alternator amps and amp hours

[peppster66]

On my 97 roadtrek, how would I find out what the amps/output is?, I would be interested in case I ever had to replace my alternator,,,,,,,, thanks

[avanti]

Originally Posted by peppster66

On my 97 roadtrek, how would I find out what the amps/output is?, I would be interested in case I ever had to replace my alternator,,,,,,,, thanks

Get a Trimetric meter or equivalent. Well worthwhile.

[markopolo]

Originally Posted by peppster66

On my 97 roadtrek, how would I find out what the amps/output is?, I would be interested in case I ever had to replace my alternator,,,,,,,, thanks

The 1997 RT brochure indicates 136 amp in the Dodge vans and 124 amp in the Chevy vans.

[markopolo]

I've started to think of the amperage out of an alternator as throughput. That's my take on the info Booster has posted on this general topic over the years.

It's voltage based meaning that the alternator via regulation will maintain a set voltage (as best it can and up to its rating) in the system. If it is trying to maintain 14.4 volts and it takes 5 amps to do that then that's the output. If the system requires 40 amps to maintain 14.4 volts then that would be the output.

Maybe Booster or anyone else here will comment on the basic accuracy of the above paragraph.

With already fully charged batteries in my van, it starts out as 14.7v which drops to 14.2v probably due to heat. I often run the 3-way fridge on AC via the inverter while driving and that further drops the system voltage to 13.9v while underway and that fridge load is present (180 watts iirc). That's an easy and convenient way for me to prevent the already charged batteries from being subjected to 14.2v continuously during long drives. If and when I need the batteries to have the higher voltage I can turn off the inverter and/or switch the fridge to a different cooling mode.

[markopolo]

I should add that it doesn't take much of a load to bring the voltage down to 13.9v in my van but it would take a very large load to cause a further drop.

I run the microwave oven via the inverter off the alternator with the engine idling while cooking meals and the voltage never dips below that same 13.9v.

[booster]

I would totally agree with how Marko described the alternator output in his first paragraph.

The drop to 13.9v under larger loads, that is stable at that point, is most likely the voltage the alternator drops to when it gets hot, to limit the output and keep from overheating. OEM alternators often heat drop very quickly as they are not really designed for long term high output. Some of the aftermarket units can run much higher output % when hot as they are made for it, and have better cooling, but they still drop some.

[avanti]

You can't go far wrong by thinking in terms of a hydraulics analogy when trying to get your head around electricity:

Voltage is analogous to water pressure
Current ~ water volume
Alternator ~ water pump
Battery ~ holding tank
Wire ~ pipe
Wire resistance ~ pipe diameter
Load ~ turbine

These analogies are not perfect, but they will get you a long way.

So, for example:
--An alternator trying to charge a battery is like a pump trying to fill a tank.
--The amp/hour capacity of the battery is like the size of the tank.
--The amperage of the alternator is like the volume capacity of the pump (amps = coulombs/sec ~ gallons/minute).
--The voltage of the alternator is the pressure it is producing.
--You can see that the bigger the "pipe", the easier it is to pump a large volume of water. You can get an equal volume from a smaller pipe, but this requires high pressure to move the water faster. This is "Ohm's law": Voltage=current*resistance ~ Pressure=volume*pipe diameter. If you raise the pressure of the pump, you will force more water through a given pipe. Of course, if you take this too far, the pipe will burst (wire will overheat and melt).
--Pumping water into an empty tank is easy: The tank will accept lots of water (amps) without pushback. But, as the tank gets fuller, you need greater pressure (voltage) to force more water into the tank (as your battery fills, you need higher voltage to force amps into it). Eventually, the tank is full, and it won't take any more water, no matter how high the pressure).
--Using a battery is like draining the tank. The battery's voltage is the pressure of the water it stores. The pressure will slowly drop as the tank empties.
--Put two identical tanks in parallel, you will double the amount of water it can provide, but the pressure stays the same.
--Put two identical tanks in series, and you double the pressure.

You can play this game for a long time. Eventually, the analogy runs out of steam (e.g., surface charge). But exploring the analogy can really deepen your understanding.

[markopolo]

Booster - thanks for adding the protection aspect (voltage drop down) as causing and/or contributing to the voltage drop - that explains another piece of the puzzle.

The water analogy make sense to me. Probably could add:

Alternator regulator ~ water pressure regulator

[booster]

Originally Posted by markopolo

Booster - thanks for adding the protection aspect (voltage drop down) as causing and/or contributing to the voltage drop - that explains another piece of the puzzle.

The water analogy make sense to me. Probably could add:

Alternator regulator ~ water pressure regulator

And we all have chased that "leaking faucet" parasitic drain in our power systems!

[cruising7388]

Originally Posted by booster

Also, in calculations, you need to factor in how much the alternator maximum output will drop when hot. It can be a lot, depending on the alternator design, so that 300 amp alternator might be only 200-250 amps when at full output for a little while. And even lithiums will taper a bit in how much they will take as they near full. Lead acids taper a lot and need extra hold time besides.

30% drop in rated alternator output when hot is typically the case for OEM alternators. There are aftermarket exceptions like Balmar alternators marketed for marine applications that are "hot" rated with respect to their output.

[avanti]

The Balmar regulator monitors the alternator temperature and reduces the load accordingly. So, it is in effect controlling the alternator's temperature.

[gerrym51]

so any alternator charging claims are 'the possible best' and not what actually happens

[booster]

Originally Posted by gerrym51

so any alternator charging claims are 'the possible best' and not what actually happens

Pretty much. You might get full output for a very short time at cold start, but it will drop very quickly, especially with OEM alternators.

[papab]

One thing that the alternator specs never seem to mention is the volts at max current? It might keep up to the regulated voltage up to a point, then the voltage would start to droop. I wonder if the spec amps is at 12 volts which doesn't do much good for battery charging.