A better way to power car audio in home

Published on June 21, 2016 . Updated on: -
Usually, car audio systems like CD player or cassette player have good audio amplifiers, with four channels (front L/R and rear L/R) of enough power for in home use. The radio tuner is also of high quality, with automatic best frequency selection and RDS information. Not all car audio systems have audio auxiliary inputs, although with a little tweaking, you can add audio line level inputs to a cassette player so you can use it as audio amplifier with any audio source.

There are plenty of how-to's about this subject online. Yet I couldn't find one that handles the memory loss problem. Car audios are designed to be continuously powered by the battery. Therefore, most don't have a permanent storage memory for settings. Upon disconnecting the power, the device loses all audio settings and stored radio stations.

This article will deal with the memory loss problem. Unless you will never unplug the DC adapter from the mains, you will need something similar to what follows if you want to use a car audio device in home. Let's start with the basics.

The proper way to power car audio in home - with backup battery

Car audio devices

There are mainly two types: cassette receiver and CD player. Cassette players are usually older and have lower audio power than CD players. Yet, they have some important advantages: you can get them at bargains at very low prices and you can disable the cassette reader, easily fool them to think there's a cassette inside and use the tape head connector as audio input. On the other hand, CD players may have higher audio output power, USB drive reader and AUX inputs. They are more expensive and if you get one that doesn't have auxiliary audio inputs and you want to use it as audio amplifier, you usually can't trick it there's a CD in if you disable the CD reader.

Basic wiring

Car audio devices have a wide voltage range. You can power most of them from 11 to 16 V D.C. I don't know why all the how-to's regarding this subject suggest using a computer power supply for this purpose. They are big, they have multiple outputs, even though you only need one. I consider laptop power supplies and external LCD monitor power supplies a much better fit fot this purpose. This kind of power supplies have output voltages ranging from 12 to 20 V at high current of 3...7 A.

You can't use power supplies with output voltage higher than 16 V, but you can adjust the feedback circuit of any power supply with, let's say 19 V, to decrease this value to 14...15 V. I will not describe this operation here. It is not that difficult: 1. unplug, open the case and discharge big electrolytic capacitor; 2. locate a transistor looking thing marked with 431 (i.e. TL431) and tweak the voltage divider that drives pin 1 of this IC.

So, if you're looking for a power supply: 12...16 V and at least 3 A will offer you good performance with most car audio players. Yet, keep reading, because there are some thing to consider when choosing the correct voltage.

So, how should you power this thing? There is a ground wire (black) and a main power wire (yellow). There is another wire (red) named "ignition". The player can be powered on only when there is power applied to both yellow and red wire. The current is drawn from the yellow wire. The red wire only signals the CPU it can exit the stand-by state.

If, somehow, the yellow wire is disconnected from the power, the player CPU resets and RAM is erased, losing all memory settings. If the red wire is disconnected, the player automatically enters stand-by mode, significantly decreasing current drawn from yellow wire. No settings are lost. In conclusion, yellow wire is considered to be always-on and also the main power line.

Basic wiring of car audio devices
Basic wiring of car audio devices
The proper powering of car audio players in home needs the following devices:
  1. back-up battery;
  2. high current D.C. power supply;
and meets the following requirements:
  1. when the D.C. power supply is disconnected, the device gets power only on yellow wire from the back-up battery. Because the red wire gets no power (is low), the device is locked into stand-by mode and draws very low current;
  2. when the D.C. power supply gets on, it supplies high current on yellow wire to device thus no current will be drawn from battery in this case. It is also connected to red wire, switching it to high and allowing the device to be turned on from stand-by.
The crucial power line needed to keep memory is the CPU one. Most CPUs operate at 5 V, so the battery voltage is down-converted by a simple regulator. If you own the schematic of your device, you could directly apply a battery back-up voltage to the CPU, but there are some risks (applying power to the wrong line, destroying the CPU or the voltage regulators by reverse-powering them).

The method I propose does not require modification to audio device's circuitry.


Well, it can be easier than you thought if the following condition is met: back-up battery voltage should never be higher than main power supply voltage. And this is easy to accomplish, because devices designed to be powered from batteries have wide voltage tolerances. So you can use 8 V battery (keep reading) with 12 V power supply or 12 V battery with 16 V power supply.

Obviously, 8V is an under-rated voltage. But is easy to obtain from two series Li-Ion batteries. And look at the previous diagram. See the CPU gets 5 V after a regulator? There are chances this regulator can supply 5 V even though it is powered at 7...8 V. Of course, not all car players are the same. But it's worth a try if you own a power supply with less than 16 V output. Below is the test circuit.

Car audio test circuit with 7 V backup voltage
Car audio test circuit with 7 V backup voltage
Trigger the switch, power on the player and make some settings (change volume, store a radio station, etc.). Now, turn off the switch, while keeping the 7 V on. Wait a few seconds, then trigger the switch on. Are the settings default or did it keep your settings? If it reset, you can't use the method I propose with two series Li-Ion cells. Keep increasing the bench supply voltage and repeat the test to determine the minimum voltage that doesn't reset memory. In this way you will find what kind of backup batteries you can use and how many cells.


I'll build the UPS using Li-Ion cells from laptop batteries. Since my car player didn't lose settings at 7 V, I can use only two series cells. The UPS circuit must reverse current direction when the car player is switched on, thus charging the battery. Below is the schematic with 2 cells and automatic charger based on LM317 (design idea from Electronics DIY).

Car audio Battery backup circuit with 2 Li-Ion cells
Battery backup circuit with 2 cells
In this situation, you can power the car player with 12 to 16 V DC. The LM317 based charger needs supply about 3 V greater than full battery voltage (8.4 V). Li-Ion batteries are full charged at 4.1 or 4.2 V. If you can't find specs for your batteries, it is a good practice to charge them to 4.1 V. Thus, you must set the charger to 2 x 4.1 = 8.2 V. To do this, power it without connecting the batteries and adjust R4 until voltage at TP reads 8.2 V. Do not use it without adjusting this voltage!

If your car player doesn't want to preserve memory at 8 V, you will need to put 3 series cells. Here is the schematic.

Car audio Battery backup circuit with 3 cells
Battery backup circuit with 3 cells
To be able to charge the batteries, this circuit must be powered from at least 16 V. This is a disadvantage, because you will have to use a power supply with this exact voltage (you can't go higher because you will break the car player). Adjust TP voltage without batteries connected at 12.3 V.

R1 is a power resistor that controls charging current. Divide 1.25 by desired charging current and you will get the value for R1. I suggest you experiment with values smaller than 6.8 ohms, because this will charge batteries faster, but the LM317 will produce more heat, thus needing a bigger heatsink. Use a resistor of 3W or more.

D1 is a high current rectifier diode. Its purpose is to prevent battery voltage reaching the red wire. If this happens, the car audio player can be turned on while being powered only from backup battery. The diode should have a current rating similar to car audio device fuse and should be heatsinked.

It is not shown in schematics, but it is highly recommended to add a fuse in series with the batteries. Keep in mind charging current when choosing one.

When this circuit is not powered, the anode voltage of D3 is higher than cathode voltage, thus allowing current flow into the car audio receiver. When you apply power through CON1, because supply voltage is greater than battery voltage, D3 is reverse biased, thus blocking current flowing into the receiver. More than this, if supply voltage is at least 3 V higher than battery voltage, LM317 starts supplying constant current to the batteries, charging them.

So if you insert an ammeter in series with the battery, you should read drawn current when not powered and a reverse polarized, charging current, when you power on. Keep in mind that these things usually draw quite a lot of current in stand-by. Mine read 50 to 90 mA. This calls for higher capacity batteries and a positive charge discharge energy ratio. It is recommended to set a charging current greater than discharge current.


The UPS I presented is far from perfect. If you don't power the device for a long time, the Li-Ion batteries will discharge completely, because there is no undervoltage disconnect circuit. This will significantly reduce their lifespan.

Car audio devices get quite hot during usage. And Li-Ion batteries hate high temperatures. After all, my idea of placing the batteries inside the device (where the cassette reader was - see top photo) may not be a good idea.

In conclusion you are free to build this as you want. You can use whatever batteries you have, either Li-Ion or Ni-Cd, just use a proper charging circuit. You can put batteries inside the car audio device or outside of it. You can include an automatic battery charger in your setup, or you can add an extra connector, where you can plug an external charger whenever it is needed. The basic schematic is similar to the test circuit, except you will replace the bench power supply with a battery power supply. When not powered, battery voltage should not reach red wire. When powered, supply voltage should be greater than battery voltage to prevent discharging.

By the way, here is a photo of what I am using for powering the device. It is a monitor power supply, capable of 4 A at 12 V. Taken out of the case in the photo.

12 V power supply suitable for powering car audio receiver in home
The 12 V power supply I use for a car audio receiver
In conclusion, use these circuits to make an idea. I said they're not perfect. Also, when building an enclosure for a car audio receiver, take into account proper ventilation. These devices can get quite hot. Also, perform current tests and compare values with battery capacity taking into account usage hours. If the car audio receiver draws tens of miliamps, you will need a high capacity battery and a high current charger.

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