[VHFcontesting] Alternator/VR question for rovers or auto guys

Mon Jun 6 08:14:42 EDT 2016

I have 2 comments or suggestions.  As Dave suggested, the voltage of the 
alternator will vary depending on the state of the battery in order to 
control the charge current.  If you are running fairly low power, less 
than 30 Amps, you can get a voltage booster/regulator from MFJ or the 
original designer whom I can't remember.  It will provide a constant 
voltage to the rig no matter what the battery voltage is down to 
something like 10.5 volts.  Also I used a battery combiner from Boat US 
that allowed charging from the automotive system but isolated it when 
the alternator is not at a charging level (or off).  This small device 
will handle up to 80 Amps. I powered my amps with this up to 400 watts.  
Also use an appropriate size wire for the current and distance,  #4 
worked fine for me.

73 and good luck roving,  Jim K3LFO

On 6/6/2016 3:55 AM, David Pruett wrote:
> Jared,
>
> After 33 years as an electrical engineer for Chrysler (now retired), I 
> guess that I qualify as an "auto guy".  To be honest, my area of 
> expertise was not charging systems but rather electronics and systems 
> engineering.  But on numerous occasions I had to go to the charging 
> system experts to get questions answered as I was often confused by 
> the behavior of a modern charging system.  I think that is perhaps 
> true in your case as well.
>
> i won't pretend to have special, secret knowledge of Ford charging 
> systems, but it's my understanding that most modern charging systems 
> are pretty much the same, differing only in minor details.
>
> You seem to believe that the charging system should keep the voltage 
> constant, but that is not true.  The purpose of the charging system is 
> to maintain a healthy state of charge in the vehicle battery.  It does 
> this by monitoring many parameters, voltage being only one. Even the 
> current going into or out of the battery is only part of the picture 
> because of variations in the current delivered by the alternator.
>
> The charging system controls the current being delivered by the 
> alternator by controlling the current in the field of the alternator.  
> But there are limits to how much current the alternator can deliver, 
> based on how fast it is spinning (e.g., engine RPM).  Few, if any 
> vehicles have alternators which can produce full current while the 
> engine is idling.
>
> Probably the biggest mystery with modern charging systems are the 
> strategies employed to do its job.  This is because the charging 
> system looks at the state of the vehicle electrical system and has to 
> try and predict what the electrical load requirements are coming so it 
> can get ready.  Some of these strategies involve temperature, which 
> might indicate that the A/C system will be turned on soon.  Also, the 
> performance of the battery itself it greatly affect by the temperature 
> in the engine compartment, which can be tens or hundred of degrees 
> higher than the outside ambient
>
> Then again, on top of everything else, there is the wiring and 
> batteries that you've added to your vehicle.  The charging system 
> doesn't know these have been added, it can only continue to do its 
> job; that is to monitor what it was designed to monitor and to 
> maintain a healthy state of charge in the battery.
>
> I don't expect these generalized descriptions to be of much specific 
> help to you.  My purpose in sharing this information was to illustrate 
> that there are many factors affecting the charging system; voltage 
> being only one. Expecting a constant, steady voltage level as a sign 
> of charging system health is simply not reasonable.
>
> Reading through your note, I didn't see any voltage levels reported 
> that would seem unhealthy so I think you're good to go for your rover 
> operation.  Just use common sense as to how you consume electricity 
> and monitor the voltage on your auxiliary batteries to watch for 
> undercharge conditions.
>
> 73, Dave, K8CC
>
>
>
>
>
> On 6/5/2016 12:03 PM, Jarred Jackson wrote:
>> I finally ran a charging cable from my car battery (2012 Ford Edge) 
>> to the auxiliary batteries in the back of the car. The system 
>> consists for a 100A fuse mounted at the battery, cable to the rear of 
>> the passenger compartment, a marine style manual disconnect switch 
>> and  two 6V 210Ah golf cart batteries in series. The plan is to 
>> disconnect the aux. batteries when the car is turned off and when 
>> starting. I cant use 6m for anything other than local contacts when 
>> the car is running, so often need to turn it off.
>>
>>
>> After getting all of this wired up yesterday, I was happy to see 
>> voltage climb to 14V on the aux. batteries and everything looked 
>> good. I left the car idling in the driveway and returned 20 minutes 
>> later to see voltage at 12.8V. Thinking I popped the 100A fuse in the 
>> charging line, I checked under the hood and saw the car battery at 
>> 12.9V and the fuse looked fine.  I turned off the car and turned it 
>> back on and voltage came back to 13.8V for maybe 10 minutes, then 
>> dropped to 12.7V. At this point there is NO load on the aux. 
>> batteries at all (the main fuse for the amp rack and radio gear is 
>> pulled). The mobile radio in the car (simply plugged into the 
>> accessory port of the car) showed 12.4V.
>>
>>
>> This morning, the car has been idling for 30 minutes and voltage is 
>> good at 13.9V. The car battery is at about 14.1V and everything looks 
>> good. No change in setup from yesterday. However yesterday was a hot 
>> day and the hood was closed when idling (which is probably not a good 
>> idea for various reasons). Today is cooler and the hood is open as I 
>> take measurements. Is it possible that the voltage regulator doesn't 
>> like heat and simply keeping the hood cracked open will solve this 
>> problem? Or maybe there is something else I dont understand. A search 
>> of the internet says I need to replace my alternator. I am not so sure.
>>
>>
>> Any rovers or car guys have any thoughts on the cause?
>>
>>
>> Thanks,
>>
>>
>> Jarred - KF2MR
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>
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-- 
Jim Erickson

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