Radio capacitor size for engine cranking?

chuzie · Jul 23, 2014

I hate it when my radio cuts off when I am cranking the engine. Does anyone know what size capacitor I need to use on the 12v line to supply enough power for 20W RMS x 4 CH for the duration of cranking (5sec max)?

Thx

roundhouse · Jul 23, 2014

I think it's probably the ign switch killing the power to the accessories while in the crank position

chuzie · Jul 23, 2014

Thanks. I get that part. That is why I need a cap. ;-)

roundhouse · Jul 23, 2014

Oh. Ok. In not sure Should work if you get the right one You need calculate the load of the radio
How much it draws

I want to do that to the tow/haul switch on my pickup

Fed from the brake lights. So when I start off from a stop it will be in the tow/-haul mode for a few seconds. Enough to no instantly shift to second gear the moment the truck starts moving. That drives me crazy

Wonder if you could hook up a direct wire from the battery. Or maybe a wire from the crank terminal thru a diode to the radio so you'd still have power but when you're not cranking the radio power wouldnt backfeed to the starter

chuzie · Jul 23, 2014

I like to have it keyed for battery protection.

The diode seems feasible.

Broncobowsher · Jul 23, 2014

It is a little more complex then just throwing a cap on the accessory circuit. IF you gave me enough numbers, it is an easy calculation. You need to know starting voltage, minimum voltage, amp draw, time (provided). You also need to do a little work on the charge side or when you go to turn the key to accessory you will arc and burn up the contacts charging the cap. There would also be the issue of when you turned the key to off you would still have a radio going until the cap was pulled below the radio on voltage every single time.

There is also issues with electronics not being happy when you dip the voltage, giving them a brownout. Radio all up and running, give a sudden dropout in voltage, that can cause all kinds of radio weirdness. In short, shouldn't bother doing it.

Steve83 · Jul 23, 2014

A cap won't work; it would be the size of the radio to store enough power to run the radio for a few seconds.

What you want is a capacitor that holds a RELAY on, and let the relay feed the "switched" power to the radio. The relay draws far less, so it will run longer on a reasonably-sized cap.

The cap needs to be isolated from the truck by a diode so it's not powering everything else on that ig.sw. feed. The relay's main input needs to be either fused, or fed by the original fused radio power circuit (137 Y/Bk).

.

But an easier, cheaper, & more-reliable solution (IMO) would be to use a common SPDT (Bosch/ISO/Tyco) relay. Trigger it from the ig.sw. "I" output (904 Gn/R, 640 R/Y, 932 Bu to terminal 85; terminal 86 to ground) so that it switches its output (terminal 30 to the radio switched power input) between the original radio wire (137 Y/Bk on terminal 87a) and a new fused UNswitched battery feed (on terminal 87). No capacitor needed, and the radio will stay on except when the key is in OFF. Here's what would happen in each key position:
ACCY - radio powered only by the original switched feed thru the relay's NC contacts
OFF - radio off
RUN - radio power is switched by the relay from the original to the new feed
START - radio powered only by the new feed, uninterrupted during cranking

broncnaz · Jul 24, 2014

The lenght some will go thru for a minor inconvience. Lets see the radio was not on before you jumped in the truck to start it. But you want it on while starting because its annoying. You might want to consider getting a remote start setup so you dont even have to get into the truck to start it. lol Sorry I just find this amusing tonight.

broncodriver99 · Jul 24, 2014

Cut the radio down before you shut it off and cut it up after you start it. It wouldn't take much of a cap to keep it going while cranking but that is a lot of trouble to go through for 3 seconds of silence. Even modern vehicles drop the radio during cranking. Do you have a gear reduction starter? you could cut the 3 seconds of silence to 1.5 seconds. Might be money better spent.

bmc69 · Jul 24, 2014

Steve83 said:
A cap won't work; it would be the size of the radio to store enough power to run the radio for a few seconds.

LOL...I was thinking it might have to be about the size of a one-gallon juice can at least.

5 seconds is a long time and the voltage differential is small.

chuzie · Jul 24, 2014

Steve, thanks. I will go with the relay.

Broncaz, I agree. It is more geared towards the situations where I am listening with engine off and then start rather than just jumping in and going. My big beef is I lose connectivity with my bluetooth on phone and it takes a while to automatically reconnect.

bmc69 · Jul 24, 2014

I know that the aftermarket stereos in my rigs cut off during cranking because the starter draws the accessory voltage below the threshold value that the stereos detect as "shutdown". 11.8V is s very typical auto-shutoff threshold for a lot of modern "12V" electronics. It's in there to protect sensitive components from over-current damage. We build our marine electronics the same way for the same reasons.

The only way to reliably avoid having your radio shut off whilst cranking is to have a second battery that is not used to crank the starter.

chuzie · Jul 24, 2014

Ugh, well i don't want to fry my receiver. The min for this one is 10.5v. After seeing what bmc wrote, I may reconsider. I would rather wait the 30-45 seconds it takes for my bluetooth to reconnect than to risk destroying my equipment.

bmc69 · Jul 24, 2014

chuzie said:
Ugh, well i don't want to fry my receiver. The min for this one is 10.5v. After seeing what bmc wrote, I may reconsider. I would rather wait the 30-45 seconds it takes for my bluetooth to reconnect than to risk destroying my equipment.

The devices are pretty well protected from low-voltage damage in most cases; that's why they shut off.

In our ship control systems, we have a power conditioning unit that not only shuts the system down clean at a min voltage threshold..it delays the re-application of power to our stuff for ~10 seconds after the input power goes back above the threshold. To avoid "flash" power outages that can corrupt solid-state data storage devices.

Be happy to sell you one of those units.

chuzie · Jul 24, 2014

Sure. They go for a mere 5k I bet

70_Steve · Jul 24, 2014

chuzie said:
My big beef is I lose connectivity with my bluetooth on phone and it takes a while to automatically reconnect.

What did you do before there was bluetooth???:?

chuzie · Jul 24, 2014

Listened to my tires

bmc69 · Jul 24, 2014

Things were so much simpler when I still had my Craig 8-track. And streamers of black recording tape blowing in the wind and hanging from trees and bushes alongside the road were a common sight.

KyleQ · Jul 24, 2014

It's how you have it wired - you have it on ignition power, but not cranking power. You could run a wire with a diode to cranking power via the ignition to simply solve your "problem".

Steve83 · Jul 24, 2014

If your starting/charging systems are in good condition, and your engine doesn't require a long crank time, the battery voltage shouldn't fall below the radio's cutoff. And if it does, it won't hurt the radio.

I've noticed this problem on my FSB & my CV recently because I put KDC-X997s in them, and the BT dropping out IS annoying.

Radio capacitor size for engine cranking?

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