I’ve been looking at external batteries for my FT-818. I noticed the DC power cable on the FT-891 has 25A fuses built into it, is that something I should consider for the 818? Would a ~13v battery ever surge?
I thought the fuse might be to prevent the 891 from damaging car electrical systems, but I wanted to check before I screw something up. Should I find a DC cable with a fuse before I connect an external battery?
73!
Two fuses, one in the positive and one in the negative. The fuses are to protect the cable from melting and catching fire.
Fuse has to be big enough to allow operation of the equipment and small enough to fail and protect the cable.
Welcome to the group!
Fuses in a DC line for a ham radio typically are not there to protect against surges, although conceivably they could protect against that to an extent.
Typically a fuse is to protect the wires to the radio from a short circuit occurring somewhere in the system. If you have a short circuit in the radio or somewhere in your DC line, you could draw enough power to melt your DC lines or start a fire—hence the fuse to blow before anything bad happens.
For portable work, many people including myself do not run fuses as fuses typically have voltage drop across them, at least at high current draw. When you are portable with your rig, you will typically be instantly aware if something bad happens and will have time to disconnect your battery before something horrible happens. In addition, many lithium packs have built in fuses anyway, so you are protected from potential battery fires.
So in my humble opinion, especially if your portable battery has a built in protection circuit, I would not bother with inline fuses on your DC cable. Of course, with fuses you are safer, but you may be somewhat limiting the voltage your rig actually sees.
73,
Tom, N2YTF
Not having double fused leads is how I nearly melted the traces on the PCB of my 817. Cost of 2 fuse holders and fuses vs. cost of repairing 817. 2 fuses and holders will win every time.
The Bienno description says, “The battery has an integrated PCM (protection circuit module) which provides complete internal cell balancing, complete integrated protection” Given the number of times they say “protection” but don’t actually say “fuse”…not sure how to interpret that.
Given that I’m still a bit unclear on how electricity works in spite of passing my Tech exam and pretty close to taking my General…I agree. At least for now. The voltage drop because of the fuses is a concern, but I’m a cautious person by nature.
As a side note - I had no idea how much I’d need to learn about electrical engineering via radio. It’s really connected a bunch of dots for me. It’s been really interesting. Feels like the license should be called “how all your gadgets actually work but companies don’t want you to understand” instead of just “radio”. 
The battery has a max Ampere delivery of 24A.
That is too much for the thin power lead wires of the FT-818.
Adding a 4 or 5 A fuse (to both wires) should be okay to protect the cables. FT818 takes about max 2,5 A in TX
For the 817 / 818 you can ignore the voltage drop. It will operate also at 9 Volt without problems.
Maybe something like this
73 Joe
There is a lot to learn—that’s what keeps the hobby interesting!
Its true that at the low drain of an 818 if you have robust fuse holders you shouldn’t have much voltage drop. If you used some sort of really tiny fuses in the interest of saving weight then voltage drop might be a problem. Its also true at least an 817 and a few other rigs have cleaner TX with higher voltage, and cold weather and other factors can lower the voltage your rig sees from the battery but certainly as you are learning all the ins and outs as you go I would suggest going with the fuses on both lines to help protect from running into any problems.
It is easy to not properly insulate a connector or to create a ground loop or a million other things that a fuse might protect you against.
I actually burned out a trace in the audio section of a 706 at home due to a ground loop and was running fuses but they did not save me…so fuses are certainly not a cure all but a “cure some” and are definitely the cautious choice.
73,
Tom, N2YTF
Lets go back to basics,
Non QRP
The battery fuse is there to prevent an uninteded short circuit becoming very spctacular!
I use 2, 3 cell X 5Ah Lipos in parallel. They are rated to deliver 30C. this means thay each can supply 150Amp…or more into a short circuit.
Do we need to fuse both positive and negative? In my opinion no, as far as the battery is concerned, what current flows in the positive line is exactly the same as the current flowing in the negative line. Kirchof’s law.
I power my FT857 with the batteries, maximum current draw 25Amp pk,
I fuse each battery at 25Amp at the battery. Maximum demand current up to 50A. They are car (auto) fuses with spade connections. I use silicone grease on the connections to combat corrosion.
The voltage drop on the fuses must be small, otherwise they would melt, they do not!
The cost of a fuse is very low. Carry a couple of spare fuses, just in case.
Bluntly, there is no excuse for not providing simple, satisfatory short circuit protection.
Regards
David
G0EVV
I use a fuse for everything! I once blew up my Walfords 80m CW kit due to some sort of fault and the power supply wiring burnt. Thankfully I managed to save the rig with a bit of rework.
I have a fused distribution board in my shack for powering stuff from the solar battery.
For my portable rigs I tend to use a 1amp fuse, I’ve had the fuse blow when I had an antenna fault when using my MTR - the rig was fine.
As a side story, I was testing my newly re-housed QCX last week and initially I’d powered the rig up with a 500mA inline fuse. I got called away, when I came back to the rig I tried to transmit some CW into a dummy load. The sidetone sounded extremely rough and the display went dim with every key down. I then realised that I’d seen that fault before - the fuse! Changing the fuse to 1amp rating cured the rig’s ills.
Colin
I power my FT-817ND with a 3 cell LiFePo and I DO employ fuses on both lines. Lithium based batteries can deliver a really large amount of current when shorted. You don’t want that current delivered into your radio or equally important not to damage your battery.
I’d like to mention one more (although unlikely) aspect: The receptive plugs of the battery are somewhat exposed. I sleep better when I know that no accidental conductive connection between the + and - connector can occur (by other conductive stuff in the transport bag) as the fuse(s) would blow immediately.
A cheap small alternative is just solder a 20mm glass fuse in the wire, and cover with clear heatshrink.
Pro tip: If you are using thin wire then run the wires going along the glass out the opposite end (under the heatshrink), so that pulling the wires puts the fuse into compression
I suggest any fuse solution needs to be field serviceable. It would be irritating in the extreme to walk several hours, blow your only soldered fuse by making a simple connection mistake, and go home with zero contacts in the log. For this reason I prefer circuit breakers that can be resettable many times. And by using Anderson Powerpole connectors, the odds of making a polarity error are reduced considerably.
The use of two fuses in radio connections is important in mobile setups, where the antenna connection is usually also a negative line connection. In the event that resistance develops in the negative line between the battery and the vehicle chassis, the starter motor current will find a lower resistance path across the PCB of your expensive radio and vaporise it. This is a real problem encountered by professionals in the mobile radio field.
However in the SOTA portable setup, where there is no external negative line connection between the antenna circuit and the battery, a fuse in the negative lead is not justified as it just adds (slightly) to the complexity, with another component to be maintained and sometimes, replaced in the field.
If however, your SOTA field antenna uses a large metal ground plane with its own connection to the battery and its own electrical circuit, you are carrying too much weight and need to re-engineer your antenna system.
Andrew VK1DA/VK2UH
I have lodged spare fuses in my rucksacks and radio cases. I tend to take a spare rig and a spare power lead, all of my MTRs use the same 1.7mm x 4mm connector.
I think I’ve only had a fuse failure once and the fuse replaced in the field also blew. Swapping rigs got me back in the air.
On another occasion, the wire on my factory FT817 fused power lead failed at the fuse holder. As luck would have it, I had a gas powered soldering iron, wire strippers and solder with me on the summit! I resoldered the fuse holder in the field (on the summit!) and the repair is still good 
73, Colin
Yet it was that missing second fuse that would have saved my activation.
I, like most of my ancestors for the last 5000 years, carry a knife. So no, a blown fuse would not prevent activation.
Wisdom of the ancestors.
Indeed, and I have saved a few activations by cobbling together bits of connectors, vhf antennas, alligator clips and bits of wire.
Still, if an activator doesn’t have tools or any spare parts, they are counting on finding a discarded soft drink can ring pull (remember that scene in the film about the kid who hacked into the missile control computer, which had a 5 letter password protected login accessible by a phone modem, as if!), not many seen recently on my summits. And the tin snips don’t usually come along, though I do have a multi tool that has come in handy.
It’s all a matter of reducing the probabilities of inconvenience.
Colin @m1buu would be able to build an entire spare transceiver between contacts.
73 Andrew VK1DA/VK2UH
A presenter to my local club some years ago was an electrical inspector from the local electricity supply authority. He showed a picture of a replacement fuse installed in one of the nearby industrial suburbs. He said a 4 inch nail would be rated as 400 amp, slow blow…
Not had one of those fail yet Andrew.
Fuses in the negative lead were useful for those occasions where the installer put the lead to the battery negative. Starting the engine would produce a decent current through the coax and the negative lead. Usually the PCB foil in the parh evaporated. Unless there was a fuse.
Installing a second battery with auto-isolation allows connection to the battery with only one fuse.
I’m unclear how Andy vapourised his FT817 on a summit. Any explanation Andy?
73
Ron
VK3AFW
Any experience with resettable polyfuse?
Well, first both you and Andrew 1DA are correct in that most times a single fuse is sufficient. The car case is the classic where a chassis bonding fail can cause huge currents to flow from the mobile antenna chassis connection through the screen to the mobile radio and from there through the mobile radio chassis and earth print traces back to the battery. Hence the negative fuse lead in addition to the positive.
My case was a derivative of that and somewhat contrived in that connectors had been removed as I forgot a battery. If you only ever run one piece of equipment off a battery it isn’t going to happen. For me, a bag of 10 IDC automotive blade fuse hoders and 10 5A blade fuses was pennies and because the setup described in the linked thread may arise again (needing to power transverter and IF radio off one battery) it seems like a pragmatic fix to me.
