Newbie Questions

[ChrisOfBristol]

I have been given a damaged Panasonic 240W 12V/24V panel. The glass has had a stone dropped on it, the panel still works, but I will need to seal the small damaged area. The rest of the glass is crazed, but I'm hoping moisture won't get in, if it does I'll have to replace the glass I suppose. The specification says 72 cells in series. I think the maximum voltage output of this type of cell is ~0.7V and 72 x .7 = ~50V which is near the VMPP in the specification. That confirms the cells are all in series and that the cell(s) underneath the damaged glass must still work or there would be an open circuit. I want to use the panel to charge a 12V battery for use in a garage without a mains connection. To provide lighting and possibly some power through an 800W inverter.
I have a few questions about it:

1. The model description is "Panasonic 240W 12V/24V" , but the specifications say "Voltage VMPP: 43.6V and Voltage Voc: 52.4V" I assume that 43.6V is the voltage at which maximum power is developed and the 52.4V is the open circuit voltage. So what is the meaning of the "12V, 24V"? Does it mean that it is suited to charging a 12V or 24V battery bank with the right regulator? Since a 24V battery needs a charging voltage of 2x14V (28V) and the regulator needs a voltage difference between input and output (say 5v), that gets us to 33V. So that sounds about right.
2. There is a small junction box at one end, which appears to have several strips running to it. I don't really understand the need for this arrangement if the cells are all in series.
3. I think the top of the junction box can be taken off, would it be practical to reconnect it inside to give me 1/2 the voltage at twice the current? It would save risking damage by taking it apart if someone told me that rewiring would be completely impractical.

[SunEagle]

It really doesn't' matter what the voltage output of the panel is as long as it is about 6 volts above the battery voltage and if you use an MPPT charge controller. The CC will provide the proper voltage to the battery as well as generate amps that would be ~ the panel wattage divided by the battery voltage or in this case 240w / 12v = 20Amp.

[sensij]

If you are using a PWM type charge controller, the voltage matters, and what you have would act as a 120 W panel in a 12V system. Most commercial panels are built with all the cells in series, but with bypass diodes that allow each 1/3 of the panel to be bypassed, which can help prevent hotspots in partial shade or in the event a cell becomes damaged. That is probably what you are seeing in the junction box. Panels wired this way cannot be easily adjusted from 24V to 12V.

It is possible that there are only two bypass diodes, which could be removed and rewired to allow the panel to create two series of 36 cells, in parallel. Quickly looking, I haven't found any Panasonic datasheets that suggest this arrangement is available in their panels, though. If you look inside the junction box, it should be clear whether there are two or three diodes.

[bcroe]

It is possible that there are only two bypass diodes, which could be removed and
rewired to allow the panel to create two series of 36 cells, in parallel.
If you look inside the junction box, it should be clear whether there are two or three diodes.

From all the panels I have seen, you can't rewire them by removing the bypass diodes. The panels are directly
wired as a series circuit, easily seen under the glass. Taps are brought back for the bypass diodes, but the
connection that would need to be broken (and brought out as a pair) is the one under the glass. Bruce Roe

[sensij]

From all the panels I have seen, you can't rewire them by removing the bypass diodes. The panels are directly
wired as a serious circuit, easily seen under the glass. Taps are brought back for the bypass diodes, but the
connection that would need to be broken (and brought out as a pair) is the one under the glass. Bruce Roe

Yes. "Possible" does not mean common or likely. If memory serves correctly, it was suitcase-style panels that were discussed with this kind of wiring, with a hinge between the two 36 cell halves and all series connections between them were made in the box. I don't mean to suggest that the OP's panels are wired this way, just that a general answer to the 3rd question in the first post allows that the possibility exists.

[ChrisOfBristol]

I've had another look at the connection box, and although there are indentations which might contain locking tabs, they don't want to move even with a fairly hard push, so I think I'd better leave it alone rather than risk breaking something. I'll make a charger to run of 40-50V.

Is it possible to use the panel to make up a shortfall in voltage from a partly discharged battery?

[sensij]

Is it possible to use the panel to make up a shortfall in voltage from a partly discharged battery?

If you connect a panel to the battery (as in a PWM type charger), the panel will operate at the battery voltage, which usually isn't the panel's maximum power point. An mppt charger will keep the panel at its maximum power point (in the bulk stage), regardless of the state of charge of the battery.

If you intend to make your own charger, do you have a good grasp on the panel's IV curve and the factors that contribute to its shape?

[ChrisOfBristol]

I've got a 250W panel to charge a car battery which will be used for light in the garage occasionally, so I expect I can buy or make something adequate without getting too technical.

[ChrisOfBristol]

bcroe you convinced me not to mess with the junction box - thanks!
When I started this thread I was a bit put-off by the prices of controllers I saw on a specialist solar panel suppliers site. I've since had a look on ebay and can see that controllers are available for under 15 quid, so I'll just buy one of those.