Powering the QCX 50 amp via LIPO battery #sota

Evan - KN3O

I'm currently building a QCX 50w amp for 20m to match my QCX transceiver.

Will a 6S Lipo battery be too much for the amp? The 6s packs are 25.2V fully charged.

Curt wb8yyy


the main factor might be the current capacity of those batteries - I recall something like 5 amps may be required?  check Hans clear instructions on voltage and current. 

note the amplifier is a huge factor if you only have 13.8 volts available just now - 3 watts to 25 watts is a much huger jump in dB than 25 to 50 watts.  the former is around 2 s-units, the later is about half an s-unit, assuming 6 dB per s-unit. 

my amplifier is nearing completion .... I plan to initially use it at the lower voltage, until I am confident all is well with it.  and that may be enough of a boost. 

73 curt wb8yyy

Evan - KN3O

Thanks Curt. I'm not too worried about the 5 amp current requirement. If you go high enough on the C rating of the battery you should have plenty of headroom.

I'm just curious how hard the upper 22V upper limit is on the amp. If necessary I can pick up a buck converter (again with enough amperage rating to cover the application) to drop it down to an appropriate voltage. 



Checking would be good.  5A at 20V would be 100W and at 70% efficiency minimum
thats 70W out.  I did that as a does it makes sense number.

Typical input is abut 3.5A at 20V or 70-72watts DC and 50 out.'

Big thing for a battery is total watt-hours in the battery . The LiFePo4 is
just lighter for the total watts.

So to be useful you need maybe 140WH battery so a 4S needs to have
cells of 7AH and with 25% duty cycle rate that will give about 8 hours
maybe longer.  That means with 2800mah 18650 cells thats a 4s3p
battery with a battery management board.  OR a packaged battery of
20V 7AH or larger.  FYI the cells need a 2C (or 5A for the battery)
max discharge rate so that usually easy to satisfy.

A lower voltage battery and upconverter has the issue of high peak
current so the discharge rate maybe far higher.  A higher voltages
battery 24V (6S) nominal and buck converter would have a lower peak
current but the converter noise may be an issue and the converter
needs to be 4A peak.

So 6S requires down converter, and the battery has to to not
less than 4A peak.   Raw 6S power is too great and will likely 
fry the devices.

Also the 6S pack needs to be at least 140WH capacity or more
for useful run time as your wasting power in the buck converter
(8 to 25%).

If you thinking portable you need to assure the antenna is known good
before going to high power and it can safely operate at that power or
failure is likely.  Also SWR is low (measure and correct) as its pointless
dragging an amp and then frying it.

For field gear aggressively using fuses and bring spares is a good idea.

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