs that was very informative. I have recently purchased a Power Top from Bain tech power products and I am now looking for a suitable solar supply. This is for car/ trailer.

I've never seen on in the flesh, but it looks like an easy way to get a turn-key solution. I did notice that the manual on Baintech's website says it has a voltage warning at 11V - it varies a bit with the design of battery, but normally with a gel battery 11V would be regarded as fairly well completely flat! It might be worth asking if this is an error in the manual, or if you can adjust it, because having a warning a couple of minutes before the lights go out, and well after the battery has got to a longevity reducing state, seems rather futile!

If you're going to locate this unit (or any other battery) in your trailer, and want to charge it from the vehicle, you'll need to do some thinking. It's a long way from the alternator to the trailer, so voltage drop will come in to play, if the alternator is producing 14.7Volts that's great, but by the time it reaches the trailer that might have dropped to a lower voltage that won't fully charge your battery! So you need thick cables, and ideally a DC-DC charger in the trailer, which will take whatever voltage it is fed and "boost" it up to the correct voltage to fully charge the battery.

On my Prado there is another issue, being that Toyota (for fuel economy reasons) chose to limit the output of the alternator after the engine has been running for long enough to recharge the standard battery. That's great, but my setup has dual batteries in the Prado, plus batteries in the trailer.......

So I have a Redarc DC-DC charger in the Prado, which charges the second battery in the engine bay. It takes power from the alternator when the engine is running and boosts it to the correct voltage, no matter what the Prado's management system decides to do, but will also allow connection of solar panels when the vehicle is parked. I then have a basic (also Redarc) automatic solenoid ("dual battery") switch which, when the engine is running, supplies power via a heavy cable to an Anderson socket on the tow-bar. This connects to another Redarc charger in the trailer, which voltage boosts and charges the trailer batteries. When the engine is not running, the solenoid prevents the trailer from flattening the car battery without having to disconnect it, and the trailer's Redarc also allows connection of the solar panels in the same way as the car mounted unit if the trailer is unhitched. There is also a small solar panel on the trailer tool box, which trickle charges the trailer battery when nothing else is happening.

That all sounds complicated, but it means I have a fully automatic system that charges everything to 100% (not limited to 90% because of voltage drop, or 70% because of Toyota's management system!). It means that when travelling everything is charged by the vehicle, with no user input
(other than connecting the Anderson plug when hooking up), and when parked both the vehicle and the trailer can be charged by solar, and there's no risk of the trailer or fridge etc. draining the car battery.

The Redarc chargers I highly recommend. Mine have been all over Australia and never given me an issue!