Why 120/240 Volt Generators Are Not Good Battery Chargers

Using a 120 volt or 120/240 volt single phase generator to charge your batteries is very common, inexpensive to purchase and horribly expensive to operate.

Expect to pay anywhere from $2.50 to $5.00 per kWh when using a single generator to operate your home and charge your batteries.

If you are going to use a single phase generator to charge your battery bank go here to learn how to get the most charging power out of your fossil fuel powered generator.


If you are using a Power Factor Corrected battery charger, the rest of this article does not pertain to you.

It will still be expensive to charge your batteries using a single phase 120/240 volt AC generator compared to a 3 phase alternator but the power factor correction will help significantly.

All Magnum Energy inverters use power factor correction in their built-in battery chargers as well as the Xantrex RV converters to name a few. There are many more. Please check your manufacturer’s documentation.

If you think about a typical gasoline (petroleum)/propane/diesel powered generator and how it charges your battery bank you can see why it is not the most efficient:

First Problem: Your generator is producing 120 or 120/240 single phase which must be reduced to 12/24 or 48 volts with the use of a sometimes huge and inefficient transformer (which is typically seen in a large off grid system) and then be rectified later into DC.

Second Problem: Single phase 120 volt power is only charging your battery bank when the output voltage is above the voltage of the battery bank.

Typical single phase, 60 hertz power generators actually charge the battery bank less than half of the time the generator is running…far less.

I’ll show you why. This is a typical sine wave one would expect from a good quality single phase 120 volt power generator.

Typical sine wave produced by a good quality 120 AC generator.

When charging your battery bank with a 120 volt AC generator in combination with a battery charger (either built in to the inverter or a separate unit), the first thing that happens is the voltage is reduced using a massive transformer to a voltage low enough to charge the battery bank. For the purposes of this demonstration we will assume the battery bank is a 12 volt battery bank.

Now the sine wave looks like this.

12 volt AC sine wave produced using a 120 to 12 volt transformer.

Next, the 12 volts AC must be rectified to DC using a single phase rectifier. Let’s take a look at the wave now and see what has happened.

In a sine wave the voltage is above zero voltage half the time and below zero voltage half the time.

When you rectify this sine wave, only the voltage above the zero line remains. Anything below the line is useless and gone.

For you geeks the actual voltage is higher when rectified so it looks closer to this.

So 50% of the power you are generating is not used to charge a battery. But wait…IT GETS EVEN WORSE!

To charge a battery you need a higher voltage than the battery voltage. If you have a 12 volt battery that is almost full, the voltage might be 13-15 volts. If the battery voltage is 13.4 you will need at least 13.5 volts DC or more to charge that battery. If your battery’s voltage is only 10.5 volts (virtually dead) you will need over 10.5 volts to charge this battery.

The diagram above has a black horizontal line representing roughly 12 volts. Any voltage above that line will be useful for charging a 12 volt battery bank. Any voltage below the line is useless and wasted when charging a 12 volt battery bank, unless the battery bank is extremely dead.

The diagram below shows when the battery bank is actually receiving a charge.


As shown in the diagram, the only time the battery bank is being charged is the area shaded in blue.

If you life off the grid and have to use a fossil fuel powered 120 volt AC or 120/240 volt AC generator you are likely familiar with the problems they cause.

1.  They rarely make a nice clean sine wave unless you are lucky enough to own an EU Honda or similar. And while the EUs and equivalents are nice, they are less efficient than than a normal “off the shelf” AC generator. They produce wild 3 phase AC, convert it to DC and then invert it to clean 120/1240 volt AC. Every change from AC to DC and DC to AC their are losses involved.

They do make up for their inefficiency by doing an excellent job of idling down when there is no load. However in this article we are talking about charging batteries with a generator, not operating a cottage or home with a generator. There will always be a significant load on the generator defeating the auto idle down feature.

2. No matter how good inverters are getting at synchronizing with the generator before flipping the auto transfer switch, the lights still dim and the fridge compressor makes a “clunka”, “clunka”, “clunka” sound which can’t be good for it. Some are so bad that your computer reboots. Fortunately the newer models don’t reboot computers if you have them programmed properly.

About 5 or 6 years ago my brother decided we could do better and it was surprisingly simple…a small diesel (or gasoline or propane) engine coupled to a Ford 100 amp alternator with a belt. It consumes a third to one half of the fuel a single phase generator consumes. As it is simply a DC charging source the inverters do not have to synchronize and operate any transfer switches.

We will write an article as soon as possible about our homemade DC generator (alternator) but look at the sine wave a Ford alternator makes and think whether or not it would charge a battery bank better than a single phase 120 volt or 120/240 volt AC generator.

Obviously this 3 phase electricity will work better as there are three times as many peaks…

If you think about it:

  • Almost all alternating current (AC) electricity is made as 3 phase power.
  • All utility made electricity is generated as 3 phase.
  • All electricity in the automotive industry is generated as 3 phase
  • All industry operates using 3 phase electricity.