Bypass Diodes?

[dovelivelihoods]

Hi Newbie question.....I am building an 18v approx. 140 watt panel that consists of 3 panels @ 6volts - each from an array of 12 cells. I am using a charge controller with a blocking diode to charge a 100ah 12v leisure battery.
My question is do I need to insert bypass diodes between each of my 3 panels? (I will connect them in series) I have built the panels but am unsure about wiring them together at present. Thanks in advance for any help. Mark

[inetdog]

Hi Newbie question.....I am building an 18v approx. 140 watt panel that consists of 3 panels @ 6volts - each from an array of 12 cells. I am using a charge controller with a blocking diode to charge a 100ah 12v leisure battery.
My question is do I need to insert bypass diodes between each of my 3 panels? (I will connect them in series) I have built the panels but am unsure about wiring them together at present. Thanks in advance for any help. Mark

Normally a bypass diode is used to allow high current to flow through the panels even though a small part of the panel may be shaded. The problem with that in your situation is that once the bypass diode is forced to kick in the voltage from the panel will be too low to charge your battery anyway, so the diodes serve no useful purpose except to allow you to measure a high current when you short the panel, even when it is partially shaded. That measured current will not do you any good.

If you bypassed at the 3 volt (six cell) level you might still be able to get a useful combination of current and voltage from a very slightly shaded panel, but the expense and trouble would probably not be worth it. A lot depends on how many cells and in what combination the shade hits.

[dovelivelihoods]

Normally a bypass diode is used to allow high current to flow through the panels even though a small part of the panel may be shaded. The problem with that in your situation is that once the bypass diode is forced to kick in the voltage from the panel will be too low to charge your battery anyway, so the diodes serve no useful purpose except to allow you to measure a high current when you short the panel, even when it is partially shaded. That measured current will not do you any good.

If you bypassed at the 3 volt (six cell) level you might still be able to get a useful combination of current and voltage from a very slightly shaded panel, but the expense and trouble would probably not be worth it. A lot depends on how many cells and in what combination the shade hits.

Thanks for that comprehensive answer, so can I just wire the 3 panels (6 volt each ) together without any diodes in between and they should be ok? I guess I should just view the three as one whole panel for my purposes?. Or do I need blocking diodes in between . Or will the Charge controller deal with the blocking. sorry but diodes are a completely new concept to me and I'm struggling to get my head around it! I currently have a single factory made 100w panel on my van already and I just run it straight to the charge controller and it runs my laptop happily for a few hours each evening charges 2 phablets, and runs 3 lights led. (that is in the summer in the uk) I'm trying to make a home made version similar.

[inetdog]

Thanks for that comprehensive answer, so can I just wire the 3 panels (6 volt each ) together without any diodes in between and they should be ok? I guess I should just view the three as one whole panel for my purposes?. Or do I need blocking diodes in between . Or will the Charge controller deal with the blocking. sorry but diodes are a completely new concept to me and I'm struggling to get my head around it! I currently have a single factory made 100w panel on my van already and I just run it straight to the charge controller and it runs my laptop happily for a few hours each evening charges 2 phablets, and runs 3 lights led. (that is in the summer in the uk) I'm trying to make a home made version similar.

For your application you should not need bypass diodes, no. If you were to use a design in which two or more of your 18V panels were put in series to an MPPT CC, then I would advise you to install bypass diodes.
And the CC will take care of the blocking. Since the subpanels are in series, one blocking diode (or blocking transistor in the case of the CC) will prevent back current for the whole string of subpanels or panels.