Stop equipment powering off when starting motor - off-the-shelf solution

Discussion in 'On the Road' started by ZL4NVW, Dec 20, 2020.

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  1. ZL4NVW

    ZL4NVW Ham Member QRZ Page


    I'm looking for an off-the-shelf commercial product that will ensure 12v power to a device (say 1.5A max at 12v) when the supply voltage from the battery drops whilst the vehicle motor is being started.

    This seems like an issue that others will have encountered with vehicle-fitted radio equipment - which is why I'm asking here - hopefully others have solved this already.

    I can go for a full auxiliary battery arrangement - but a second full-size car battery would be overkill for what I need - maybe 10 seconds of power at 1.5A max. I was hoping for a small lithium ion or super-cap based solution but to my surprise I can't find anyone supplying such a thing.

    Price is important.


    I know that I can build something myself to do this - there are dozens of circuits out there in electronics forums to do precisely this - so please don't suggest DIY solutions at this stage.


    Matt - ZL4NVW
  2. KA9JLM

    KA9JLM Ham Member QRZ Page

    A BIG cap that is normally used for audio amplifiers mounted in the trunk should work fine.

    But you may need to use a diode, So it does not try to power the automobiles electrical system.

    Are you connected directly to your battery ?

    Good Luck.

    K0UO likes this.
  3. KB3WFV

    KB3WFV Premium Subscriber QRZ Page

    Hi Matt,

    I am not aware of an 'off the self' or a plug an play solution to prevent 12 volt equipment from powering down nor could I find one. Depending on how the equipment is connected a cigarette / accessory socket adapter with a small 12v sealed lead acid battery and a diode would work.

    12 volt equipment can power down during engine cranking for a couple of reasons.

    1) The equipment 12v power supply is connected to an accessory circuit or switched. These circuits such as a cigarette lighter / accessory socket, are powered when the ignition key is in the accessory or run position. These circuits lose 12 volt power when the key is turned to the crank / start position.

    Solution: Move the equipment 12v power supply to a 'Hot At All Times' (direct from battery) circuit.

    2) The equipment 12v power supply voltage when connected to a 'Hot At All Times' circuit drops below 10.5 to 11.8 volts and automatically shuts off to protect itself from low voltage. This occurs when the vehicle battery voltage drops too low during cranking.

    Solution: Test - Replace -Repair, weak or under-rated engine battery, bad electrical connections, excessive starter motor current drawl, weak charging system or eliminate battery drain that may bleed off battery voltage when the engine is not running.

    Alternative Solutions, depending on when the equipment needs to be on or off and how it is wired to the vehicle.

    If the 12v equipment needs to be off when the engine is off and is connected to an accessory circuit, then installing a simple off the shelf standard relay will provide 12 volt power to the equipment during engine cranking. However IF the engine battery is weak or under rated, or the engine starter drawls too much current while cranking the 12v system voltage may still drop low enough to shut off or even possibly damage the equipment.

    Assuming the 12v system voltage maintains proper voltage while cranking. Then purchase a standard SPDT relay (Bosch/ISO/Tyco) Trigger it with a 12v power source when the key is in the run position to Terminal 85. Terminal 86 to ground. Connect the original 12v power source for the equipment to Terminal 87a. Connect a 'Hot At All Times' power source to Terminal 87. Connect the equipment to Terminal 30. This will keep the equipment powered when cranking and turn off the equipment when the key is off.

    If the equipment needs to stay on even when the engine is off, then use a small 12v battery and a diode. A LifePo battery by itself will not like being charge by the car's charging system. Use a small sealed lead acid12v battery instead. These can be found as replacements for emergency exit lighting, back up batteries for UPS units, such as computer back ups or alarm panel backups. These 12v batteries are small. Connect the 12v power source to the diode, the diode to the positive battery post. Connect the negative battery post to ground. Connect the equipment to the small 12v battery. The battery will charge while the engine is running. The diode will isolate the small battery from the engine battery voltage drop while cranking.

    Matt, I'm sorry that I didn't have a plug and play solution for you. There may not be such a solution because the answer is simply how things are connected, a simple relay or an auxiliary battery with a diode.

    PY2RAF and KB2SUJ like this.
  4. KF5LJW

    KF5LJW Ham Member QRZ Page

    Auxiliary battery with electronic isolator is what professionals and competitive A/V folks use. Those guys are light years ahead of ham radio.
    K0UO likes this.
  5. KC0LDH

    KC0LDH Premium Subscriber QRZ Page

    How about an MFJ-4403. I used one on a vw, and it just has a bunch of caps in it, with a diode. Off the shelf... 16086768796513485322224788456745.jpg 16086769213568513414046997410285.jpg
    N0TZU likes this.
  6. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    Good ol’ MFJ!
  7. AG6QR

    AG6QR Premium Subscriber QRZ Page

    That is an interesting device. The manual is available on MFJ's website, and it has a schematic.

    It looks to me like, once the capacitor bank is charged and the relay closes, the input becomes directly connected to the output, with the 4 Farad capacitor in parallel. This would mean the device can allow the radio to draw peak current much greater than the supply would otherwise be able to provide. But if the input voltage drops due to something like the draw of a starter motor, the capacitor remains directly connected to the input, at least until the voltage across the capacitor drops below the level required to keep the relay energized. Only the resistance of the wiring feeding the device (or fuse) limits the capacitor from giving its energy to the starter motor.

    This might work, and in fact it might make the car's engine easier to start. But I suspect it would do a better job of maintaining voltage to the radio if a diode were placed on the input, to prevent a starter motor from robbing the capacitor of its charge.

    Edit: if a relay in the car disconnects the power to the radio during starter cranking, this device should work well and it won't be supplying current to the starter motor.
  8. ZL4NVW

    ZL4NVW Ham Member QRZ Page

    Thankyou for all the replies. It's been a busy week - so only getting to read them now.

    Yes - My equipment is on an 'always-on' circuit - as is the commercial Tait RT (PMR) in the same vehicles - the PMR radios also all reset during engine start. These are fleet vehicles and they do this from brand new (radio resets) so it is not a case of an out-of-condition battery or corroded connections. It is simply that the supply voltage drops too low during cranking.

    The SLA battery and diode is the obvious cheap solution and the one I'd favoured. However, there is an organisational policy of no lead acid batteries in the passenger compartment. So either I would need to plead a special case (as it's a _sealed_ lead acid) or get the battery mounted in the engine compartment which is not ideal in a fleet vehicle. I'd personally query the policy of allowing Lithium Ion but not SLA batteries in the cab as not based on a true-risk approach - but sometime it's easier not to fight these things (knowing my luck I'd just end up getting LI batteries banned too!)

    The MFJ device looks like a good commercial option - though the price landed in NZ is probably as much as the rest of the kit put together! A diode in a fuse-case or similar container inline on the power lead should be acceptable if we need to avoid back-powering the vehicle from the device.

    The BOSS capacitors look slightly less suited to sitting free in the cab and more for hard-mounting in a vehicle. But at the price look like a cheap option. Will definitely have a chat to our local car audio installers and see what options they suggest are available in NZ.


    I was deliberately vague on what I am powering in my initial question - as I wanted to know specifically about 12v solutions. However, to fill in the details: the device I need to stop resetting is a Raspberry Pi (the fact that my radio also resets is not really a big issue). This clearly opens up the options of providing the 'battery backup' at 5v rather than 12v. There are several RPi 'hats' out there that do this - though all at prices similar to the MFJ device mentioned above. I've been trying to avoid making the RPi any bulkier and heavier than it is by keeping the battery backup device external - but these are also an option (though they'd require me to find a new case and modify it for the various connections I have going to the RPi).

    I had tried to use a USB power bank to provide the power-bridging service - as these are cheap and neat. I have found only one able to 'pass through' power whilst itself charging (most switch off the output when charging). And this one A-Data device has a short break of a few ms in power supplied when it switches from pass-through mode to supplying from the internal battery. Bridging that short gap in supply at 5v is my current preferred solution - but I wanted to have some other options up my sleeve if that fails.

    Thanks all for you help.
  9. K6CPO

    K6CPO Ham Member QRZ Page

    Why not just wait to turn on the device until the vehicle is running?
  10. ZL4NVW

    ZL4NVW Ham Member QRZ Page

    The vehicle could be stopped and started anything between 20 & 200 times in a typical day. I have been shutting the RPi down for each start but that's a real pain.

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