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I have a voltmeter. I can see drops in voltage for landing lights and strobes.

I don't really get any drop for charging my tablet.

Perhaps measuring volts is too "coarse" for this.

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Your voltmeter is helpful but won't always save a bad outcome.

For instance. When my aircraft's factory carried out a repair, they replaced a single 'landing' light with a neat pair of lights. Muchlater, on. A. Cross country, I had inadvertantly l left them turned on. Landed and shut down. Noted that voltmeter showed 12v. (Should have been more like 14). Couldn't restart the Rotax.

Problem was that my voltage regulator couldn't supply two 50w lamps and the battery had drained to a marginal level.

 

The message is: voltage does not indicate state of charge.

 

PS, if fitting was shunt to allow current monitoring, it's best to have it in a insulated, protective enclosure. Historically, as an unfused part of wiring, it is a fire hazard.

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I think you're a bit wrong on that nomadpete. I think battery voltage is a very good indicator.

 

Pinched from car battery world below.

A fully charged car battery has 12.6 volts. There is a very big difference when a car battery drops even a small amount of voltage. ... At 12.4 volts, a car battery is 75% charged while at 12.2 volts its 50% charged. Consider your car battery charged at 12.4 volts or higher and discharged at 12.39 volts or less

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Battery testers would often use a high discharge test. Just a voltmeter reading doesn't tell much about a batteries condition or how much It's GOT IN IT. Amp Hours..The acceptance of a high charge rate from about 1/2 charge is also a good test but hard for the average person to organise. On most batteries today you can't test individual cells and if one is a dud you are in strife. Dropping batteries can fracture one or more plates and you'll still get a normal voltage indication.. How your starter spins on a cold morning is a good indicator of a batteries GUTS. Your ammeter reducing after a short flight time MAY also be used but will not tell you much about the batteries condition. The charging voltage regulator is set at 14.2 volts for a 12 volt battery.. That's the figure normally showing on your Cars voltmeter. Nev

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The voltmeter is at best, a rough guide.

The ammeter doesn't really tell much about the battery either. It only confirms that the charging circuitry is supplying power. It cannot confirm that a battery is effectively storing the energy.

The only way to confirm how much energy is stored in a battery is to discharge it at a known rate (usually the ten hour rate), until an agreed minimum battery voltage is reached. (For a 12v battery this is usually 11.8v). Clearly, this cannot be done 'on the run', and shouldn't be done too often, because even 'deep cycle' batteries don't like frequent deep cycling.

There are cheap digital A/H Meters available now, but they only measure A/H in and out, to estimate the stored energy. Since they don't know the storage efficiency of the battery, they still aren't foolproof. And are not suited to our aircraft anyway.

 

For better accurate info, go to your battery manufacturer's website and hopefully there will be discharge voltage graphs. As well as max charge current, float voltage and equalising voltage. But remember these discharge graphs only apply when the battery is fresh from factory.

 

Car Battery World is trying to simplify a complex issue.

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I have a digital voltmeter in my plane. All you need to know is the voltage at rest (which is about 12.4v). When the engine us running then anything higher is a charge. Anything lower is discharge. With everything on it should still be higher than 12.4v. ideally around 14v. When I was an auto elec when testing charging we considered anything over say 13.5 to be a win because there's so much variation in different systems.

 

So your voltmeter can give you similar information to an ammeter, just not as dramatically.

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If you are using an aircraft battery be aware that they are different to car batteries. The charged voltages are different! Car chargers don't work as well on aircraft batteries.

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If you are using an aircraft battery be aware that they are different to car batteries. The charged voltages are different! Car chargers don't work as well on aircraft batteries.

My Odyessey (AGM) is 12.85 v fully charged and ideally requires 14.7v charging.

There is a special setting on some battery chargers for this.

I think alot of problems people have with AGM's are from using "normal" lead acid chargers...... which reduces the life of AGM's.

Mine is over 5 yrs old and in perfect health. Load tested etc....

The aircraft system can't charge to 14.7 so it requires regular charging with a dedicated charger.

I do this most flights when home hangar based.

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Nomadpete, I have recovered some sulphated batteries by repeated discharging and recharging SLOWLY. Never leave ANY battery discharged for long. AGMs are more fussy. If they ever get hotter than normal, renew. They get bubbles in the paste and it's permanent damage..Nev

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If you are using an aircraft battery be aware that they are different to car batteries. The charged voltages are different! Car chargers don't work as well on aircraft batteries.

 

 

My statement still stands- find what your battery voltage is when charged and not being used. With your digital voltmeter you can easily see if at any time is higher or lower than at rest.

 

So, for Down under s battery it is 12.85v. after start it should show more than that. Note what it is under normal running conditions and you can discern most problems. Overcharging, no charge, dead short etc.

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How is what you are saying and different to my statement? I just wanted everyone to know what I found out, the hard way. I was not countering anything you said. Some aircraft have car batteries, some aircraft batteries. They are slightly different.

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How is what you are saying and different to my statement? I just wanted everyone to know what I found out, the hard way. I was not countering anything you said. Some aircraft have car batteries, some aircraft batteries. They are slightly different.

 

Apologies, didn't read it carefully enough

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I Think by altering even the SG of the acid you can change the voltage figures. I reckon there are emerging issues with batteries in aircraft . In a GA (certified) plane I've always stuck to the original part number and its a strong cased lead acid battery with special caps that restrict acid leaks They are usually very heavy for their size and have a good cranking performance. Marginally more expensive than a car battery . Nev

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I had a case split open in my Mooney. The acid destroyed the case and some of the airframe. Cost a considerable amount. It taught me a lot about batteries. We think that the regulator was faulty and cooked the battery leading to overheating and cracking of the case. Regulator was replaced.

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I fitted a AMP meter to see what amps are being pulled during normal flights and trips when accessories are plugged into the USB and outlets for ipad, camera, charging things etc.

 

ammeter is a good testing/adjustment instrument, it can give information "who sucks all the power from system" and "how much power I really have in my generator", but generally useless as indicator "something is wrong" - voltmeter is much more sensitive when things come to "not enough power". For ammeter it is difference between 40 and 45 Amp (if generator provides 42), for voltmeter - between 13.5 and 12.0, and even more useful rate of change, if you rev engine/generator from idle overloaded generator will rise voltage veeeeeeeery slow (as it has to charge the battery with tiny excessive current). Center-zero ammeter (connected in battery lead) is a bit more useful as indicator, but still very voodooistic in interpretation of its data.

 

Also - we are in 21st century! What is it on the photo? BALLAST????? heavy resistor to measure voltage drop????? Nightmare. For 20 years all this is made with contactless Hall sensors, providing good output voltage without any additional voltage drop in cable itself, without any additional joints, screws, lugs etc. Just the same cable pulled thru the hole inside the sensor - and thats it! google "hall dc ammeter" and be happy!

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Just measure voltage drop along a length of existing cable and then calibrate it for an ammeter. Thick copper cable is heavy and you don't want unfused conductors all over the place. Rarely is starter current measured anyhow. It's too high for the usual instruments used. If you use a + and - ammeter (charge discharge) you can play with your switching and watch the changes and get a good idea what's going on. (or off). Nev

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To measure the cables resistance accurately you have to use a Wheatstone bridge. Another problem, for accurate readings that would not be required for an approximate calculation, is the temperature of the cables when carrying a high current. It is the maximum temperature that the insulation can take that determines the maximum current that can be carried by the conductor. From memory 125 degC for PVC.

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Just measure voltage drop along a length of existing cable and then calibrate it for an ammeter.

 

extremely unreliable and unprecise way. Cable resistance vary with temperature (due to load and enviroment change, 100C is real change range for space under car bonnet, for aircooled engines even more) too much to be counted stable, also this layout creates significant magnetic loops to catch all external EMI. For DC circuits it is not so significant, but anyeay strong EMI will affect your instrument.

 

as I said above - use hall sensors!

 

hall dc ammeter 100A 1%

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Thirty years ago I bought a whole heap of instruments, a few were hall effect measurements instruments. If I remember correctly you have to zero the instrument before use. Is that still the case?

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Thirty years ago I bought a whole heap of instruments, a few were hall effect measurements instruments. If I remember correctly you have to zero the instrument before use. Is that still the case?

 

NO. Thirty years ago it was not a hall sensor but inductive loop sensor, hall meter could be there only as zero magnet flux indicator. This type of sensor does require zeroing, as it is able to measure only CHANGE of current (DC current produces zero magnet flux change so zero inducted current in coil). It is acceptable for tester/hand meter/etc, but not for permanent instrument. These hall ammeters measure not inducted current but flux itself, even if it is constant.

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Hi Scott

 

Only made on flight With engine off it shows the electric carby heat draws 5.2 amp, Fuel pump draws 0.7 amp, strobes show no real effect, should have turned on landing light (next time) In flight fuel pump draw of amps does not show. I'll do a few more flights and see what its doing. May try the Halls type as must be like a clamp meter. My shunt is in the negative cable. Does the Halls ring go around the positive cable? Cheers

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May try the Halls type as must be like a clamp meter. My shunt is in the negative cable. Does the Halls ring go around the positive cable? Cheers

 

industrial hall sensor is usually a solid ring, not like clampmeter - this significantly increases precision and reliability, but you have to pull the cable thru the hole, not just hang the sensor on existing cable as is

 

It does not matter, you can measure current at any point of circuit. Sensor measures just magnet field, not potential of the cable, so it can be used on 1000s V cables or on earth connection without any need for level convertor. Its outputs are completely isolated from the circuit, that is very convenient for data inputs and signal processing.

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