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This question already has an answer here:

I'm building a custom radio system for a friend by using a car-audio setup built into a boom-box-like container. I have everything wired up, and it works well enough. In testing though, I've run into an annoyance... the radio system (being designed for use in vehicles) depends on a constant battery power to maintain its settings and memorized channel presets.

Here's how I've hooked up the power so far:
LCD monitor AC adapter (output: 12V, 6A, 72W)
This source is directly connected to the radio via 2.5mm jack.

There is a power switch between the main power hookup for the radio and the source. (The radio wiring harness has provided a memory backup wire which is connected to the source parallel to the main power hookup for the device... this allows you to cut off the radio (and its face lights) while maintaining power consumption necessary to keep the internal memory alive.

I'd like to hook up some batteries parallel to the incoming 12 VDC source current such that the main source can run the system while it's plugged in, but the batteries would operate the system without interruption if the main source were disconnected long enough to move to another wall outlet.

My main questions are:
Can this be done safely and efficiently?
Are there any considerations I've missed (like power regulation or special charger hardware); it seems pretty simple as I'd be using 12v batteries and a 12v source.
If this is feasible, which type of batteries could best handle potentially frequent 80-90% discharge/charge cycles without suffering damage?

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marked as duplicate by Kaz, placeholder, Keelan, Olin Lathrop, Anindo Ghosh Apr 19 '13 at 15:48

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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That would not be feasible. Firstly, a 12V DC battery does not supply a consistent 12V. As the power gets drained off, the voltage drops. When the voltage supplied by the battery drops, there will be a potential difference between the two positive ends of the voltage sources and that would cause huge heat to dissipate through the wires. That would also mean that the wires would burn.

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  • \$\begingroup\$ Some chargers are high impedance if output is lower than external load, ( diode protected or feedback to semi switch) Others would drain the battery. Designer care and choice as I indicated is essential on charger or simply add power Schottky diode. \$\endgroup\$ – Sunnyskyguy EE75 Dec 20 '12 at 23:46
  • \$\begingroup\$ Yes that is right. I was referring to standard batteries. \$\endgroup\$ – elwc Dec 26 '12 at 1:30

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