I am trying to find some factual information on using different size solar panels. I have been told that all panels used must be from the same manufacture and same model.

I have a small fiberglass trailer that has a BP SX 320 panel attached to the roof with a Heliotrope HPV-22B MPPT charge controller with a temperature compensation sensor attached to the battery. The controller will handle 400 watts of input with 22 amps of charging output. Much more than I need!

I want to add an external panel that I can move with the sun attached to an extension cord. I understand that attaching the panels in parallel will keep the voltage the same and add to the overall current. The BP 20-watt panel specs are 1.19 amps at 16.8 volts. I want to get a 40-watt BP 340 that has 17.3 volts at 2.3 amps. Is it true that the voltage in this arrangement will be no greater than the lowest, 16.8 volts and the total amperage could be as high as 3.99? Physical size is the restraint that I have to live with. I could get the 30-watt panel that has 16.8 volts, but I can get the 40-watt panel in the back of my truck. There will times when the 20-watt panel will be in deep shade.

If I do this will I have to add a blocking diode to each panel before I attach it the terminal block? The current panel is wired with 10 gage marine wire to a terminal block and the wire from the terminal block to the controller and from the controller to the battery is wired with 6 gage marine battery cable.

My goal is simple. I want to keep the battery charged as much as possible when we do dry camping for up to a week or more. The night time use will be interior lights, LED and a 0.8 amp florescent light, water pump for the plumbing when needed, a refrigerator that has an electronic panel and a DC solenoid valve for the gas, a water heater that has electronic ignition and DC gas valve, and the furnace fan. We keep the interior at about 50F to keep the pipes from freezing in cold weather.

Thanks for any info!

Keith

Kieth short answer is you can add the panel in question in parallel without a lot of losses. It is not perfect but will work in a parallel configuration.

The BP SX320 panel will pull down the 340 panel down a bit, but doable.

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Dereck, PE, MSEE
Moderator

Sunking,

Thanks for the reply. Do I need to add a blocking diode on each of the panels? The reason I am asking is for the time the smaller panel would be in deep shade. Would the active one in the sun try to push some voltage into the other one and damage it?

Keith

Yes, You will want a blocking diode for each panel. When the one is in the shade it will rob power from the other panel but this will not damage the panel. Just a big waste of expensive power.

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I would check the panel spec first as most already come with the blocking diode built in.

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Dereck, PE, MSEE
Moderator

Please forgive my ignorance about this stuff! Here is my confusion and questions about a blocking diode. The following is from the Instruction Sheet that came with the BP panel.

"If two or more crystalline modules are connected in series, a bypass diode is required to protect each cell series string(typically 18 cells.)
All Modules with rated output above 49W are factory equipped with bypass diodes.
On smaller modules(<49W) bypass diodes must be installed by the customer.
Bypass diodes must be axial-lead Schottky diodes with a rating of at least 150C junction temperature and short circuit current and open circuit voltage as shown on the module label."

-------------------- 20W panel - 40W panel
short circuit current ---- 1.3 ------ 2.5
open circuit voltage ---- 21.0 ---- 21.8

The only information in the Instruction sheet about parallel installation is the following.

"If more than 2 strings are to be connected in parallel, then a series fuse is required for each string in each non-earthed pole. Only DC fuses rated at the maximum fuse rating should be used, maximum fuse rating should not exceed the maximum system voltage rating stated on the module label. When calculating the minimum fuse size then multiply the short circuit current by the module by a factor of 1.56."

So if I understand this I don't need a blocking diode for a parallel installation and it will not harm the panel in the shade, but the output from the one in the sun will be diminished to some degree which could be considerable. So I should put a blocking diode on each panel even when there is loss using the diode.

Keith

I purchased the 50 watt BP 350J instead of the 40 watt panel. We camped for three days and four nights with the set up and was able to get the battery charged or so said the blinking light on the controller in about 2 hours after each night. Usage was the following, lights, refrigerator, water heater, water pump, and the furnace. The refrigerator, furnace, and water heater run on propane. The refrigerator has an electronic control panel that requires 12V to run and has a solenoid gas valve with spark ignition. The water heater also uses a solenoid gas valve with spark ignition. We kept the interior at 50F at night and then raised it to 70F in the morning. Each night the outside temperature was hovering around 30F. The furnace with the fan going must be the biggest usage of power. I also bought a 150 watt pure sine wave inverter to power a few devices. With both panels in the direct sun there is no voltage drop using the inverter with a laptop or power to a 19" LCD TV. But not both at the same time. I have since changed all of the lights to LEDs to reduce battery consumption. I am a happy camper!

Hello Kieth and welcome.

No that is not entirely correct. It all depends on how the strings are configured. If you say install two panels in series to make one string, and then use another or multiple strings and connected each string in parallel, you will need bypass diodes in the series strings. Hope I sad that in a way you can understand?

For example 2 x 2 configuration.

__________________
Dereck, PE, MSEE
Moderator