Adding battery capacitor for GMRS radio?


Active member
I recently installed the Midland MXT275 radio in my vehicle, but have run into a bit of an inconvenient issue.

When running trails, I often get out and scout sections on foot, usually from 2-10 minutes. Being a long time Land Rover owner and not trusting their cooling systems to not spontaneously explode, I don’t like to leave vehicles idling when I’m not sitting in them.

If the MXT275 loses power, obviously it turns off, but it doesn’t come back on when you start the vehicle again like a stereo does, you have to manually turn it back on each time.

So far it’s just been an annoyance, but it has the potential to be a more serious problem if you forget to do it before proceeding and somebody else is trying to get you attention over the radio that you’re about to back into a tree, a dog just ran in front of you, etc.

At this point I just have it plugged into the 12v socket, I will be hard wiring it into an accessory fuse block that’s ignition controlled by a solenoid, so it will work the same way. I have a switch to the relay that will allow me to override the ignition trigger and keep it hot constantly, but the radio may still lose power and cut out during cranking.

My thought was to wire in a small capacitor between the fuse block and the radio that would allow it to stay powered up in standby mode for 10 minutes or so with the key off, and keep it from shutting off during cranking.

It’s a 15 watt radio so it should only draw around 1 amp, especially when not transmitting. However I’m not sure what size capacitor I would need or if this is even a good solution to it.

I’m good with wiring but my electrical engineering experience is limited. I’m familiar with the 12v ones used for car audio systems, but I’m hoping to find something smaller and cheaper.


Active member
From googling it, I don’t think I’m calculating correctly, it’s been a long time since physics classes.

The formula I found was Capacitance (measured in farads) = charge stored (measured in coulombs) / voltage.

It also gave me the conversion of 1 amp hour = 3600 coulombs, meaning 1 amp for 10 minutes would require 600 coulombs. Divide that by 12V and you get a 50 farad capacitor requirement.

Those are around $150 and around the size of a 2000w car audio amplifier, hence thinking I’m doing it wrong. I’d assume something the size of a 9v battery or smaller would suffice.


Active member
I suggest forget the capaciator idea.
Use a small SLA battery. 7Ah or so.
Wire it up like any other solenoid controlled dual battery system.

I do a similar thing with my Hilux but use the built in relay what controlls its keyswitch controlled accessory sockets.
My SLA battery is connected to one of those sockets. It gets charged when my motor is running. When motor is off, SLA backfeeds 12V into the accessory circuits but wont backfeed anything else because the keyswitch controlled relay is open.
I had considered that but those batteries are junk for the most part (feeder and power toy types), and it would be run down flat constantly whenever you didn’t manually shut the accessories off. It wouldn’t harm your starting battery, but I just don’t see it lasting more than 3-4 months. Maybe for long enough to accomplish what I’m wanting it would be fine.

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Robert Bills

You are overthinking this.

Most two way radios consume very little power in Rx mode. Your Midland MXT275 pulls less than 300mA on Rx, less than 1750mA on low transmit power, and less than 3500 mA on high transmit power (per Midland USA).

If I were in your situation I would wire the MXT275 directly to the vehicle battery and call it a day. That way it stays on until you turn it off.


Well-known member
Seriously, my radio doing APRS packet TX every 15m in my driveway goes like a week before I gotta jump start the vehicle.. there's no reason to shut it off unless your not gonna drive it in the next 24-48h.. If you wired it up to a low voltage disconnect set to like 12.4v it'd never risk you not being able to start and would likely RX only for hours if not days after shutting engine off.


Active member
Thanks for the input. I’m not interested in wiring it to the battery for a variety of reasons. I’m anal about wiring stuff and don’t want multiple ring terminals on the battery, extra wire running across the engine bay, in-line fuses, etc.

I found this one I think I’ll try. From reading, it’s a good idea to use a capacitor rated for double the voltage you plan on running through it for longevity reasons. They don’t act as transformers and step up voltage, so there are no worries about it supplying too much power. It accepts spade terminals rather than being soldered in, is under $3, and 1”x2”.


Robert Bills

. . . I’m not interested in wiring it to the battery for a variety of reasons. I’m anal about wiring stuff and don’t want multiple ring terminals on the battery, extra wire running across the engine bay, in-line fuses, etc. . . .
Ham radios and CB's are typically wired directly to the battery for a variety of reasons.

There are many ways to cleanly and easily wire new circuits directly to the battery. This is one:

Mine, with circuits for on-board air compressor, communications (CB and ham), a miscellaneous 12v circuit that doubles as the input for my Battery Tender 3 amp trickle charger, with one more for future use:

Blue Sea Fuse Block.jpg


Well-known member
As suggested, wire it to an auxiliary fuse block that's directly connected to the battery. A Blue Sea Systems SafetyHub would be a good option for under the hood, since it's designed for wet locations.

Between the fuse and radio, install a timing circuit so power will be cut to the radio after x minutes.

Use quality wire:

A good crimper:

And good terminals:


Active member
A cap will charge up to the voltage you have it connected to, so you need to select one with a voltage rating at least as high as the highest voltage you expect to see and it's wise to select one with enough margin to eliminate manufacturing variance and transients as issues, so two times your nominal voltage is pretty safe.

A 15,000 μF capacitor is 15,000 / 1x10^-6, or 0.015 farad. Since 1 C = 1 V x 1 F, so when charged to 12V this will represent about 0.18 coulombs.

Since 1 C = 1 A x 1 s, 0.18 C is 180 mA-seconds or 50 μA-hours. So that 15,000 μF capacitor will keep your 300 mA radio powered on for less than half a second.

You need a super capacitor, which are measured in full unit farads.

A 1 farad super cap charged to 12V is 12 coulombs which is 12 A-seconds. So it will keep your radio on for 40 seconds. The bigger super caps are 500 farads, which at 12 V is 6,000 coulombs. Now that is getting somewhere. About 1.6 A-hr and will power the radio for about 5.3 hours.

It's only $600 for a 500 F AVX SCM 16V super cap.

In comparison a 7 A-hr 12 V battery will keep your 300 mA radio on for 23.3 hours and costs about $20.
Thanks for the helpful reply. It appears my math was in fact correct then, that a 50 farad cacaotor would keep it going for the time I need. It just seems odd given the physical size compared to a rechargeable battery that will keep a handheld radio going for days. Shows how little I know about capacitors.

I suppose I’ll stick with my plan B, and simply use the manual override switch to keep the fuse block powered up with the ignition off. Hopefully cranking won’t interrupt it for long enough to kill the radio.

I appreciate the other wiring suggestions, I’m pretty tied to what I have planned. It’s all Blue Sea components, Ancor marine wire, woven nylon loom, appropriately sized, fused, and crimped, etc. I’ll post a writup of it in the electrical forum when I have a chance to take some better pictures.


Active member
To follow 50% rule of thumb, that radio should only remain on 11 hours, probably less. Simply put, Its about surface area.
Batteries and capacitors are very similar in how they work. But to acheive near instant charged to discharged properties of capacitor, its plates must have large surface area exposed to each other and extremely close to each other.
Batteries plates may have similar large ’surface area’ but physically much smaller as their ’surface area’ is exposed to each other by chemical action.
That chemical action takes relative long time. A reason why batteries cant go from charged to discharged almost instantly like their capaciator cousins.
That’s the issue with using a small battery, I don’t want to have to manually remember to turn the radio off, that’s what the solenoid is for, but letting a battery run flat every time will kill it in short order. Wiring in a low voltage disconnect is overkill for what I’m trying to do with this.

Hopefully I can just use the manual override switch when I’m running trails and the engine cranking won’t kill it. If I take power for the switch from my main battery lead extension, I don’t see why it would.


Well-known member
A very simple one is under $10.

Mounted in a waterproof box next to an engine bay fuse block, that little gem could be used with a relay to power down several circuits.


Active member
I was once toying with the idea of getting a UPS charge controller and a small (6Ah) LIFePO4 battery for that purpose.

I looked around a bit for a charge controller but couldn’t really find one that would work with both the vehicle’s voltage and the battery’s voltage requirements.

I do have to remove my seat again to install a nice, new ID-5100A I recently procured. I might look into it again.

More to the point, I concur with Mr. Bills and recommend not using a capacitor for what you want to do, it really isn’t really practical for the job at hand.

Sent by electrons or some crap like that.

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A very simple one is under $10.

I love the Engrish on that listing.

“This product is a new digital display using 12v delay module can be widely used in a variety of control switch position.”

“With protection against reversal diode, better protection while wrong connection.”

Sent by electrons or some crap like that.


While I get the intention of the OP, this sounds like a lot of unnecessary time and money being spent when simply wiring it directly to the battery (which really is the best way to install a 2 way radio) would have had you all done by now.

A capacitor is not the way to go for something like this. Capacitors are really meant only for smoothing out power or acting as part of an oscillator circuit. Yes, even the multifarad power capacitors.

While you could substitute a battery instead, any battery powerful enough to power the radio for a meaningful amount of time will either be bulky and difficult to place, or be of lithium chemistry and require proper charging management.

Just wire it straight to the battery and if you're really concerned about it running down the battery with the vehicle off (it won't unless you leave it for weeks or longer) then use a timer. But frankly I would not bother. Easier and cheaper to just be in the habit of turning it on and off when you need to.

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