Hi everyone, and happy holidays!

I'm helping my dad connect 2 of his old solar panels to a Mercer Axpert 3000VA inverter. Everything works well except that it seems that neither the panels nor the inverter's MPPT has a blocking diode installed so the battery of course drains during the night.

I Google'd but I cannot seem to find any blocking diode products available locally. From what I could find, it seems like this diode (https://www.digikey.co.za/en/products/detail/smc-diode-solutions/15sq100/6022445) should work (it falls within the voltage and amp ratings), and I guess I can install it in an MC4 holder on the positive side? (It is only 2 solar panels connected in series).

I am not 100% sure if I am correct... any advise will be greatly appreciated! 

1 hour ago, necrolingus said:

Hi everyone, and happy holidays!

I'm helping my dad connect 2 of his old solar panels to a Mercer Axpert 3000VA inverter. Everything works well except that it seems that neither the panels nor the inverter's MPPT has a blocking diode installed so the battery of course drains during the night.

I Google'd but I cannot seem to find any blocking diode products available locally. From what I could find, it seems like this diode (https://www.digikey.co.za/en/products/detail/smc-diode-solutions/15sq100/6022445) should work (it falls within the voltage and amp ratings), and I guess I can install it in an MC4 holder on the positive side? (It is only 2 solar panels connected in series).

I am not 100% sure if I am correct... any advise will be greatly appreciated! 

What size panels are you connecting to the inverter?

Hi @Scorp007 Thanks for the reply. The panels have the following ratings (I have 2 of them connected in series):

• Brand: Fivestar (two old 265 watt panels)
• Pmax (Maximum Power): 265W
• Vmp (Maximum Power Voltage): 32.4V
• Imp (Maximum Power Current): 8.96A
• Voc (Open Circuit Voltage): 39.3V
• Isc (Short Circuit Current): 9.47A
• Maximum System Voltage: 1000V
• Maximum series fuse: 15A

 

 

2 hours ago, necrolingus said:

Hi @Scorp007 Thanks for the reply. The panels have the following ratings (I have 2 of them connected in series):

• Brand: Fivestar (two old 265 watt panels)
• Pmax (Maximum Power): 265W
• Vmp (Maximum Power Voltage): 32.4V
• Imp (Maximum Power Current): 8.96A
• Voc (Open Circuit Voltage): 39.3V
• Isc (Short Circuit Current): 9.47A
• Maximum System Voltage: 1000V
• Maximum series fuse: 15A

 

 

The inverter will use 600Wh per 24h so if left on during the night it will drain the battery. The 2 panels however will charge about double what is being drained. 

Not sure if the panels are draining the battery. Try switching off the panels after sunset to find out if it is the panels without diode or the self consumption doing the draining during the night time. 

Thanks! It's definitely the panels draining the battery overnight, already tested it over the last few days. 

I think any 10A (1000V) diode should work. I would solder it in circuit. Perhaps a higher rated diode will produce less heat at peak production. 

A poor solution: Wire in a contactor that only closes when the sun shines.

1 hour ago, frivan said:

I think any 10A (1000V) diode should work. I would solder it in circuit. Perhaps a higher rated diode will produce less heat at peak production. 

A poor solution: Wire in a contactor that only closes when the sun shines.

 

1 hour ago, frivan said:

I think any 10A (1000V) diode should work. I would solder it in circuit. Perhaps a higher rated diode will produce less heat at peak production. 

A poor solution: Wire in a contactor that only closes when the sun shines.

I would also connect a timer to a suitable relay that can break the low DC voltage at low current in the evening and make early in the day. The potential power loss by being 30min out is negligible. 

I have a similar clone inverter and with 4 year old 270W panels and no drain from the battery. Schottky diodes will have a lower drop but again 0.3V difference the loss is very small. 

Diodes add another risk if there is lightning and need to be changed when blown and can go into short circuit when blown. 

Edited December 25, 20222 yr by Scorp007

Thanks everyone for your replies! Definitely solutions I did not think about but will investigate, especially the relay mentioned by @Scorp007 For now the diode will have to do until I get around to a better solution.