Using Your Generator to Charge Your Batteries

September 28, 2013 · 6 comments

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There are going to be many times when the sun isn’t shining, the wind isn’t blowing and your batteries are low. The only choice you might have is to charge your batteries using your fossil fueled generator.

As you may already know, there is not a more expensive method of battery charging than using a 120/240 volt 60hz (230 volt 50hz) AC fossil fueled generator.

There are 3 ways to charge your battery bank and they are listed below in order of best to worst (most efficient to least efficient) methods of charging your battery bank. Most efficient just means you get more kWhs per dollar spent (or liter of fossil fuel consumed).

  1. Ideally you would have enough solar, wind or micro-hydro power (but that can be almost impossible in some climates).
  2. The second best would be a DC generator made from a fossil fueled engine and 3 phase alternator that is rectified to DC.
  3. Using an off-the-shelf or industrial 120/240 volt 60hz (or 230 volt 50hz) fossil fueled generator.

Why is a 120/240 volt 60hz (230 volt 50hz) fossil fueled generator not the best choice for battery bank charging?

But there are ways to get more kWhs from your 120/240 volt AC “off-the-shelf” or industrial generator.

Kubota Diesel Generator

Making your own electricity with a gas, propane or diesel generator can cost anywhere from $2.50 t0 $5.00 per kilowatt hour compared to the $.05 to $.60 per kilowatt hour most utilities charge. It is horribly expensive and environmentally a disaster. Most fuel powered generators have no or very little emissions controls and are made to wear out in a matter of months when used to charge batteries in an off grid power system. Read more about the downfalls of using a 120/240 volt AC generator to charge your battery bank.

Below are the steps to charging your batteries with a generator while using as little fuel as possible:

STEP 1: Bulk charge at 10% of the battery bank”s C20 rate

The first step in setting up your system to charge batteries efficiently is to program your inverter/battery charger to charge at 10% of the battery bank’s C20 AH rating.

This sounds complicated but it is not. What is a C20 amp hour rating?

Consult your battery manufacturer’s literature for the C20 amp hour (AH) rating.

For example:

Trojan’s L16s have a C20 amp hour rating of 370 AH at 6 volts.

If you were to take four of these L16s and place them in series you would now have a 24 volt battery bank rated at 370 amp hours.

Bank of Trojan L16s wired in series to get 24 volts

Simply take the C20 AH rating and multiply by 10% or .10. This is your bulk charge current setting.

In the above example you should try to put 37 amps (370 multiplied by 10%) at 24 volts  into your battery bank. This would require about 880 to 1110 watts depending on the actual battery voltage.

While some manufacturers allow up to 15%, they are usually referring to the C3 AH rating. What is the difference between C3 and C20?

To get the most “bang for your buck” try your best to achieve the 10% of the bank’s AH rating. Obviously there will be times when this is not possible such as having too small of a generator or too small of a charger. You can charge at a lower current, it just won’t be as efficient.

If you charge with more than 10% of C20, you risk damaging the batteries due to over heating and over gassing.

Let’s try another example:

This time we have sixteen Trojan T105s (225 AH @ 6 volts) wired in series and parallel to make a 48 volt battery bank. There are two banks of eight batteries that are paralleled to make 450 amp hours (C20) at 48 volts. This is pretty common battery bank and is shown in the picture below…

how to charge a battery bank with a generatorIn this example, it is best to set your inverter/charger to bulk charge at 45 amps (450 amp hours multiplied by 10%). Forty five amps is approximately 2160 to 2700 watts depending on the real voltage of the battery bank.

Even if you have a much larger battery charger/generator, 45 amps is the target charge rate for this particular battery bank. Any less will result in excess generator run time and fuel consumption, any more can result in damage to your batteries.

STEP 2: Program correct bulk voltage into inverter/charger

Consult your battery manufacturer for the correct bulk voltage setting. Next examine your inverter/charger’s manual to learn how to program the bulk voltage.

You need to be careful here as some manufacturers list:

  • the bulk voltage per cell
  • the bulk voltage per battery
  • the absorption voltage (same thing as bulk voltage)
  • the bulk/absorption voltage (same thing as bulk voltage)
  • the bulk voltage as a range (i.e. 2.35-2.45 volts per cell)
  • the daily charge voltage (same thing as bulk voltage)

All of the above terms like absorption, daily charge and bulk voltage are all referring to the bulk voltage setting.

If your bulk voltage is listed per cell, think of each cell as being two volts.

  • a 6 volt battery or battery bank has three cells
  • an 8 volt battery or battery bank has four cells
  • a 12 volt battery or battery bank has six cells
  • a 24 volt battery or battery bank has twelve cells
  • a 48 volt battery or battery bank has twenty four cells

Once you know how many cells you have simply multiply the volts per cell by the number of cells you have. If your battery bank is 48 volts you know it will made up of 24 cells.

For our first example we have 4 Trojan L16s. Trojan lists the bulk/absorption voltage as 2.45 volts per cell.

As we have a 24 volt battery bank we simply multiply 2.45 by 12 to get a bulk voltage of 29.4 volts.

Trojans L16s have a bulk voltage of 2.35 to 2.45 volts per cell.

Let’s try our other example:

In this battery bank we have sixteen Trojan T105s (225 AH @ 6 volts) wired in series and parallel to make a 48 volt battery bank. There are two banks of eight batteries that are paralleled to make 48 volts at 450 amp hours (C20). According to Trojan, the best bulk voltage is 58.8 volts as seen below…

Setting the bulk voltage for a 48 volt battery bank made with Trojan T105s.

In the above diagram we know the recommended bulk voltage is 2.45 volts per cell.

As it is a 48 volt battery bank we simply multiply the cell voltage by 24 to get 58.8 volts.

What is bulk charging?
Bulk charging is when you apply a predetermined amount of current (10% of C20) to your batteries until a specific voltage is reached. This specific voltage is known as the bulk voltage. If your battery bank is low all of the charging capability (amps) will go into the batteries. As the batteries begin to fill up, the voltage will rise until the bulk voltage is reached.

As soon as the bulk voltage is reached, the absorption charge begins. The absorption charge gradually reduces the current while holding the bulk voltage steady until the batteries are full. Once the batteries are full, the batteries can be float charged.

If you are using solar, wind or micro hydro to charge your batteries, the three stage charging is great, but if you are using your generator, you SHOULD only be performing a bulk charge.

Remember our goal is to charge our batteries but use as little fuel as possible. Use the generator to keep the batteries from going dead and use the solar, wind or hydro to top off the batteries later.

STEP 3: Program the absorption charge time setting on your inverter/charger

timerAlthough this step is not necessary we must discuss it. Normally we would use the absorption charge to add the extra 20% to your battery bank but to save fuel we will not.
But most inverter/chargers require a time setting for the bulk charge program. The standard setting would be 4 hours. ‘This is a good backup in case your automatic generator shut down (from STEP 4) malfunctions. It will take about 4 hours (if using 10% of C20) to bring your bank to a full state of charge once the bulk voltage is reached.

STEP 4: Design a system that will shut down generator once bulk voltage is reached.

The bulk charge (when done correctly as shown above) will bring your battery bank to 80% state of charge (SOC). Using your generator for anything but a bulk charge is very inefficient, wasting fuel. You can obviously complete the absorption charge, and float charge if you wish to add the extra 20% to your batteries but it is a waste of fuel.

It would be better to use your generator as a bulk charger and your solar, wind or micro hydro to bring the batteries to 100% SOC.

Most high end inverter/chargers or charge controllers have programmable relay drivers that can stop your generator once the bulk voltage is reached. Refer to your inverter/charger, charge controller and generator’s owner’s manuals to learn how to set this up.

If you follow the steps above you can get the most energy input for the least amount of fuel.

Here are a few more suggestions when using a gen set as your backup battery charger.


Use a battery temperature sensor with your generator and inverter/charger.This is extremely important. As a battery’s temperature changes, so does the voltage required to charge it. Cold batteries need a higher bulk voltage and warm batteries must use a lower bulk voltage. You cannot babysit your batteries and charger to make changes with temperature swings. Temperatures change many times a day inside a battery depending on ambient temperature, how hard the battery is working and how many amps are going in or coming out. Install the temperature sensor below the electrolyte level and on one of the batteries in the middle of the bank.

Do not buy a portable generator from Home Depot, Walmart etc…

Inexpensive generators that can be purchased at your local big box store are great for the camping enthusiast, carpenter, RV dry camper but they are a horrible waste of money and fuel for the off grid homesteader. We all have made this mistake.

Instead, spend the money and buy an industrial, water cooled, low rpm (1800 rpm, 1200 rpm, 900 rpm) generator like Kubota, Onan, John Deere etc.. They cost a lot more money but will last your entire lifetime if maintained compared to anywhere from 6 months to a couple of years with a cheap 3600 rpm air cooled gen-set.

Always perform maintenance on your generator yourself.

High quality generators are expensive. Repairs and extra parts are also very expensive. Doing your own maintenance is always preferable to shipping your generator out for repairs or paying someone to make the repair on site. If you can make minor repairs and perform basic maintenance yourself, you will be able to catch problems before they become a big deal and make sure the repairs are done right.

You are now living the independent “off grid dream” and it is time to become as self sufficient as possible.

Your generator’s manufacturer will recommend what maintenance needs to be done and how often. However they never dreamed you would be abusing their gen-set like you are. In the “real world”, a generator is simply idling waiting for someone to start a power tool, light or heater.

In your world, your generator is working like a dog and often producing as much power as it was designed to for hours at a time with no break. If you are abusing your gen-set you NEED to perform maintenance even more than the manufacturers recommend. If you keep your oil super clean and keep your air filter super clean, you generator set will last longer and use less fuel. Engine oil is inexpensive compared to the extra cost of fuel, extra repairs and the cost of replacing your generator set.




Enough said.

Leave a Comment

{ 6 comments… read them below or add one }

miriam s. January 27, 2017 at 11:36 pm

Has anyone else who is off grid ever experienced generator problems occurring in tandem with battery death? I have a 48v system with Surrette batteries and an LP Onan standby generator. The system has worked automatically and flawlessly for ten years until earlier this month. Generator failed suddenly (not related to the generator’s starting battery). At the same time, as I began troubleshooting the off grid system it became clear that it was time for new deep cycle solar batteries. Easy to replace batteries, that’s happening in a few days. But we got the generator manually working (and sounding great BTW) for only one day after having been fully serviced and new control panel installed before it tanked again and trashed the new $1,200.00 control panel that had just been installed. Could there be some issue with back feed of current??? Or anything related to the depleted batteries to make the generator fail to manually run again? My highly qualified generator mechanic is so stumped that I’m ready to replace my quality generator with a new one. But since we already went through two control panels, I’m afraid that a hidden problem in the system will cause the new generator to fail, too. Anyone have any ideas on where I should look for a problem? Charge controller (Outback) and inverters (Xantrex) all acting normally as far as I can tell. Thanks.


Jody Graham January 28, 2017 at 8:41 am

Hi Miriam,

Thanks for your questions.

First you have done well to get 10 years out of the Surrete batteries. Great job.

How many do you have and were they S460s or S530s?

As far as the generator goes it would be impossible to cause back feed damage as long as you have it connected to the AC input of your SW4048s (which I am assuming) with the age of your system and the fact that the rest is your system high quality.

Maybe someone after the repair is connecting the generator to the AC output of the inverters instead of the AC input destroying the control panel?

Please let me know the model of your battery bank batteries (S460 or S530), how many batteries you have, your inverter model numbers and how many you have and the generator model and I will do my best to help…Jody

A few photos would really help of the entire system, and the wiring terminals inside the inverters showing the wiring connections. You can email to jody@solar Thanks.


Ron July 9, 2016 at 9:08 pm

How do you charge a 48 volt system with 12 volt charger?


Jody Graham July 13, 2016 at 10:54 am

Hi Ron,

Thank you for your question. You cannot charge a 48 volt battery with a 12 volt battery charger. You need at least 60 volts available to charge a wet lead acid 48 volt battery bank as voltage flows from highest to lowest and depending on the battery and ambient temperature you could need up to 60 volts just to bulk charge a 48 volt battery bank.

If you referring to the 12 volt alternator charging a 48 volt battery bank it is a different matter. Automotive alternators without internal voltage regulators are only current limited, not voltage limited. The faster they turn, the higher voltage they produce. When you connect them to a 12 volt battery bank they are a 12 volt battery charger with a 100 amp current limit. When connected to a 48 volt battery bank, they become a 48 volt charger with a 100 amp current limit assuming you have a large and fast enough engine to turn it. I have seen people operate 120 volt incandescent lights with a small lawnmower engine connected to an alternator (without an internal voltage regulator. Very efficient and inexpensive if you can build it yourself.

There is a limit of how much voltage an alternator can produce and that is only due to the limitations of the insulation on the windings and other wiring inside the alternator. I would not want to push any more than about 500 volts from a typical old style Ford alternator as I assume they are using 500-600 volt insulation on their windings. If you allow the voltage to go higher than the insulation allows, you will eventually get a short circuit and meltdown. Even using them for 12, 24 and 48 volts, the insulation can fail and short the windings but this is more due to the environment (rain, snow, dirt, duct) and vibration than voltage.

Small micro-hydro and wind turbines work the same way…the faster they spin, the higher the voltage they produce unless they have internal voltage controllers or diversion controllers…Jody


Ben Mendoza October 27, 2015 at 8:29 am

Hello, I planned on putting together a 24V battery bank to power my basement and work shop and run portable heater. I’ve seen the Permanent Magnet Alternator (PMA) that comes in 12, 24 and 48V rating. I was thinking of driving a 24V version to charge the battery bank I will use.

Can you tell me if these PMA will do the job? The PMA will be driven by a steam engine I built. Thank you.


Jody Graham October 29, 2015 at 1:22 pm

Hi Ben,

Using a PMA can be a good way to charge your battery bank. How many amps is the PMA designed to produce and at what RPM? How many horsepower is your steam engine and at what RPM? The PMA (load) must match the output of the steam engine. That is why we like using a Ford alternator for these kinds of projects as you can increase or decrease the field of the rotor (the spinning part with the magnetic field) using a rheostat. You cannot change the power of the magnets in a PMA. Some PMAs have a mechanism that will bring the magnets closer or farther from the stator (wire windings) which you can use to match the PMA to the output of the steam engine. The other option is use an MPPT charge controller. That would match any PMA with your steam engine. Hope I have not added more confusion. I would love to see some pics of your steam engine or videos. Congrats on making your own steam engine. Impressive. Jody