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53 Diodes - Blocking & Bypass, What do they do?
We have been asked many times what is the difference between
a blocking diode and a bypass diode.
Let's try to straighten it out.
If a photovoltaic module is directly connected to a battery and a
blocking diode is not in the module or the controller, it will backfeed
power from the battery to the pv module at night.
In the early eighties, Solarex connected some pv modules to batteries
without blocking diodes and photographed them at night using light
A glow could be seen on the edges of each pv cell in the module.
Early pv charge controllers were either on/off or shunt type.
At night, when the battery voltage dropped, the controller would
re-connect the pv modules to the batteries(s).
This would allow the pv modules to drain power from the batteries.
In this case a blocking diode, a one-way gate, would be used between
the pv modules and the charge controller.
Later pv charge controllers had either a blocking diode built-in or
disconnected the pv module's, when their voltage dropped below that of the
batteries - as in night time hours.
Most current production pv charge controllers do not need a blocking
diode in the system.
A notable exception would be the Flexcharge NC25 and NCHC controllers
which do need a blocking diode.
These are hybrid controllers which can accept solar, wind, hydro or a
combination of power inputs.
In this case, a blocking diode is needed on the pv modules to prevent
night time battery drain as well as to protect them from voltage spikes
when another incoming power source is in the system.
Do your pv modules have a built-in blocking diode.
You can connect the pv module to a battery, I would recommend fusing
the positive side for safety, at night and cover the module.
Using a sensitive amp meter, check for reverse flow of current.
If none shows, your module has a blocking diode in it.
When purchasing a blocking diode you will want to look for the
low forward voltage drop (voltage is lost passing through a diode)
amp rating of the total pv module(s) plus 25%
maximum voltage rating of the open circuit rating of the pv modules
(Voc) plus 25%
Bypass diodes are located in the pv module itself - usually in the
"J" (junction) box.
Sometimes pv modules are wired in series to add up the voltage.
This is often seen when using an MPPT charge controller to take
a high voltage pv array located some distance from the battery
bank and drop it to the normal charging voltage.
When you see a pv module that is UL Listed and has a high system
series voltage, usually 600 volts d.c., it has a bypass diode in it.
If one or more pv modules is shaded in a series string of modules,
the unshaded modules can direct their power to the shaded
This can be destructive to the shaded modules as well as present
a fire hazard.
When a module has a bypass diode in it the diode is parallel to
the module output.
Since the diode has a higher electrical resistance than the cells in
the module, when the module is producing power the diode does
When the module is shaded the cell resistance goes up higher than
the bypass diodes' resistance.
Power from the other modules in the series string passes through the
diode without back-feeding into the cells of the shaded module.
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