# Supercapacitor added to radio or amplifier



## SoNic (Feb 14, 2021)

Today I got around and installed the supercapacitor that is meant to keep the audio settings on my radio, when I disconnect the car battery for maintenance.
It is annoying that in 2021 the car radios lose the station and clock every time the battery is disconnected. If I unplug my computer or my TV, even for a year, they don't lose settings or even the clock.

The schematic is attached, if anyone wants to give it a go.

The capacitor is board with six supercapacitors (3.0V each, for a total of 18V) and their equalization circuits, a 20A Schottky diode (low voltage drop, fast), a 10 ohm/20W ceramic resistor and an 80A relay (with wired base).
The circuit works this way:

when the battery is connected the capacitor charges with limited current. Those capacitors can take huge spikes of current when empty, so charging needs to be limited. The 10 ohm gives a charging time to 98% of voltage T=5RC=80 seconds.
when I turn the key on Acc or Run, the relay contact short-circuits the resistor and diode, providing full voltage to the radio. This might not be important for the radio followed by an amplifier (my case) but if the radio actually drives speakers, it needs the full voltage. This will be good also if I will decide to put a supercapacitor close to Amp's power connection (for "stiffness", better low frequency/bass attack handling).
the diode prevents the supercapacitor to reverse discharge when I disconnect the car battery. That would negate the isolation of the battery during maintenance and can be dangerous for electronics, airbags (it can generate high short circuit current for a short time). When the battery is disconnected, even if the Acc contact is forgot on "on" (it shouldn't ever be like that), the relay contact will drop.
after every battery disconnection, assuming the capacitor got emptied (it won't), I need to wait some 1-2 minute before I put the ignition key on "contact" (Acc or Run), so the relay contact doesn't get overloaded with a huge spike of charging current. I choose an 80A relay because that's what I had, and also to have a low contact electrical resistance, so low voltage drop when feeding a big amp.
colors are for the standard non-OE radio harness, but in parenthesis are also the Toyota harness colors.

My Radio fuse is 30A and the JBL Amp fuse is 40A, so the values of the diode and relay are not exaggerated. If anything, the diode should be a 30A one (if there is no Amp to help the Radio driving the speakers). The Acc fuse is 7.5A and in Toyota audio system is used only to signal the Radio and the Amp to "turn on".

The only issue that people might run into is that, at first connection to the radio, the radio it might need to be reset (low speed rising voltage, when capacitor is really empty, might confuse the radio internal CPU reset).
My JVC aftermarket radio seemed to be "working", but no audio was coming out of speakers. It has a small Reset hole in the faceplate, so that was easy to fix with a paperclip. It should not be needed again, now that the supercapacitor was initialized.










It doesn't look pretty, but it's soldered good, the insertion in the yellow harness wire is done with isolated spade connectors (male-female, for easy removal), the 10 ohm resistor is actually two 20 ohm / 20W connected in in parallel, for mechanical stiffness. I left the capacitor in the plastic bag that came in, for insulation.
The electrical tape over resistors won't heat up, the heat output is minimal during charging.
Only if the capacitor would internally short, then the resistors would dissipate 10W (together).


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## Bob Sanders (Nov 10, 2013)

I just connected a rechargeable 12v battery to the yellow and black with a diode on the yellow so the backup battery can't feed back into the vehicle when the main battery is disconnected. But I'm using external amps so the power draw on the yellow is not that excessive. I didn't need a stupidly large diode.


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## Domo (Nov 9, 2018)

You can also use any 12 VDC source simple clamp onto the positive and negative cables - and then disconnect the battery. The external 12 VDC will keep the radio's memory alive. As and alternative, if you have an "always hot" cigarette lighter socket, you can configure a 12 VDC source (few AA batteries or lantern battery) attach a lighter plug and just plug it into the socket - then remove your battery.

Here's a vid that might work for some.

how to keep car radio for forgetting when battery is disconnected - Google Search


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## SoNic (Feb 14, 2021)

Domo said:


> The external 12 VDC will keep the radio's memory alive.


That's great only when you want to replace the battery, because it is bad. In that case, all the electronic systems connected on the constant power bus will remain powered.

However, there are lots of of cases when you need to disconnect the battery, while working on the car, so you don't accidentally damage the electronics. Like when you replace any sensor on the engine. Or work around airbags.
The diode and capacitor insures that the voltage will remain confined to the radio.

Also, I am planning to do the same for the amplifier connection. A stiffening capacitor is beneficial there too, but the diode is needed also IMO, to prevent discharge of that capacitor into the rest of the constant power bus (when battery is disconnected for maintenance).


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## Domo (Nov 9, 2018)

SoNic said:


> That's great only when you want to replace the battery, because it is bad. In that case, all the electronic systems connected on the constant power bus will remain powered.
> 
> However, there are lots of of cases when you need to disconnect the battery, while working on the car, so you don't accidentally damage the electronics. Like when you replace any sensor on the engine. Or work around airbags.
> The diode and capacitor insures that the voltage will remain confined to the radio.
> ...


It is obvious you do far more work on your car(s) than I do!

Heck, I've changed the battery once and hooked up power to my trailer socket. But I will attest that when my battery was "going down" and failed to start the car that the volts got low enough that my climate control became stupid. I'm guessing my radio must have a greater tolerance or different memory that could retain down to zero volts for a short duration (5-10 minutes).

Accolades on the circuit it certainly isolates the radio from the rest of the car. - I'm an EE and guess you must have good experience in the field.


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## SoNic (Feb 14, 2021)

Haha, yep, I'm an EE too.


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## Bob Sanders (Nov 10, 2013)

Not sure why you would need a back up connection on the amps? I have never seen an amp with volatile memory... unless you're just talking about a big cap for the extra thumper value?


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