I'm planning to build myself a small portable speaker. I'd like to include a Li ion battery and a jack for an external 12V-ish power supply. I'd also like to add a jack that will be used to charge the battery from an external Li ion charger.

If the first one turns out well I might make more to give as gifts. As such, I'd like to make the design as dummy proof as I can. I'd like to wire the power supply jack and the charger jack so that:

1) When the power supply is plugged in, the battery is disconnected from the circuit to prevent the power supply from damaging the battery.

2) When the battery is charging, the battery is disconnected from the amp so that I'm not drawing current from the battery at the same time that I'm trying to charge it. (Again to protect the battery)

3) I can run the speaker from the power supply while simultaneously charging the battery with the charger.

I think I can solve the first issue using the switched power jack as in the following (poorly drawn) diagram. enter image description here

I'm not sure how to do #2, but I imagine I would have to include a switched jack for the charger as well. Just not sure how to hook it all up. Anyone have any ideas? Thanks!

  • \$\begingroup\$ Connecting the battery to a dedicated Lion charger as you suggest in your OP is a Good Idea, as it saves having to create one from scratch, and possibly damage your battery. \$\endgroup\$
    – Neil_UK
    Commented Feb 10, 2016 at 8:11

2 Answers 2


To implement 2) break the vertical wire above the battery -pin (above the first junction). Then reconnect the upper wire to the charger jack pin that you labeled as ???. So now when you insert the charger plug the battery -pin will be disconnected from the upper circuit, and when you remove the charger plug the battery -pin reconnects to the upper circuit. (See revised circuit below)

Your drawing does not show any over charge protection for the battery. If you are going to use a Li Ion battery this is absolutely required. It may be best to just buy a battery assembly that already has a protection/charging circuit built in. For most any other type of battery you should still have some minimal over charge protection, either directly in line with the battery or in the charger module itself. What ever battery type you use be sure to read the manufacturer's recommendations for properly charging the battery.

Lastly be absolutely sure that the Power Supply and the Charger you use are simple isolated types, with no common earth ground. Since you are switching the ground pins in the circuit you do not want any extra ground paths making a connection. After assembly carefully test the circuit to verify that the battery connects and disconnects properly.

Revised circuit

  • \$\begingroup\$ Thanks! This is much appreciated. I was planning to just purchase a battery with protection circuit from battery space along with the recommended charger. Since this will be the most expensive single component of the whole build, I wanted to take extra care to get the most life out of it. \$\endgroup\$
    – Clengman
    Commented Feb 10, 2016 at 14:30
  • \$\begingroup\$ Should I plan to add a diode in the path between the battery + and the power supply +? \$\endgroup\$
    – Clengman
    Commented Feb 10, 2016 at 14:31
  • \$\begingroup\$ Adding a diode at that position, while not absolutely needed, can provide an additional margin of safety, for example if you do not fully trust the opening contacts of the input power jacks, or if the ground to ground connection of the supply devices is questionable. Such a diode would need to be selected to handle the expected maximum current and voltage levels of the system. Also be sure that your system can tolerate the small drop in battery voltage that the diode will cause. Some medium power Schottky diodes can have minimal voltage drops. \$\endgroup\$
    – Nedd
    Commented Feb 21, 2016 at 11:06

This will be a fun project! Maybe rethink your drawing and use a DC power supply to power and charge the unit. DC power supplies are inexpensive and have already been rectified from AC to DC. So try drawing this again with one power supply that will handle the following, one amp that operates at your desired voltage range, one battery that will operate your unit and a charge controller. These items are easily found on E-bay and remember SPECIFICATIONS are everything when dealing with electronic components(mins./max, Volts, Current, Watts) look up the definition of each so you will understand better . I want to help you learn. Also, in reference to what you posted, using the battery while charging it WILL help protect it by causing a slower/longer rate of charge that produces a more thorough charge with less heat... a batteries worst enemy in this application. Research electrons, where they come from and how they work moving from and to a valance ring as well as more or less electrons on this ring. You wont be sorry!

  • \$\begingroup\$ Oh, by the way. A charge controller is how you want to protect your battery. They can work in many different ways. \$\endgroup\$
    – Daniel
    Commented Feb 10, 2016 at 3:25
  • \$\begingroup\$ Using the battery while charging will confuse the charge controller, not smart. Electrons dancing on valence rings is totally irrelevant to this application, however fun to bend your head with. \$\endgroup\$
    – Neil_UK
    Commented Feb 10, 2016 at 8:10
  • \$\begingroup\$ Wouldn't the easy solution be to Have a DC Step Down to near what your circuit is going to run at, and have that go to your circuit and to your dedicated battery charger, and then put a diode from your battery to your circuit. It would keep the charger from getting confused, and let you charge and play at the same time. \$\endgroup\$
    – HilarieAK
    Commented Feb 10, 2016 at 8:26
  • \$\begingroup\$ Maybe I should have been a little more clear about my background. I've had my 3 semesters of physics back in college. I even took one class in EE, so my fundamental understanding is fairly solid. This was all a quite a while ago, though and I've never done much with putting any of the ideas into practice. Thanks for taking the time to answer. \$\endgroup\$
    – Clengman
    Commented Feb 10, 2016 at 14:35

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