Low solar output?

[kita]

I have mobile solar PV installation that I use to power an outdoor cinema based in the UK. The vehicle is a mercedes sprinter 312, with the panels mounted flat on top. I have six 'monolith' batteries 12V@ 230Ah hours each that Hugh Piggott helped me source, charged by two 140W Solarworld panels via a Steca Tarom 285 charge controller. All the main loads are run off a Studer 2800-12 inverter, which is connected to the opposite pair of battery terminals to the charge controller. The batteries are equalized and cycled regularly thanks to the charge mode of the Studer inverter, and all their voltages are about equal. The panels and the inverter are on anti-vibration mounts which is pretty important on the Romanian roads we are driving on at the moment....

The equipment draws an average of 500W, 800w peak. Because we show infrequently (about once a week) we should have enough time (3-6 summer days) to charge the 2-3kwh we need per screening. I have been watching the display of the charge controller closely and the peak current displayed (of the modules and the charging current) has been 14A. The maximum charge in one day has been 115Ah. Both these figures seem low to me given that they were taken during full sun (in May in Romania), and that the controller measures the 'working' current which is at 12-14v, 14A therefore giving a maximum power of 196W. That seems like quite a long way from 280W, which is the combined power rating of the panels. On the panel label the max current is given as 7.8A but that is at the open circuit voltage of 18V.

I don't know whether my maths, my interpretation of pv panel specs or my installation itself is wrong. I have tried covering one panel, and then the other, and the current drops by half exactly in each case. The voltage seems to follow the same pattern so I don't think that either panel is defective. I have the panels wired in parallel, with one solder junction and one crimped for each side (positive and negative) before the charge controller. The controller is connected to the battery bank with a 50A fuse and a manual isolator switch on the positive line. All connections are soldered, except at the battery terminal which uses a nut bolt and washer.

Is there a problem, and if so, what are the next steps I should take for diagnosing the cause? The panels are mounted horizontally, but I could make them tilt 45 degrees (when stationary). Would this make much of a difference?

One other smaller thing is the 0.7V volt drop I have measured across the 200A fuse on the positive line to the inverter, when the inverter/charger is charging at its peak of 130A. There such a heating effect taking place aroung the fuse that I have turned the power output of the charger down. The problem is that 0.4v of the volt drop seems to be a result of the fuse itself, and 0.3V due to the connections. I am working on resoldering the connections to the fuse holder but I guess there is nothing I can do about the fuse itself short of getting a bigger fuse?

If you are interested other specs and details can be found at www.projectortank.org.uk

[wdyasq]

Kita,

Solar panels are usually rated at output voltage(close to 20V), not battery voltage. Your panels seem to be performing about like the manufacturer's claims. The closer your panels are to 'normal' to the sun, the more power they will put out.

Ron

[ghurd]

It sounds fine to me.

About the best amps you can expect is a little bit over the amps at peak power, or about 8A.  7A is better than I would expect laid flat.

Tipping the panels up will help.  Tipped up panels and turning the car for morning, noon, and evening sun will also help.

G-

[Flux]

I agree with the others that 7A per panel is pretty much what you can hope for.

The only way that you may be able to do any better would be to connect the panels in series and use a MX60 or similar mppt charge controller. Even then you will only gain if you can keep the panel temperature low.

I don't think there is much you can do without very considerable cost to increase the output and personally I would rather to spend money on another panel than a fancy charge controller.

I do wonder how much of your available energy you are managing to get into your battery . There was some discussion here a few weeks ago and someone posted a useful link to work done by one of the big labs ( was it Sandia?). The final stage of charge is very slow and you may not be using much of your Ah in the final stage.

Flux

[ghurd]

"I do wonder how much of your available energy..."

I would think almost all of it. 1380AH of battery and PVs at peak making about 14A, or only 1%.  

Must be hard for the PVs to get the batteries past 13.85V?

G-

[Flux]

Agreed, I missed the fact that he has 6 batteries in parallel.

On the subject of the fuse, that volt drop seems excessive. Few commercial holders are really satisfactory for low voltage. Fork lift blade fuses with connections bolted to the lugs should in theory get you below 100mV at rated current.

Flux

[Nando]

The charging system seems to be OK, the basic problems is the specifications readings and the charge controller.

The Panels have Voc = Voltage open circuit and Vmp = voltage at maximum point, Isc = current short circuit connection.

Since the Voc is around 20 and the battery is around 14 volts, there is a difference of 6 volts lost in space with its corresponding watts ( Volts * charging current) ( not really, lost in the panel as heat).

The culprit is the charge controller that does not have MPPT ( Maximum Power Point Tracking) capabilities which converts the extra 6 volts ( really watts as above indicated) to a higher current to add the additional watts that the panel is using to heat itself up.

The available solution, for you to obtain the higher charging current, is to get an MPPT controller.

Nando

[bob golding]

cant really add much apart from from making sure you keep the panels really clean. if you have been driving along dusty roads in eastern europe you could pick up quite a lot of crud on the panels.

bob golding

[claude]

Well, allow me, I'm living in this dusty part of Europe... Unless you're touring along the countryside Romania, there's no extra reason to worry about dust but  panels safety. No, no one will steal them but a few days now there has been some big hail storms around the central Transylvania, I wonder how those solar panels will cope with that.

If I were you I would monitor the panels output while cruising, that way the panels will get cooled by the airflow and the panels efficiency will rise. And perhaps you'll be happier about their output.

Also, check the prices around here, you could find a 1000 Watts gas generator for under 150 USD (and you're not picky or care that it's a no-name brand) if you go to one of the "METRO" superstores, just to keep your batteries charged up.

As a last resort, ask some smiling citizen to let you use one of his power plugs at home to charge your batteries. You'll be surprised how nice some of them can be as long as you keep them from seeing the power meter. :-))

That's it from Central Transylvania, for fieldlines.com, this was Claude.

[kita]

well thank you everyone for your useful comments. In a way I am glad that the problem isn't so much my installation but my interpretation of the specs (and not using MPPT). On the other hand I know now that I am going to be about 30% short of anticipated power  output (as always!) and either need an extra panel or an equivalent reduction in my power use. Problem is, no space for another panel (I will post photos soon), and we're already on a tight budget for the juice.

As for the charging curves, yes, its true that the voltage climbs steadly between 50 and 85% of batt voltage, but then slows down. Only the mains charger can really push it to the top of that hill, but in terms of my power usage, I only need a maximum of 40% battery Ah capacity.

I am using a quality forklift fuse and holder but I will check them again. Maybe ensure that the fuse and cable lug are in direct contact, not separated by a washer.

As for Romania, I can confirm that its not all dust! There is still alot of dust in the countryside, but its the potholes that give me more trouble as my van is weighing a little more than it should at four tonnes. Half a tonne of battery. Next time I think I'll go for a trailer installation. Anyway all of that is more than made up for by the amazing scenery, very friendly people and the best tasting fresh produce I have every eaten. 90% of what you buy in the markets here is local, seasonal and grown using compost rather than chemical fertilizers. That makes for some ugly looking but very ripe and tasty fruit and veg. People's responses to our screenings have generally been positive, especially in the countryside where we tend to turn from a  mobile cinema to a mobile disco as the night goes on. I will definitely be coming back.

No hailstones so far....

One last technical question. Since my loads are connected to the inverter rather  than the charge controller, the controller cannot measure the S.O.C. accurately (the net of all charging and discahrging currents into the battery). As a result I have the regulation set to voltage mode. However it would be nice to have an accurate S.O.C. display, as well as the amps going in and out of the inverter. I contacted Steca and they supply a shunt to be attached to the Tarom controller, but the English distributors are asking for 260UKP plus carriage! The controller only cost £50 so it hardly seems worth it. Anyone know a DIY workaround?  

thanks

[sh123469]

You might want to consider one of the second battery charge relays that are sold for RV's.  They aren't all that expensive and you could use the vehicle's alternator especially for bulk charging while driving.  It disconnects from the vehicle's electrical system when not running so you won't risk discharging your starting battery.

Assuming full output of a 100 amp alternator, you would get approximately a C/10 charge rate.  You will, of course, have alternator power used for the vehicle while it's running so probably more like a C/15 or C/20 charge rate.  Bulk charging should work pretty well like that and then let your panels and charge controller handle topping it off and the equalizing charge.

I am not saying run your vehicle more to charge the batteries, although that would be an inefficient option.  Just use the power when you're driving to give the panels some help.  You're already running the engine and the extra power taken to turn the alternator shouldn't be noticed.

Don't know if you've considered this or not, just a thought for your consideration.

Steve

[ghurd]

I am not familiar with the Steca Tarom controllers. At all.

It doesn't seem very likely the batteries will over charge from the solar.  Sounds like 260UKP for a more accurate reading of exactly how undercharged the batteries are at the moment?  Must be a better use of the money.

Yes, Hardly seems worth it.

Be careful in central Transylvania.  Put garlic on the PVs, or something may suck the juice out of them.  ;-)

G-