[Mobile-Portable] Big Farad versus Hybrid Caps?

[N1KHB at aol.com]

Hi Doc,
   Once again, you should check the dc operating voltage spec for  the 
amplifier. Most ham gear wants 13.8 volts + or - 10%. The lower limit then  
would be ~12.42 volts AT the amplifier terminals without any other power  
connection losses. There are often some surprisingly high connection losses  with 
high current draw designs. 
    A "12" volt lead-acid battery resting voltage (no  load) is ` 12.6 
volts. That means there is only a margin of 180 millivolts  from the battery 
voltage to the possible trouble point in operating the  amplifier (and the 
radio/exciter if operating from the same source). Way too  close for a good 
design even if the battery voltage doesn't sag under load with  or without the 
super-cap. As the battery charge depletes, the situation gets  only worse 
while the battery output voltage reduces even further. Please  check carefully 
what you plan to get into here. This sounds very risky  technically 
speaking, and after spending the money to follow this path you  may well find 
yourself very disappointed.
   In a running vehicle the alternator can handle the current, so  you 
won't really be drawing from the battery under usual circumsatnces.
   And once again I ask why not just use a power supply if ac  power is 
available? Wanna talk by phone? Email me privately for my phone  number.
 
Best,
Joe N1KHB
 
 
In a message dated 6/19/2009 1:53:59 P.M. Eastern Daylight Time,  
doc at kd4e.com writes:

If my  battery is fully charged and is rated at 155 Amp Hours 
(20 Hr Rate)
and  can provide 75 Amps for 77 Minutes and I put a 20A (or 
larger)  float
charger across it *and* buffer it from surges with a huge  
capacitor why
would I still see low voltage?

In a mobile  environment I would have an even larger source 
of float-
power across  the battery (when the engine is running).

In SSB (intermittent service)  I don't see a reason for 
concern - perhaps
in digital  (semi-continuous/pulse service) I could see a 
concern.

What am I  missing?

>    It would be wise and prudent to check the  amplifier specs for  
operating 
> voltage range. With that level of  current draw, the battery  voltage may 
> well sag under load to  less than what the amp wants in order to  operate 
> properly and  an extra outboard capacitor probably won't help  enough.
>   Another (better IMHO) approach would be to use a battery  voltage  
boost 
> device. I assume that some are made in that current  range.
>    Are you trying to avoid using a power supply  because of cost,  or are 
> you just trying to be independent of AC  mains?
>  
> Joe N1KHB

-- 

Thanks! & 73,  doc, KD4E
FS/Swap/Wanted: http://kd4e.com/swapn.html
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