Reusing PV panels in new configuration

You can get regulators that work on 48V.

Google "tristar solar regulator" or similar.

You will of course need four series-connected 12V solar panels unless you want to step the voltage up from 12V to 48V using a separate step-up converter (but this is an expensive and inefficient way of doing it).

How many solar panels do you have?

Most are 12 volts nominal, so if you have 4x you just need to wire them in series and the same with 4x 12 volt batteries. It is pretty trivial to design a regulator for any battery and cell voltage. Usually these work on the maximum voltage across the battery on charge. This would be typically about 14.4 volts with a 6 cell lead acid battery. As to inverters, well I have seem 12 and 24 volt models and very possibly there are 48 volt ones on the market. However they are probably more expensive to buy. But of course you could always use the batteries in pairs to run 24 volt inverters for diffferent loads.

However, why on Earth would you want to run a 75 watt system on 48 volts?

My 160 watt system for lighting originally ran at 12 volts and has been upgraded to 24 and works well at both voltages, though theoretically the losses are less on a 24 volt circuit since wiring losses =I2xR. of course there the load is 230 volts AC and is fed from a 400 watt inverter.

I can only assume that your load is designed for 48 Volts, because nothing else would be reasonable.

Would it indeed be possible to change the voltage of the system which is fed by the PV's and generator, that would make things MUCH easier.
crite 40

This existing system is a hodge podge created by several people in the past who had differing ideas as to how to proceed. The residence is off grid. Unfortunately the owner/resident is not really into conserving energy or going green or whatever. It's the family homestead and he is the third generation born and raised there. Its too far from power utility lines to make electrification by utility a low cost option. In pioneering days you had kerosene lanterns and swedish steam power. In those days electrification meant getting your own generator lighting plant.

There are six solar panels ( Siemens 75 watt) salvaged from a mountain top radio repeater site. A wind mill was salvaged from somewhere else but the normal winter gales blew the props off the hub. Last year the son in law recommended getting an Outback inverter system and ordered the 48V option.

Now he is too busy or not interested in finishing up the system. Now its my turn. I might add the owner is also my wife's uncle. So it has become a family thing.

Perhaps you consider it trivial to design a simple regulator, but when you have multiple charging sources simultaneously feeding into one battery bank it gets somewhat more interesting. I wondered about the series wired arrangement but if these panels have blocking diodes does that make a difference? There is a 0.7 volt drop across a silicon diode; or do they use Schottky diodes with only a 0.4V drop? In any case, is a diode between each panel necessary or can you just install one on the final output before the wires go to a battery. Plus; I need something better than a simple voltage sensing regulator. They merely shunt output to a dump load when hi V is reached. I need something like an MPPT controller and which can compensate for early stages of absorption stage charging from the other chargers.

A second consideration is the age of the panels. If I understand the date code correctly (24 -93) these panels were built in week 24 of 1993. Even though they are guaranteed for 25 years, I think I can expect to see output degradation as a result of ultraviolet exposure. How much remains to be seen. After all these panels originally saw service for 10 years on a mountain top.

Regarding why I would even consider using a 75 watt panel on a 48 volt system. Charging by genset is fine for bulk stage and part of the absorption stage. However this is not cost effective - especially now we pay $5.60 per gallon for diesel fuel. I need something for trickle charging. The house bank is 375 amp hours at 48V. It consist of quantity 8 size L16 batteries wired in series. Even a 5% trickle charge should be 18 amps. Normal practice is to shut down the generator when the battery completes the Bulk stage and begins the absorption stage. Without a trickle charge sulfation would build up and lead to premature battery failure.

The genset is 10kW and below 50% loading it becomes inefficient. There is also the problem of wet stacking which means increased wear and service issues. The genset ran 5100 hours last year and burned more than $8000 in fuel. Reducing run time to 3000 hours would be a major improvement.

If I could use a non engine charging source ( like the PV panels) for trickle charging the battery bank from 80% to nearly float voltage completion, it would improve battery life, reduce sulfation and also reduce the amount of run time needed by the genset.

There is nothing in the system using DC directly. The house is equipped with all the usual AC appliances so the system DC voltage is solely determined by the inverter which is 48V

You are right. Losses are reduced going from 12V to 24V and likewise losses are further reduced going to 48V. An added benefit which many people ignore is the lower heat produced at higher voltage for the same wattage. As the battery voltage decays the inverter pulls more DC current to compensate. This current manifest itself as heat in the semi conductors in the inverter. I have found I can deliver more power for longer periods of time by running at higher voltages. The inverter is less apt to shut down from thermal overload. In the power systems I work with ( 3 -10 kW) this makes for a significant difference.

Changing the system DC voltage at this time is not an option. It looks like the best option at this point is to rip out the solar panels and wind mill and use them in trivial applications.

As a foot note, most newer systems, especially off-grid, seem to be 48V these days.

are you seriesous?

You should revolt.

Hi, maybe i can assist you. Check with the manufacturers data sheet on the panel for maximum circuit voltage. Connecting 4 of them in series will give you a nominal open cct voltage of about 84v, just right for 48v battery pack. The connection to the battery pack and genset should be OR'd as in an or gate. Two diodes forward biased with common cathodes together and one anode going to the panel bank and the other to the genset. Take care on the piv(peak inverse voltage) and current rating of the devices. I would suggest around 10A 600v piv on a suitable heat sink for the solar side on a trickle charge. If higher spec parts are available i would defiantly over spec the diode (small price saves a lot of damage). Alternatively you could always manually switch in between the genset and panels but you would still need at least a forward diode and pref a controller for accidental overcharge.

I'm not sure what a "MPPT" controller is. Could it go by another name? Pls advise so i can follow up. You may want to use LED's for lighting they have really come a long way and have a long burn lifespan provided they are biased to the correct spec

MPPT = Maximum Power Point Tracking

Automatically adjusts the operating point on the PV's I-V (current-voltage) curve to keep the system operating at it's max power output. It's a DC-DC converter.

Hi

I would be very careful with connecting the panels in series; it most likely will go over the current rating and will result in panel heating and therefore performance decay (especially considering the panels age). The best solution is to separate the batteries to 12v banks and charge them directly. Second best is getting a DC/DC converter – for 75W it should not be too expensive and will do the work right.

Well that's very interesting! It appears you have a real "Topsy" on your hands.

<>A question, do the panels generate 12volts@75 watts each
or 75 watts total?

<>It certainly makes life hard when you have to design around such an unsystematic system.

<>With 6x 12 volt panels you of course have another problem, how to use them efficiently on a 48 volt system. A previous poster who appears to think that 4 panels give over 80 volts surprises me. My panels give ~17 to 18 volts off load in bright sunlight, dropping to around 15 volts at max load. This charges a 12volt battery to 14.3 volts (under charge) which is fully charged. That only adds up to about 70 volts with 4 in series.

<>

<>However to use your panels optimally your idea of a DC to DC converter may not be a bad one at all. There are small DC/DC converters on the market and I am sure that one could be "beefed up" to give you an output of around 57 volts to charge your system. If your panels total 75 watts this would only be around 2 Amps and if you have 6x 75 watts then around 12 Amps, both allowing a safety factor.

<>So you would wire your panels to give a voltage suitable for your DC/DC converter input. With 6 panels you could have 12volts, 24 volts, 36 volts and of course 72 volts.

<>

<>Though I imagine that 12 or 24 would be easiest, then your converter would double or quadruple this voltage to feed your batteries through an isolating Schottky diode.

<>Charging voltage regulation could be done by setting the output voltage of the regulator to suit the fully charged batteries on charge voltage.

<>At least this would give you some useful input from these panels, though I guess You wouldn't get much juice in the Canadian winter.

<>Here in Northern NZ I run my house lighting through most of the year using mains CFL's on solar energy but then we are at 36 South and even then I back up to mains in winter when we get a few days of dull weather

<>and I'm not trying to run all my household appliances from it!

<>

<>

<>

<>

<>

<>

<>

Of course you can wire the PV modules in series. That's usually how it's done.

I would buy a charge controller to put between the PV and the batteries. You can get one from Outback. Good ones are also made by http://www.morningstarcorp.com and http://www.apollo-solar.net . Just make sure you match the charge controller input voltage with the expected range of the PV string. The PV voltage will vary with temperature. The max voltage will occur at the lowest temp in your area.

I would give Outback a call since you are using their inverter. They can be helpful if you explain what you need.