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.   
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