Author Topic: Way to modify Alternator Voltage Output  (Read 6944 times)

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kittysmitty

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Way to modify Alternator Voltage Output
« on: September 23, 2014, 12:04:06 PM »
I use what I call my lawnmower generator when the batteries are low and no sun. It's an old lawn mower engine with a Ford alternator. When I had a large forklift battery I used diodes in the voltage sensing line  to fool the alternator that the battery voltage was lower, so the output voltage would be higher when I wanted to equalize the battery. Things have changed. My battery bank are AGM's and I'm concerned about having too high a voltage. The alternator output is 14.8 volts and I would like to lower this. Any ideas how this might be done?
Thanks

Flux

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Re: Way to modify Alternator Voltage Output
« Reply #1 on: September 23, 2014, 01:31:47 PM »
Not too difficult to make a regulator for lower voltage but if you can arrange a voltage supply above 16v you can use the diode trick to fool the regulator into seeing a higher voltage to force the output down by a diode drop.

You need the diode the other way round but to get it conducting you need a bias resistor to a higher voltage so that the diode clamps to battery volts but holds the sense wire a diode drop above it. The current needed from the higher voltage supply is small so adding a small car battery to get 24v would work. You would need to charge it occasionally by some means.

Flux

dnix71

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Re: Way to modify Alternator Voltage Output
« Reply #2 on: September 23, 2014, 06:05:37 PM »
Or you could just add a diode or two to the output and facing the battery. Diodes all have a small forward voltage drop, usually about 1 volt for a silicon diode. A 1n3289R diode is rated 200v / 100 amps with a forward voltage drop of less than 1.5v.

http://www.ebay.com/sch/i.html?_nkw=1n3289R&clk_rvr_id=701560308783&mfe=search  $20-30 new and used on eBay.

kittysmitty

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Re: Way to modify Alternator Voltage Output
« Reply #3 on: September 24, 2014, 07:02:28 AM »
Thanks, had not thought of a diode on the output. The alternator is rated at 60 amps max, so even something a little less robust would work, I think. I usually see around 40 amps output.
http://www.ebay.com/sch/i.html?_from=R40&_trksid=p2047675.m570.l1313.TR0.TRC0.H0.X70hf10.TRS0&_nkw=70hf10&_sacat=0

Flux

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Re: Way to modify Alternator Voltage Output
« Reply #4 on: September 24, 2014, 12:28:26 PM »
Yes a good stud mounted diode on a decent heat sink should be ok. If you can mount the heat sink in the cooling air of the alternator it will be a much more manageable size. The 35A bridge things are probably not up to this job even with 2 diodes paralleled.

Flux

kittysmitty

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Re: Way to modify Alternator Voltage Output
« Reply #5 on: April 01, 2015, 06:05:33 PM »
http://www.princessauto.com/en/detail/90a-battery-isolator/A-p8235673e

would a Battery Isolator work in reducing the voltage?

Thanks

Flux

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Re: Way to modify Alternator Voltage Output
« Reply #6 on: April 08, 2015, 11:16:28 AM »
Yes its just a couple of diodes on a heat sink. Probably schottky so will drop about 0.6v at high current.

Flux

kittysmitty

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Re: Way to modify Alternator Voltage Output
« Reply #7 on: December 13, 2015, 05:04:51 PM »
Hi Folks

Just thought I would give an update. Finally had a chance to give this a try. bought a diode wired it in and no luck. Then I bought a battery isolator, used for 10 bucks. Still didn't work. I then realized the battery voltage wasn't getting back to the alternator through the diode. Once I shorted across the diode momentary, it worked great! Thanks

OperaHouse

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Re: Way to modify Alternator Voltage Output
« Reply #8 on: December 14, 2015, 04:01:05 AM »
You could do that with back to back diodes ( opposite directions) in parallel.  That would give you an auto start.

Ungrounded Lightning Rod

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Re: Way to modify Alternator Voltage Output
« Reply #9 on: December 21, 2015, 03:49:53 AM »
Yes its just a couple of diodes on a heat sink. Probably schottky so will drop about 0.6v at high current.

Flux

Not necessarily.

Some battery isolators are diodes.  Others are relays, pulled in when the generator is running, dropping out otherwise.  The power lost to the relay coil may be substantially less than that lost to the voltage drop of the diode, the lack of a voltage drop means they don't foul the regulation and reduce the charging rate, and they don't pull leakage current when the genny is off.

Of course when fooling the regulator is what you're after, using a relay-type battery isolator is not effective.

dr_zogg

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Re: Way to modify Alternator Voltage Output
« Reply #10 on: March 25, 2016, 09:11:41 AM »
I have been using a slightly modified version of this circuit it really works well.
I am sure you probably have your issue sorted out by now but it might be useful to someone.

http://chemelec.com/Projects/Alternator/Alternator-Reg.htm

Cheers,
           Zogg