Recently someone asked about installing a 50Watt radio in an RV and would the cigarette lighter plug be enough? I would guess after 40 some years of ham radio I should of known better than to immediately insert foot in mouth and answer.
This is what I responded with:
Use what you know from study first. 50 watts divided by 12 volts = 4.2 amps (+/-) add 1 amp for normal receive, misc loss, etc and you have 5.2 amps required on full key down, you won’t be key down for more than 3 minutes (normal time-out timers), so it should be ok..
Next find the fuse box and see what fuse is used for the cig lighter plug – normally I find they are 10 amps , maybe more.
That indicated the cig lighter plug would handle a 5 amp load with out much issue.
Your situation may be different
All that said, I use the cig lighter for a lot of stuff since mine have 10 amp fuses in them. some may ever have up to 15 or 20 amp fuses, but pays to check first. I do try to find a way to get direct power to the battery with ground as well but sometimes you just can’t or it is too much hassle to get to it. I like to have my radios on a separate circuit fuse if not direct to battery so in my motorhome I have them wired to the fuse box, use an open slot and put in a 15 amp fuse (I have a IC7100 and at 100 watts I use twice the power. ) although each leg of the power to the radio also has a fuse in it. Safer is better.
It was only partially correct and not at all accurate for the actual case. The actual case here should have been to look up what the radio requires, not just do a simple power formula. In reality the radio requires 10 amps on TX full power not the 4.2. Why? because it takes more power to create that 50 watts out than just the simple power formula says.
Next checking the fuses was totally correct in that cars today have anything from 5 amp to 20 amp fuses on the cigarette lighter line and all that is based on how the manufacturer wired it up so the fuse may be set due to limits of wire and length and other things.
Thank goodness that the email was sent to the club email address and we have several way more knowledgeable Electronic engineers and instructors than myself and they corrected my miss-calc method. Bob, K0NR, went on to link in a post by K0BG that covers this topic in some extensive details. Who knew that new vehicles now have things like BMS, EIS and ELD’s. (Battery monitoring systems (BMS) are now a universal subsystem in every modern vehicle, due in part to the fed-mandated Engine Idle Shutdown (EIS). Also the load current is measured with an Electrical Load Detector (ELD). Yes, look those up.
I must say that I have been “hacking” my way through mobile operation for over 40 years according to some of this info. In my defense – it works. That said, I am going to go back and try to update some of my wiring. I have already done that on my Jeep with direct to battery runs and the addition of my home made timed cut in/out control (it uses a relay so it is not great but not $250 in cost either). I built 3 of them from an ARRL article back in 2012.
So going mobile? read up, check the fuses, make sure you can handle the load and wire it up the best way possible.
WD0AJG
WD0AJG - Hamming for over 40 years 