How Fast Will a Generator Charge a Battery? 

Generators are reliable sources of electricity, but if you want to store some electrical energy at a remote location, you will want to use batteries in tandem with generators. How fast a generator will charge a battery depends on several factors, such as the generator’s power output, battery size, and battery charge speed.

High-wattage generators will charge batteries more quickly than low-wattage generators. A 3500-watt generator can charge most batteries in four to six hours. You could consider a higher wattage generator if you need to charge a battery faster. 

Charging batteries with a generator can be a dangerous proposition. Before doing so, you should know:

  • Does a generator charge its battery
  • How do I keep my generator battery charged
  • How often should I charge my generator battery
  • How long does it take to charge a deep cycle battery with a generator
  • Can you charge a car battery from a generator
  • Charging boat batteries with a generator

After going through these topics, you can confidently do your battery maintenance and charging.

Generator plugged in to a battery

Does a Generator Charge its Own Battery?

Some generators with easy-maintenance batteries can charge themselves while the generator runs.

Charge your generator’s battery every month to keep it from dying. Charging your generator battery every month is a good habit if your generator spends most of its time in storage. If you regularly use your generator, it is easier to keep the battery alive.

When Generators Charge Batteries

  • Generators with sealed batteries in regular use only need to be charged every couple of months.
  • Manual charging with an external power source is the only option if you don’t have a sealed battery.
  • It usually takes about five to seven hours for a generator to charge its battery. For many generator owners, external charging is the quicker, easier option.
Car battery connected to cables

How Do I Keep My Generator’s Battery Charged?

Keeping a generator’s battery charged requires one of two actions. Either you run your generator a few times a month to let it self-charge, or you charge your generator’s battery with an external power source.

Whichever action you take, you should keep your generator’s battery level close to 100%.

Some generators, especially higher-end models, come with a battery health indicator. Consult your product user manual to find the location of your generator’s battery readout.

How Often Should I Charge My Generator Battery?

Generators sitting in storage for prolonged periods should have their batteries charged monthly. Generators in regular use can get away with less frequent charging. You can get away with charging the batteries every two to three months. 

Non-sealed batteries need charging more frequently.

As generator batteries age, they will require more frequent charging and will be more prone to dying. Aim to change your generator battery every two to three years.

How Long Does it Take to Charge a Deep Cycle Battery with a Generator?

Deep cycle batteries are longer-lasting than traditional batteries. Additional energy storage is ideal for long remote trips or powering large machines or equipment. But the convenience of deep cycle batteries comes with a caveat: they take longer to charge.

If your deep cycle battery is in regular use, it should easily accept a charge and have a full charge in a few hours.

How many hours your deep cycle battery takes to charge depends on how dead it is. Batteries that are mostly full will only take an hour or two to top off. In contrast, depleted batteries may take as long as ten hours or more to charge.

You can also purchase a battery charger to plug into your generator’s AC outlet to increase charging efficiency.

How Do I Know If I Have a Deep Cycle Battery?

All batteries come with a manufacturer’s label that includes information about the battery’s make and model. If the labels have become worn or unreadable, look around for a model or serial number printer embossed into the battery case.

After finding the brand and number, look it up online to find your battery specifications.

Battery being charged

Can You Charge a Car Battery From a Generator?

Yes, you can charge car batteries with a generator, provided your generator produces direct current (DC). Most generators are made to output alternating current (AC) because that’s what modern household appliances run on, but DC generators exist.

If your unit produces DC, you’re in luck – you can hook up your generator directly to your car battery.

What To Consider

  • It may take up to an hour or more for your car battery to receive enough charge to turn the alternator over.
  • If your generator produces AC, you can still charge your car battery with your generator. You just need a currency converter or battery charger.

Charging Boat Batteries With a Generator

You can charge boat batteries with a generator. Most boat batteries are deep cycle batteries and take longer to charge with a generator than standard batteries. 

Unless your generator produces direct current, you will likely need a battery charger for your generator to charge your boat’s batteries. 

Consider purchasing a charge control device to prevent overcharging your boat batteries even if your generator produces DC.

Car battery terminals one covered and one open connected

Battery and Generator FAQ

Here are some commonly asked questions.

What size generator do I need to charge a battery?

You will need a generator capable of producing at least 3500 watts over several hours to charge most twelve-volt batteries. As a general rule of thumb, your generator should be able to produce twice the amount of wattage you need.

How long does a 2000-watt generator take to charge a battery?

If your battery charge is at 50% or greater, it will take anywhere from four to six hours to top off your battery. If your battery is older, it will take longer to charge. If your battery charge is below 50%, it can take anywhere from six to twelve hours to recharge.

How long does it take to charge a 100ah battery with a generator? 

It takes six to eight hours to half charge a 100ah battery with a generator of at least 3500 watts. It can take thirty-six hours to recharge a dead 100ah battery fully.

Most 100ah batteries are lead-acid, meaning they can only accept so much charge simultaneously. Since lead-acid batteries limit the amount of charge, they can accept, using a generator with a higher wattage output will not charge your battery faster.

How long does it take to charge a trailer battery with a generator? 

Charging most trailer batteries with a 3500-watt generator will take four to eight hours. Expect charging to take longer for batteries whose charge has been severely depleted.

How does a generator charge its own battery? 

A generator charges its battery through routine use. Many generators will recharge their batteries through normal operation, but some models require manual charging, which can be done with conventional outlets.

How often should I charge my generator battery?

If your generator spends most of its time in storage, you should charge it monthly until the battery is full. If your generator sees regular use, you can get away with charging your generator battery every couple of months. 


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