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I have several small solar panels. I would like to configure them in series to build up enough voltage for a project. But some panels are much smaller than others.

Is there a problem of connecting them in series if they have very different voltage outputs?

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  • \$\begingroup\$ Yann is essentially correct but due care is needed. Series connection is acceptable but, as he says, the current output of the lowest current cell in the string controls the current output of the whole string. Also, if one cell in a sring is shaded or receives less light the output of the whole string will be affected. | Cells may also be connected in parallel but must have islating diodes or other electronics to prevent "backflow" of higher voltage panels into lower voltage ones, or of battery current into the panels when there is low or no sun. \$\endgroup\$ – Russell McMahon Mar 9 '12 at 18:39
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The voltage output of the panels is not a problem for series connection. The result is just the sum of all the voltages. After all, the different panel voltages are a result of different numbers of cells in series.

However, the current ratings should match because the same current will be going thru all the panels. Another point that is very important is to put a Schottky diode in reverse accross each panel. That allows the other panels to still push current past a panel that is in the shade. Otherwise, a single panel in the shade will kill the whole output.

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  • \$\begingroup\$ Possibly better a std Si diode than Schottky if doing whole panel as at panel temperatures Schottky reverse leakage can be severe. \$\endgroup\$ – Russell McMahon Mar 9 '12 at 20:15
  • \$\begingroup\$ @Russell: Yes, reverse leakage of a Schottky needs to be considered. On the other hand, the higher forward drop of a regular silicon diode is a issue too. However, you're right in that a silicon diode is better unless you know what you're doing and understand the tradeoffs. \$\endgroup\$ – Olin Lathrop Mar 9 '12 at 21:58
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Solar panels typically contain cells both in series and parallel. Take a close look at the metal bands going from slate to slate in a larger panel (or even small cheap ones, which may be made from fragments). While current is limited by the least powerful series element, there's no problem connecting more panels in series.

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