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This question is an exact duplicate of:

I am building a Bluetooth speaker and will have two power options running into a DPDT Switch. One side of the switch will go to an 11.1 volt 2200 milliamp battery which will be regulated to 12v. The other side of the switch will go to an AC adapter receptacle. This will give me the option to run off the battery or a 12v AC adapter but neither at the same time. The battery will be charged separately off a different circuit receptacle.

My question is would it be a good idea to put a diode on the positive battery line running to the DPDT in case the switch ever failed while running off of AC? This would be a safety measure since I know LiPos can be dangerous and I wouldn't want power going back into the battery from the AC adapter source. If this is a good idea what type of diode should I get and would it only go on the positive side and before or after my regulator before going into the switch?

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marked as duplicate by Ignacio Vazquez-Abrams, uint128_t, Dmitry Grigoryev, Daniel Grillo, Community May 11 '16 at 12:05

This question was marked as an exact duplicate of an existing question.

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Yes, you can use a diode for added protection. Go with a Schottky diode that has a low forward voltage (to minimize power loss) and ensure that it is capable of handling the power (I*Vf) that you require. You should also be sure that you have in place a way of stopping the battery from becoming over-discharged. This irreparably changes the internals of the battery. This is not usually a problem when it is discharging, but past a certain point (~3.0V/cell), it becomes increasingly hazardous to continue charging and using it.

It's worth mentioning that if your setup looks like this

schematic

simulate this circuit – Schematic created using CircuitLab

then A) the DC/DC converter will always be using some amount of power and B) installing a diode is as much about protecting the converter as the battery. If you install a diode, you would do so after the converter but before the switch. If there is a voltage feedback pin on the converter, make sure it is connected after the diode, otherwise the speaker will receive 12V - the voltage drop across the diode.

Lastly, as Jason says, there are other battery chemistries that are better suited for low power loads (such as your speaker, most likely). If you really want to use a LiPo battery though, there are several with integrated battery protection (over-charge, over-current, over-discharge) which would take care of much of the battery safeguarding for you. In any case, charging your battery to about 80% and discharging to about 20% will significantly extend your battery's lifetime, particularly if the battery is only sparingly used. (LiPo long-term storage charge is typically 40-60% of maximum charge)

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  • \$\begingroup\$ Thank you for your thorough explanation and I will get a Schottky diode that fits my specs and install it per your direction. I am quite familiar with LiPos as I have been using them for years in the RC Airplane hobby and have all the proper charging equipment. \$\endgroup\$ – Hartman9 May 11 '16 at 12:07
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Aside from the forward loss in the diode, yes. Make sure your diode has the proper voltage and current ratings, and try to get one with a very low forward voltage, and you should be fine. Will your load be able to tolerate lower voltages? 3S Lipos vary from about 11V when discharged, to about 12.5 when charged. This means that with your diode, you'll get 10.5 to 12V. LIPOS are probably a bad choice for an application where you are going to be leaving the battery charged. Have you considered a regular Li-Ion, or NiMH?

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  • \$\begingroup\$ Another question: What kind of LIPO are you using? High discharge LIPOs shouldn't be stored fully charged, as they are dangerous in this condition over time. \$\endgroup\$ – Jason A Soares May 11 '16 at 0:28

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