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Re: QMX+ with 100 amp/hour lithium iron battery

 

Let's not get too carried away with special $150 fuses and such, that only discourages the use of these batteries.
Large high current batteries should be treated with respect, but are very useful for the amateur.
Used properly and with some understanding of what's going on, they are not dangerous.
Far less dangerous than the 2000 kV plate supplies of an old school transmitter.
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My advice for powering a QMX is to just use something like16 AWG wire, not 0000 welding cable.
16 AWG is 0.004 ohms/ft, so 6 ft of wire round trip is 0.024 Ohms, at 1A that's only 0.024 of voltage drop.
A 2A automotive fuse in the power supply wires is fine, and the wire will go if the fuse does not for some reason.
Try not to drop a screwdriver across the battery terminals.
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As an experiment, I donned my safety goggles and shorted out our 30kwh 48v off-grid battery bank
using two feet of 6 AWG copper wire and a 7.5A ATO automotive fuse, curious if the fuse would arc over.
It did not, just a nice pop and flash of light inside the fuse.
I am not confident that this would always be the case for a 48v supply, for example this may depend
on how large a gap gets established by the initial current surge, or who made the fuse.
I am confident that such an automotive fuse is adequate for a 12v supply.
After all, a lead-acid starting battery can also deliver several thousand amps when shorted.
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Here's a quick video that drives home why we care about high voltage DC arcs.
https://www.youtube.com/watch?v=Zez2r1RPpWY
The AC arc self quenches whenever the sine wave crosses zero volts, at 60Hz*2 = 120 times per second.
The DC arc has no such quenching action, the ionized air in the arc remains conducting till the arc is quite long.
In this case it's 240 Volts DC, not 12 Volts.
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LiFePO4 is a very good choice.
Safer than most other lithium chemistries, and has reasonably good power density.
A 100AH battery is good for half a week with key down at 5W out on a QMX.
Excellent for emergency backup communications.
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Jerry, KE7ER
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On Mon, Apr 14, 2025 at 07:52 AM, Bruce Akhurst wrote:

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On Thu, Apr 10, 2025 at 02:52 PM, Bruce Akhurst wrote:
Hope you’ve got your fault protection sorted.?
Prospective short circuit will be 10kA and an auto fuse ain’t going to stop that arcing ..
It can be as simple as a fusible link.? Go down 4 wire size numbers, so for example #16AWG link in #12AWG conductor, a foot long, connected in series with the main 12AWG line with compression crimp splices NOT solder.?
The link including splices and at least an inch beyond (I cover about 2-3 inches), is covered with fire sleeve.?
I use an 18 inch long sleeve for a 12 inch link; the idea is to contain and cool the hot gases if the link fuses so leave room


Exactly.? ?I see a lot of DIY/ RV stuff where 100's of Ah of lithium batteries are merrily connected to quite low consumption projects in inflammable locations without properly thinking through the prospective fault/short circuit currents and consequences.? ?
Large current DC arcs are hot and self sustaining and something you only want to see once and not in your workshop /office /Van/ home.?

In theory the better batteries may have BMS or other internal protection but it's hard to test that without damaging the battery and it may not be there or not reliable anyway.? ?
T grade fuses are $150 for a high interrupting current fuse+holder so cheaper alternatives are useful.? ? ? ?
Also don't ignore marine grade Switch/Breakers which are resettable, re-usable? and decent ones by Buss etc (no not ebay clone/copy ones) can typically carry and switch 15 - 100A or more as circuit breakers and interrupt 6000 amps in fault conditions for less money than a T fuse . .??
Note a 'battery switch' or 'breaker' or 'resettable fuse' while cheaper,? are not necessarily able to interrupt large DC fault currents and this figure usually missing from their specs?


Bruce
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