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Author Topic: Filtering for battery charger...  (Read 3776 times)
KT0DD
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Posts: 277




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« on: November 28, 2011, 09:27:36 AM »

Hello, I bought an automatic battery charger/conditioner from Autozone to keep my emcomm station battery charged. It is always "on" and just reduces the charge rate as needed.  I am wondering if I need to install a large filter capacitor between the charger/battery and radio power leads to prevent unwanted hum or feedback, and if so, what size cap should be used? I will be running a Kenwood TS 480 HX 200 watt rig, drawing approx 42 amps peak maximum.

it is inconvenient to just unplug the charger if I have problems, as the plug is behind a very large bookcase.

Thank you for any help. 73. Todd - KT0DD
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AA4PB
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« Reply #1 on: November 28, 2011, 09:43:19 AM »

It's possible to introduce hum on the transmit signal from a charger if the battery charge is low. If the battery has a decent charge on it then it will act as a big capacitor. In addition, the automatic charger is likely to contain some filtering. Any added capacitor would have to be quite large to have much of an effect with a 42A peak load.
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K9KJM
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« Reply #2 on: November 28, 2011, 10:30:57 PM »

It's possible to introduce hum on the transmit signal from a charger if the battery charge is low. If the battery has a decent charge on it then it will act as a big capacitor. In addition, the automatic charger is likely to contain some filtering. Any added capacitor would have to be quite large to have much of an effect with a 42A peak load.


Yep, I agree.   Just charge your battery up and give it a try!  I normally do not use any type of filter at all.    Many years ago when trying to milk out a little extra life from an old battery that was starting to fail I did some experiments to see what took that nasty AC hum out.   A really large choke in series with the charger worked out the best, With some large capacitors across the positive and negative terminals also helping out..........    but with a good battery, Just not needed, At least with my Schumacher 10 amp automatic chargers.
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W8JX
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« Reply #3 on: November 29, 2011, 05:41:57 AM »

Given that a charger has unfiltered output, a 20 or 30kufd or bigger electrolytic cap in parallel with charger and battery would help a lot. To suggest that the charger is clean enough or that "battery" will act as a capacitor to clean up all hum is bad advise. It is not clean nor does it have good regulation and a big CAP would help here a lot.
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AA4PB
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« Reply #4 on: November 29, 2011, 06:57:51 AM »

Based on my experience I'll stick with my "bad advise" Grin  It depends totally on the quality of the charger and the condition of the battery. If you've got one of those cheap chargers that consists of nothing more than a transformer and a half wave rectifier then you may have an issue. If you've got a "trickle" charger or an automatic charger and the battery is not deeply discharged then you won't have an issue. At any rate, before purchasing a 20,000 to 30,000 uFd or larger capacitor I would just give it a try and see if I had any hum on my transmit signal. Odds are that you'll find that the capacitor is not necessary.

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W8JX
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« Reply #5 on: November 29, 2011, 05:16:01 PM »

Based on my experience I'll stick with my "bad advise" Grin  It depends totally on the quality of the charger and the condition of the battery. If you've got one of those cheap chargers that consists of nothing more than a transformer and a half wave rectifier then you may have an issue. If you've got a "trickle" charger or an automatic charger and the battery is not deeply discharged then you won't have an issue. At any rate, before purchasing a 20,000 to 30,000 uFd or larger capacitor I would just give it a try and see if I had any hum on my transmit signal. Odds are that you'll find that the capacitor is not necessary.

If it was as simple as you suggest you could save a lot on power supplies for rigs. I would not run a unfiltered charger on a battery while using radio. Even without hum factor, rigs cost too much to risk running them with a cheap unfiltered car battery charger powering rigs.   
« Last Edit: November 29, 2011, 09:23:58 PM by W8JX » Logged

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K1CJS
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« Reply #6 on: December 01, 2011, 07:35:32 AM »

I'll agree with AA4PB.  Unless you have really bad humming or interference, you don't really need a capacitor there.  You can also always turn off the charger while operating if you find you have excessive interference.
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W5LZ
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« Reply #7 on: December 08, 2011, 12:22:35 PM »

... Pssst!  Add a switch to that charger, then just turn it off.
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K1CJS
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« Reply #8 on: December 21, 2011, 06:03:47 AM »

Most modern three stage chargers have perfectly adaquate filtering.  They have to so the charger sensing circuitry can do its job.  And as AA4PB said, the battery, when nearly fully charged has a capacitive effect on the charging current from the charger already.  The added capacitor isn't needed at all.
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