[aprssig] Increased Diode Efficiency

Stephen H. Smith wa8lmf2 at aol.com
Fri Jul 5 17:06:52 CDT 2013

On 7/5/2013 3:06 PM, AC wrote:
> On 7/5/2013 10:00, aprssig-request at tapr.org wrote:
>> >
>> I donno whether they can work as distribution diodes too, (isolation
>> diodes for batteries etc,) but it looks promising for higher power 12v
>> stuff.
>> Bob, WB4APR
>  From the data sheet it is designed exactly as described, to replace a
> four-diode bridge rectifier and not a single diode.  So no, you can't use this
> chip to replace the Schottky diodes that are used for things like the PowerGate
> battery isolator.
> If you want to use active switching for battery isolators, you need the LTC4355
> chip which performs the same task using MOSFETs instead of Schottky diodes.
> This chip also can monitor for power faults (too much power across the MOSFETs)
> and fuse faults (blown fuse).

For the past 10 years, I have been using a power FET battery isolator device 
made by Hellroaring Technology at:


to maintain a second battery in my mobile that I use for all my radios and APRS 
computer gear.

This device, originally made for RVs,  connects between two sets of 12-volt 
batteries.  It contains a power FET switch capable of 150-300 amps and an 
op-amp voltage comparator.   When the alternator and main (starting) battery 
voltage rise above 13 volts (i.e. engine running) the FET is turned on to 
connect the main battery and alternator to the secondary battery(s).  When 
saturated on, the voltage drop is less than .05 volts at 100 amps.

If the secondary battery is severely deep-cycle discharged, which would result 
in an immense inrush current, the device automatically goes into a pulsing mode 
to limit the average current until the secondary battery rises to within a volt 
or so of the main battery.

When you shut the engine off and the main battery terminal voltage drops below 
13.0 VDC, the FET switch automatically opens to separate the radio battery from 
the starting battery.

Additionally, the device has a pair of terminals you can short with an external 
switch to force the FET switch ON even with the engine off, that allows you to 
jump-start the main battery from the secondary battery.

I originally used this isolator to charge a 55AH Optima Yellowtop Spiracell 
from the 12 VDC system of my 2005 Prius.  (The OEM 12 VDC system in the Prius 
has a really small motorcycle-sized 12 VDC battery since the gas engine 
starting is done with the 240 VDC traction battery and gene-motor. The OEM 12 
VDC battery system in the Prius is only used to boot the computer control 

The isolator and second battery has now served in my 2006 Jetta TDI diesel for 
the last 140,000 miles.

Highly recommended.....



Stephen H. Smith    wa8lmf (at) aol.com
Skype:        WA8LMF
EchoLink:   14400  [Think bottom of the 2-meter band]
Home Page:          http://wa8lmf.net

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