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I have an interesting project and I don't know whats the best solution.

Basically I have a charger (charges one battery at a time) and 2 batteries.

What I want to do is to have the charger charging battery 1 (while battery 2 is feeding clean power to an audio system)

With a voltage sensor (when battery is low) , I want to switch the charger to battery 2 and have battery 1 power the audio system. (the voltage sensor its not the question)

My question is : The battery that feeds the audio system must be completely isolated from the charger and ground (so no power noise is present).

Is there a way (except SSR) to do that ? PCB is quite small and space is at a premium. Also take in consideration and charger is 3A 5.5v and batteries are 5.4v

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    \$\begingroup\$ you could also use good old relays ^^ \$\endgroup\$ – PlasmaHH Sep 9 '15 at 12:32
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    \$\begingroup\$ Will the 2nd battery charge more quickly than the 1st will discharge? \$\endgroup\$ – HandyHowie Sep 9 '15 at 12:58
  • \$\begingroup\$ NMOS transistor, but you will need a charge pump. PMOS is easier to implement, but it has greater Rdson for same size. There are also dirvers that have bulilt-in threshold monitor. Try to google: PMOS cahrge pump battery threshold monitor. As for the ground isolation: this is not true, install the minus wire from battery to the audio equipment and ground it near battery. \$\endgroup\$ – Marko Buršič Sep 9 '15 at 13:06
  • \$\begingroup\$ Yes battery will charge faster \$\endgroup\$ – Johan B. Sep 9 '15 at 20:12
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A relay would really be the only way to completely isolate the the charging battery subsystem from the consuming battery system. Using solid state components to switch the battery connections would probably be good enough, even with a shared ground, for most applications and users. But technically this still leaves a connection. On the other hand for some audio purists, and it seems that you fall into that category because you are even talking about using batteries to power your audio system, you would have to use a multi-pole relay to connect both the + and - sides of the battery to one system or the other.

Such system could very likely provide your intention of "clean battery power" to the audio system. There are some short comings though -

  1. During the switch over from one battery to the other there would be an annoying loss of operational mode of the audio subsystem for a moment.

  2. Some audio equipment may give loud pops when the power source is suddenly connected or disconnected.

  3. If the charger is not capable of charging one battery faster than than the consumption rate off the other battery then you maximum listening time, with one annoying battery switch over event, would be the time afforded by two charged batteries.

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    \$\begingroup\$ And (4): You would need to incorporate a circuit to ensure that, in the event of both batteries being depleted, you don't enter a race condition such that the relays are constantly switching from one battery to the other. \$\endgroup\$ – CharlieHanson Sep 9 '15 at 13:03
  • \$\begingroup\$ @CharlieHanson - Yeah. :<) Now we are into the design side and there would be all kinds of things to consider including trying to see if capacitors can somehow prevent most of the pops and sudden power loss that I've mentioned. \$\endgroup\$ – Michael Karas Sep 9 '15 at 13:11
  • \$\begingroup\$ Anyone can point me to smallest multipole relay (SSR) since mechanical relay will make too much noise, defeating the point of having high fidelity audio \$\endgroup\$ – Johan B. Sep 9 '15 at 13:21
  • \$\begingroup\$ @MichaelKaras Having each relay pole make-before-break will allow for the power to be constantly applied. A capacitor will have to soak up the voltage difference to ward of any nasty transients, but it should be reasonably seamless. That said, Op has not yet alluded to full/dead battery voltages or current consumption, so it's anybody's guess. \$\endgroup\$ – CharlieHanson Sep 9 '15 at 13:23
  • \$\begingroup\$ @JohanB. Farnell's range of MOSFET SSRs \$\endgroup\$ – CharlieHanson Sep 9 '15 at 13:27

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