AC amps - DC amps

26 Posts
Mar 3, 2009 01:03 pm
AC amps - DC amps

Hello all,

I've been trying to wrap my head around a couple of things lately and was hoping someone could shed some light.

AC amps to DC amps....whats the formula?

I understand an amp is an amp but isnt there a difference when dealing with the AC and DC and different voltages, watts, etc...

My system is as follows:

Lister generator 6 hp, 4k head, w/ two legs of 120v for 240 volt sent to a 240 - 120 volt transformer.
This is sent into a midnight solar e-panel with a modified magnum 3924 watt inverter/120 amp charger hanging on the front.
The battery bank is made up of 16 Trojan T-105's hooked up in strings of 4 for a 24 volt bank.

If my generator puts out 33.3 amps at 120 volt and half that amperage for 240, what amperage is actually being sent to the charger and what is the formula to determine the maximum amount of amps being deliver into the battery bank?

Note: I have not discharged my batteries below maybe 75% so I have never seen the charger display read higher then 60 amps or so. 

163 Posts
Mar 3, 2009 04:26 pm
Re: AC amps - DC amps

AC and DC amps are the same. What you are doing is transferring power (volts x amps) from one source to another.

You have a 3924 watt inverter which is 120volts x 32.7amps to give 3924. If you have a 24V output, then the maximum current during the transfer process is 3924/24 or 163.5 amps.

The reason that you are only seeing 60 amps output is the "charging rate" for your batteries. The more discharged the batteries are, the higher the initial charging rate will be, but it should not exceed 10 percent of the amperehour rating of the batteries and will taper off as the batteries move to full charge.

Hopefully your charger has been setup properly and you have nothing to worry about, but the more you discharge your batteries the higher the initial charging current you are likely to see.
26 Posts
Mar 3, 2009 09:29 pm
Re: AC amps - DC amps

I guess I'm still a bit confused.
Lets just say I have a regular 100 amp battery charger plugged into the 4K generator putting out 120 volt and charging a 12 volt battery.
What would be the maximum output amps of the charger if the batteries were low?
Mar 4, 2009 01:08 am
Re: AC amps - DC amps

There is no "formula" for ac amps to dc amps.
If one had a load of 75 watts at 120 vdc the amps (.63) is identical to the amps for the same 75 watt load at 120 vac.
A clue is the "v" for voltage at the front of vdc and vac.

As for "isn't there a difference when dealing with the AC and DC and different voltages, watts, etc..."
No. But there are losses. Most of those losses end up as waste heat. While your system is running and charging batteries, feel the gen. head. Now the transformer and then the inverter. Chances are they will all feel good and warm depending on how long they have had current flowing through them.

I am curious about the gen. head. Is there no way to tap the gen. head for a full 120 volt circuit? "Weed" out the transformer.

As for "what is the formula to determine the maximum amount of amps being deliver into the battery bank?"

There probably is a formula but it would only be a "somewhere in the ballpark of" kind of figure because the efficiency's from one generator to the next can vary widely as well as transformers and battery chargers.

Chances are you may never see much more than 60 amps going into the batteries from that charger. Its not uncommon for manufactures to advertise higher ratings. Those advertised ratings probably came from some formula not a real world situation where KVAR might come into play literally lobbing off the peaks of the sinusoidal wave coming off you generator. Does your gen. head have a capacitor bank on it?

26 Posts
Mar 4, 2009 09:40 am
Re: AC amps - DC amps

Thanks but I'm still not clear on one is my generators maximum amperage of 33 able to go to the 60 amps I have seen?
I guess forget the sentence "formula for ac amps to dc amps."
What is the math, theory, idea of having a capped amperage output from one source go through another to be increased. (if that makes any sense)   

I understand heat, wire, and inefficiency losses and that no "formula" is absolute but there has to be a simple "rule of thumb" or "ball park."

The gen head is tapped for a full 120 volt through the transformer by reducing the voltage from 240 down to 120.
I could of hooked up one leg of 120 from the generator but I didn't want to wear down just one side of the head.

The generator doesnt have any sort of meaningful capacitors. Maybe just a few for the underside of the digital display.   
Mar 4, 2009 08:10 pm
Re: AC amps - DC amps

Oh, okay.
Anytime voltage is decreased with a given wattage, amperage goes up. Likewise if the voltage is increased with a given wattage the amps go down.
4000 watts / 120 volts = 33 amps
4000 watts / 24 volts = 167 amps
Of course the reason your only seeing 60 or so amps is because of the windings ratio of the chargers transformer.
60 amps x 24 volts = 1440 watts

Go here
and scroll down to such things as;
Faraday's Electromagnetic Induction Experiment,
AC Generator Action,
DC Generator Action,
Ohm's Law, (Watts can be construed as resistance at work.)
How a Transformer Works

I don't know. Maybe you know this already.
 This wavelength is drawn two dimensionally. In real life or 3-D its more like a Slinky kinda strechted out.

Its interesting to note that the small portion of the electromagnetic spectrum we humans know is like a grain of dust to the planet Earth. Its a continuum.

Sorry. Getting off the subject. I'll let you digest this and hope that I haven't skipped to much. Its all kinda like that Star Trek movie, the one with the Whales. When Dr. McCoy ask Mr. Spock what it was like to be dead and then come back to life. Mr. Spock's reply was, "Without a common frame of reference I would be speaking in gibberish."

I've yet to get into harmonics and RMS voltages. I think I touched a bit about kvar - Kilo Volt Amps Reactance. Maybe next time.
« Last Edit: Mar 4, 2009 08:30 pm by Thomas Allen Schmidt »
Mar 4, 2009 08:28 pm
Re: AC amps - DC amps

I thought about this after I posted.
Its educational as well as entertaining.
You might still want to put on a pot of coffee. Just in case.
26 Posts
Mar 4, 2009 09:43 pm
Re: AC amps - DC amps

Thats what I was looking for! Thanks.
I wasnt sure if it was that simple in fact and I also think I had a difficult time determining the passage of power through all the components.
Another lesson for me that the simplest answer is usually the best/ correct.
The first link is great.
The others I would have to read more. Except maybe the universal wavelength one. I think I've had great conversations with trekies and hippies alike on that one and now makes a little more sense in hindsight.
Thank you, Thank you,
Thanks you! 
26 Posts
Mar 4, 2009 09:48 pm
Re: AC amps - DC amps


I had another thought along the same lines as my first question:

Running an adjustable 140 amp MIG welder from the generator. I'd hesitate putting a meter on the output to check the voltage though. Yikes!
It's powered from a 120v wall outlet with a 20 amp breaker.
Same idea I guess.
2 Posts
Mar 16, 2009 09:56 pm
Re: AC amps - DC amps

I think you got some good advice on how to follow the power through your system.  The key is to use Power (Watts).  Watts (W) are calculated by multiplying Voltage (V) x Current (I, or often A).  For any given power value in Watts, there are infinite combinations of V and A.

Starting at your generator, it produces 4000W in the form of 120V x 33A, or 240V x 16A.  To get all 33A on a single 120V leg, you have a transformer to convert the 240V to 120.  In theory you will get 120V @ 33A out of the transformer, but there are losses involved, so if you assume that the transformer is 90% efficient then you will get about 30A out of the transformer.

The charger in your 3924 has a max charge rate of 105A into the batteries, and at that charge rate it draws 30A of AC power.  105A at 29V (bulk charge voltage) is about 3000W.

The question still stands about why you are only getting a 60A charge rate.  Looking at the Magnum manual, I see that the default configuration limits the AC power draw to 30A.  That means that the power drawn out of the generator for BOTH AC loads AND charging cannot exceed 30A.  So, if you have any AC loads that will reduce the charge rate below 105A.

Also, the default charge rate is 80% of the max, which limits charging to just over 80A.

Do you have the remote control for the 3924, and have you checked the programming for these two values?  They are likely contributing to the lower charge rate.

I had a 24V system for years with a Trace inverter/charger and a 4000W generator and I was only able to get 95A or so of charge current, with the AC draw on the generator being the limiting factor.  It's still more than 60A, but I think sets an upper limit on what to expect.
Mar 18, 2009 05:28 am
Re: AC amps - DC amps

Hey Dave.
A little while back I asked if your generator has a capcitor bank on it. Take a look at this report by Xantrex and maybe this will clear up some more stuff for you.

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