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Old 15-04-2009, 10:40   #1
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Battery as Filter

Question for the engineering types out there.

Many years ago when I had a 1 , 2 both off battery switch I experience a drop in voltage when starting my engine when I decided to motor sail. At the time some of my nav instruments would go off and in the case of Loran would have to re acquire the signal which at the time would be several minutes.

The solution to this problem was to install a small "inline" motorcycle battery for my nav instruments which acted as a sort of "filter" and maintained the 12v + which would keep the instruments on. Its charge was maintained by the main batteries since it was in line. This worked fine.

I'm wondering if this approach to voltage regulation is good, bad or indifferent or would actually function as a device to prevent down stream devices from seeing large variation in voltage associates with smart 2 stage charging systems.

For example would this make lamps last longer since would never burn real hot as 14v+?

What say you?
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Old 15-04-2009, 11:02   #2
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"Filter"

Perhaps the most reliable and elegant solution for powering your nav instruments is to use a dc/dc converter which accepts 8-18V input (or so) and delivers a filtered and regulated nominal 12V out. Measure the total power of all your respective instruments to determine the power rating of the dc/dc converter.

A less reliable, yet good, solution is to use a low-pass filter designed to limit spikes and high frequencies from passing to your instruments. Loran is most succeptible to low frequency noise.

Battery impedance helps yet is not reliable or consistent as far as filter specifications are concerned.
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Old 15-04-2009, 11:21   #3
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Defjef,

By in line was it series or paralleled. Inline to me means series? A small paralleled battery makes sense but only if you have a diode between it and you mains allowing it to be charged but isolated with a voltage drop on the mains. The diode should also have enough current capacity to power your total electronis requirement in amps.

For what it's worth.
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Old 15-04-2009, 11:43   #4
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It sounds to me as if your battery(s) were going bad or were not large enough to begin with. The motor cycle battery might have added just enough power to keep everything running while you cranked your engine.
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Old 15-04-2009, 15:18   #5
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It sounds to me as if your battery(s) were going bad or were not large enough to begin with. The motor cycle battery might have added just enough power to keep everything running while you cranked your engine.
I believe your right about bad or too small a battery. However the small battery to support the nav. and com. sounds like a good idea.

I had never cosidered that.
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Old 15-04-2009, 15:46   #6
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One "real world solution" is to have a starting battery which is completely separate from the house batteries. You should then see NO voltage drop when you start your engine.

Rick's solution (a DC-to-DC converter) is fine, too, but involves extra gear and expense. Most instruments found aboard a cruising boat aren't all that sensitive to reasonable voltage variations -- excepting the serious voltage drop in the case of a house battery used as a starting battery and the higher voltages seen during battery equalization (15.5V and up). Still, there's little doubt that such a converter would help protect some devices and possibly extend their useful life.

One thing large storage batteries CAN filter pretty well: AC and radio frequency transients. That's one good reason why HF radios should be connected DIRECT to the house batteries.

Bill
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Old 15-04-2009, 17:42   #7
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I have a new DC system with a Blue Seas 8080 switch which separates the start and house bank. I don't have the voltage drop problem and so I don't need any solution for that.

But when I do run the engine and the high output alternator is doing its thing, the DC system sees high voltages since it charges to HOUSE and the START is charged with an Echo charge. When this happens everything is seeing 14+ volts and I was wondering if this was harmful to things like bulbs as in shortening their life.
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Old 15-04-2009, 18:29   #8
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bulb life

Bulb life will be quite reasonable under high acceptance voltages AS LONG as they were already on before charging begins. Chargers will ramp up the battery voltage over hundreds of milliseconds which is a long time to a filament to warm up without shock.
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Old 15-04-2009, 18:45   #9
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So, if I understand correctly if I am pushing out high amps and high voltage turning on a 12v bulb WILL shorten its life? Does that apply to LEDs such as SensiBulbs?
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Old 15-04-2009, 19:06   #10
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Sensibulbs self regulate the current, so that you don't have to worry about fluctuations.

Chris


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So, if I understand correctly if I am pushing out high amps and high voltage turning on a 12v bulb WILL shorten its life? Does that apply to LEDs such as SensiBulbs?
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Old 16-04-2009, 10:30   #11
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FWIW,
The dc to dc converter might avoid the voltage drop problem, but in my experience, most of them are VERY noisy in the RFI sense.This would likely wipe out any HF communications on board whilst the instruments were on! Using an isolated starting battery is the only reasonable solution to your problem, and is the setup found on nearly all serious cruising boats.

Cheers,

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Old 16-04-2009, 11:13   #12
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Sorry,
I can't stay out of this. As far as the bulbs go. I would be more concerned with a low voltage condition as far as bulb life. Simply Ohm's law. R on the bulb is a constant. Drop the voltage the current goes up. The heat is a function of the square of the current. Burn out.
Yes the voltage drop on starting is probably inadequate or shot batteries. I'm not sure I like nav. tied the the house batteries. They are the ones that take a beating from lights, etc, ect,. Thats the reason for the independent start battery so you don't anchor have a good time and can't start.
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Old 16-04-2009, 11:53   #13
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Sorry,
I can't stay out of this. As far as the bulbs go. I would be more concerned with a low voltage condition as far as bulb life. Simply Ohm's law. R on the bulb is a constant. Drop the voltage the current goes up. The heat is a function of the square of the current. Burn out.
Yes the voltage drop on starting is probably inadequate or shot batteries. I'm not sure I like nav. tied the the house batteries. They are the ones that take a beating from lights, etc, ect,. Thats the reason for the independent start battery so you don't anchor have a good time and can't start.
Huh?

V=IR or R=V/I

If R is constant, then the ratio of V/I must remain constant. Drop the voltage and the current must drop to maintain the constant.

John
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Old 16-04-2009, 13:03   #14
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John,
Your right. Too much grog.
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Old 16-04-2009, 16:00   #15
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Filament resistance

Filament resistance is not constant. The resistance varies non-linearly with temperature (follows a PTC, or positive temperature coefficient) and, therefore, a doubling of voltage does not result in a doubling of current or a square of power.

Temperature increases with current thereby increasing the resistance. This phenomenon is what Hewlett and Packard used to make the Wien bridge oscillator stable in their first garage produced product.
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