Voltmeter vs. Ammeter

[RallyBob]

Originally Posted by timemachine If VDO made a same-sized analog clock to fit the rallye console I'd probably go with that, but I haven't found one yet. Has anybody seen a 2 1/8 clock out there? Jim

They still make them, in various styles. Try egauges.com or Summit racing.

[namba209]

Let me throw some more garbage into this one. Something I learned in A & P electric school. No matter how many volts you try to put in a battery it will only take what it is rated at, i.e., about 2 volts per cell. We tried to put 28 VDC in a 12 volt battery and all we got was a fully charged 12 volt battery, you just can't increase the voltage at all. If you have a dead cell and the alternator is putting out 14 volts, you'll only end up with about 10 volts So that being said, unless there is a dead cell in the battery, it will always have 12 volts. Now, if you lose the alternator, and don't have an idiot lite, you'll never know it until the engine starts misfiring from lack of electrical power to the coil, unless you have an accurate analog or digital voltmeter. With an ammeter and no idiot lite, you'll know immediately if the alternator quits putting out, by either a slipping/broken fan belt, or the alternator quitting completly. Of course with a slipping/broken fan belt the temp gage will be an indication as well. But all this info really doesn't mean a thing, if you don't know the basic functions of each system.

[timemachine]

Originally Posted by namba209 So that being said, unless there is a dead cell in the battery, it will always have 12 volts.

Ron,
You've really got it in for this thread. The voltage on a good battery will be 12 volts until it's under load, and then (unreplenished) voltage drops as the load increases. As the link in the first entry states, a volt meter measures pressure, an ammeter measures flow (only from the alternator to the battery).

"When the alternator recharges a "low" battery, the ammeter indicates a high charge rate; with a fully charged battery the voltage regulator reduces alternator output, and the ammeter is supposed to indicate a very low charge rate. But how can you really tell the regulator has reduced alternator output because the battery is fully charged? Maybe a diode in the alternator rectifier failed, or the alternator belt slipped after it warmed up, just as if the battery were fully charged. Or maybe the meter indicates a medium charge rate most of the time-does the battery want this much or could the voltage regulator be overcharging the battery? The voltmeter directly measures the result of charging-system performance. With normal alternator/voltage-regulator function, battery voltage is maintained at 14.0 to 14.5 volts-and this is reported directly by the voltmeter. In the event of alternator-system failure, voltage will be low and continue to drop as the battery discharges. In the event of an "overcharge" condition, the voltmeter will climb above its normal zone. In summary, there is no chance for misinterpreting a voltmeter's readings as can happen with an ammeter."

(The ammeter also can't tell you if you have a dead cell in your battery. The voltmeter confirms this with ignition on/car off.)

From wikipedia:

In mathematical terms, this is written as:

I=V/R

where I is the current, V is the potential difference, and R is a proportionality constant called the resistance. The potential difference is also known as the voltage drop, and is sometimes designated by E or U instead of V.

Think of "V" as the total load on the battery (to ground) from the ignition, lights etc.

Jim

[namba209]

Originally Posted by timemachine Ron, You've really got it in for this thread. Jim

Naw, Jim, like I said, just throwing out some garbage for folks to digest. It's just a matter of preference, I happen to like an ammeter over a voltmeter. Two of my wheels has an ammeter, which indicate total amp flow into and out of the battery, the monza only has an idiot lite, and the motorhome uses a non accurate voltmeter that just has a very wide "normal" green band. When I charge up my Ni-Cad wet cells for my model boats, I use a voltmeter and ammeter. I have to charge the battery cells at 2 amps to 15 total volts to insure a full charge to the battery. Ni-Cads are a really different horse than a lead acid battery. Different rules apply to them, gotta be careful to avoid thermal runaway. Just info for you, from the A & P Electric Manual, voltage regulators, internal and external, for lead acid batteries are set at the factory to 13.8-14.2 volts. Anything more will just cause the electrolyte to boil off more rapidly than desired, because of outgassing, and less will not fully charge the battery. It's the volts that pushes the amps into the battery, as you stated.

[timemachine]

Ron,
Didn't mean anything by that, sorry if it came off wrong. Like you said, it's what you like and what you're used to. I'm all for that. Plus, I don't know ni-cad. You might want to replace that non-accruate VM in the motor home in any case. Somebody pointed out earlier how a runaway voltage regulator wound up costing them a burned out car, but that shouldn't happen normally unless you purposely crank it too high.
Jim

[STUMBLINTOM]

There's a meter called the Trimetric that will measure battery voltage (either at rest or while charging or discharging) plus how many amp hours have been withdrawn vs. how many amp hours have been charged toward the battery. It will then factor in the charging inefficiency usually about 18% and tell you the status of your battery. You can select the function you want such as Battery Voltage, Amps In, Amps Out, Percent of Full Charge etc. It uses a shunt (the resistor mentioned in the earlier posts) so all that's required from the shunt to the meter is 18 gauge 5 conductor sensor wire. Interestingly, the shunt is mounted on the negative post of the battery and the negative connections are then made to the other side of the shunt. This device is used a lot in renewable energy systems to keep track of where the batteries are (in terms of charge, not location) at any point in time. Check out http://www.bogartengineering.com/ for more information on this meter. It's the best of both worlds.

[Lindsay]

Few years ago, installed OPGTS combination voltmeter & oil temp gauge to replace GT clock. Driving on Freeway in daylight with headlight on, I noticed that rate of ammeter charge was low, but not discharging. Swithched headlights off, and charge rate increased slightly, but not by too much. Continued driving and voltage slowly decreased.

Troubleshooting:
1. Ammeter warning light was functioning as it should.
2. Extra voltage regulator did not change charge rate.
3. My gut feel was correct; there was oil on alternator gnd wire!

Cleaned gnd wire and charging system was then OK.

While driving GT, I still don't know what time it is, other than daylight or dark.
Someday will get clock or radio with clock.

[STUMBLINTOM]

Hey Lindsay,
Down at K Mart they have these really neat clocks that strap on your wrist for about $20 and they have a light that you can press to see what time it is even in the dark. Gimme a good old Volt meter over an in-dash clock any day.

[timemachine]

Lindsay,

Nice job diagnosing that problem! I've had my head under the hood for too long on more than one occasion before I realized the problem was a bad ground. Nowadays, that check comes right after the primary circuit.

Jim