Battery for mobile activities

My mobile activities consist of carrying around a FT-857D in the field, it is a radio that can produce 100W, the voltage range is 13.8 +/- 15% or 11.73 to 15.87 Volt. In the ideal case you would have a power source that provides the 13.8 Volt up to something like 20 Amperes which you get during transmit. The current during receive for the FT-857D is around 0.5 ampere.

Solutions so far

I have tried a number of battery solutions, an easy one is, a standard car battery which is rather heavy to carry around. Smaller sized 20Ah AGM batteries exist, but they are still around 5 kg which is also too heavy.

The lead acid battery voltage range is too reduced to produce 100W in the field.  The measured HF output is around 80 Watt for a full battery but less than 50W when the battery voltage drops to less than 12,5 volt, at even lower voltages you will experience that the modulation quality reduces, at least, this is the feedback I got from reception reports at low voltage.

A DC-DC converter is a solution but it is a device that generates RF noise which you don’t want. There is already a lot of power converter noise in our living environment, and the reason to go outdoors is to avoid that noise. Even cars will produce RF noise, there is always something running and you can hear this on the HF bands. If you want a clean environment then avoid the switching power supplies near a receive antenna.

Design requirements

For each active hour in the field you need on average 1,5Ah and the longest continuous session in a mobile setup is approximately 3 hours or 4,5 Ah. The design requirements for backpacking are:

  1. reduce the battery weight less than 1 kg,
  2. battery capacity should be around 6Ah
  3. supply voltage range should be 11.73 to 15.87 Volt
  4. supply amperage should up to 20 Ampere peak and 0.5 Amperes in rest
  5. desire for a voltage and capacitance monitor

Lithium polymere (Lipo) batteries

A four cell (4S) lipo with a capacity of 5800 mAh satisfies requirements 1, 2 and 4 easily, it weighs 580 gram, its nominal voltage is 14,8 volt and with the 20C model you can in principle draw 20 times 5.8 Ampere, which we never do of course. There is only one drawback, the voltage range is too large, it is between 14,6 and 16,6 Volt, so when it is charged we exceed the specification of the FT-857D. The indicator on the Lipo shows the voltage, the capacity remaining is related to that voltage and a lot of other factors, such as temperature and age of the battery. There are various internet sources for this,  see for instance:

http://www.rcuniverse.com/magazine/reviews/1183/ampd14.pdf

The three cell (3S) Lipo battery could be an alternative, but in my opinion it underpowers the radio resulting in cracky modulation reports. So back to the 4S lipo with an add-on where we peak shave the 4S lipo when it is above 15.8 volt. The standard solution is to insert two high amperage diodes in series which takes away 1.5 volt, this is sufficient to reduce to 16.6 volt down to 15 volt which is within the acceptable voltage range of the radio. High amperage power diodes can be found in rectifiers, I found one that handles 35A as a maximum average which can handle peak currents up to 100A or more and voltages up to 500 Volt or more, more than  enough for our intended use, and easy to get.

image1_cleaned
The intended use for the standard bridge-rectifier
image2_cleaned
In the voltage reducer circuit it takes the following shape
image3_cleaned
Use a die cast box able to dissipate approximately 30W of heat
image4_cleaned
Use the shrink tube as a tule to protect the cable (I used the thickest audio cable I could find)

The intended use for the reducer is when the 4S lipo goes beyond 15.75 volt, so before the reducer you insert a monitor (that I got via the internet). All used connectors are EC5 which is compatible with the hacker series batteries that come from CONRAD.

Last update: 31-aug-2018

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