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I am building a simple power source OR-ing circuit using two Schottky diodes like the one below. Source 1 is a 3.7V lithium-ion battery and Source 2 is a 6V 1W solar cell. I can see that the two diodes make it such that the load is provided current from only one source at any time (ideally) and that current is prevented from flowing back into the inactive source. But say, one of the diodes is omitted. What are the consequences of current backflow?

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  • \$\begingroup\$ You need the diodes when one source is higher voltage than another. For instance it would be unsatisfactory if you applied 6V to a 3.7V battery and overcharged it, damaging the battery. Also, not all sources are batteries, and some of them have electronics that can be damaged by current going into the output. So not every source needs a diode, but some do. I would assume in your case the diodes are needed to a) prevent overcharging battery and b) prevent damaging solar panel electronics, but would have to know more about both to know for sure if diodes are needed \$\endgroup\$ – I. Wolfe Oct 15 '15 at 16:12
  • \$\begingroup\$ In your specific case, if current flows from solar panel to battery, that is unregulated charging of the battery. It would definitely lead to shortened battery life or possibly, catastrophic failure of the battery. In general, rechargeable batteries need to be charged in a controlled fashion, and non-rechargeable batteries must not be charged at all. \$\endgroup\$ – mkeith Oct 15 '15 at 16:47
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The consequences of reverse current flow depend on the power source - some can handle more current (and voltage) than others.

Your 6V panel probably consists of 12 cells in series (equivalent to 12 forward-biased silicon diodes) so it will draw excessive current when the voltage exceeds 7.2V. However a 3.7V Lithium-ion battery is only 4.2V when fully charged, so you could safely eliminate the diode in series with the solar panel.

Your Lithium-ion battery may be able to take all the current that the panel can produce, but it must be not be charged above 4.2V (or it will explode!). The panel can put out up to 7.2V, so you need a diode in series with the battery to prevent it from being overcharged.

Another reason for using diodes is if one or both of the power sources is connected externally, and you want to prevent damage due to a short circuit at the socket or source connected with reverse polarity.

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