Generator Battery

Do you own a generator?

If so, you know that having a backup power source is crucial in case of an emergency. But did you know that the type of battery in your generator is important? There are many factors to consider when choosing a generator battery, and our experts are here to help.

We understand that it can be difficult to make a decision on something as important as your backup power source. That’s why we’ve created this guide – so you can feel confident in your purchase and know that you’re getting the best product for your needs.

Read our guide on generator batteries till the very end now!

The Function of a Battery in a Generator?

As you may be aware, batteries work by double conversion of energy through electrochemical processes. This involves the conversion of electrical energy to chemical energy during charging and from chemical energy to electrical during the discharge process. This is achieved by the use of two electrodes of different metals and a liquid electrolyte that acts as an ionic medium connecting both the electrodes and allowing the transfer of electrons between them.

In a portable generator, the battery carries out the following main functions:

  1. To provide power to the starter motors of the generators equipped with an electric start feature.
  2. To power the generator control panel, engine controls like automatic choke, oil level, and pressure, idle control, solid-state ignition, etc.

Most large generators have an additional 12V stator winding in the main alternator with a full wave bridge rectifier and a battery with an engine block and frame acting as the ground return. The circuit for this arrangement is shown below.


In smaller engines, the power to the engine controls and the battery is sometimes provided by the flywheel alternator. Permanent magnets are placed or cast into the flywheel rim. These magnets get linked with the stator windings of the flywheel alternator to generate an intermittent output as the alternator usually has only a single pair of windings. Usually, a capacitor acts as the voltage regulator for the system. The diagram for this arrangement is given below.


A full or half wave rectifier converts this output to DC. At 3600 rpm engine speed, you can expect 13.5 V DC at the battery terminals. Otherwise, you must carry out the rectifier’s multimeter resistance tests.

Powering Starter Motors

The power arrangement for battery charging has been discussed in the previous section. When the key to start the engine is turned slightly to the “ON” position, the engine controls receive power supply from the battery. When you turn the ignition key to the “start position,” a solenoid closes the contact that actuates a plunger which pushes out the pinion against a return spring pressure, making it engage with the flywheel gear.

This movement of the plunger closes a contact that connects the battery power to the starter motor. As discussed later, the starter motor requires a burst of power from the battery to crank start the engine. This is because the motor is designed to generate high torque at low speed or low torque at high speed, depending on whether the system uses a direct drive or gear reduction system.

The gear reduction arrangement differs from the direct one in that it has one or more gears between the starter motor shaft and the engine, with different gear reduction ratios to generate sufficiently high torque to crank the engine. A direct-driven starter motor will be much larger in size.

Once the engine picks up speed, there is a chance that the flywheel will start driving the starter motor. At a flywheel speed of about 300 rpm, with a gear ratio of 15:1, the starter motor speed will be 4500 rpm. The drive mechanism should never be allowed to drive the starter motor. To avoid this, a one-way clutch, called the overrunning clutch, is deployed between the pinion and the starter motor shaft and allows engagement in one direction only, slipping in the other to disengage the starter motor.

The below video explains the entire process with animation.

Powering Engine Controls

As shown in one of the above diagrams, the battery provides power to various engine controls like automatic choke, oil level and pressure, idle control, solid-state ignition, etc.

What type of battery is in a generator?

Most generators use a standard lead acid battery for their functioning. The lead acid battery has been around for over 100 years, and its applications continue to grow. They are finding use in electrical and hybrid vehicles to provide the required energy for vehicle starting, lighting, and engine ignition. They are available in sizes ranging from a few AH (ampere-hours) to 10,000 AH ratings.

Common advantages of Lead Acid Batteries

Their high electrical turnaround efficiency of about 75 to 80% makes them an ideal choice for these applications. They are available in all sizes and designs around the world at a very low cost. These batteries have reasonably good low and high-temperature performance. A lead acid cell has an open circuit voltage of 2V, which is higher than any other type of battery. The individual cell components can be recycled.

Common disadvantages of Lead Acid Batteries

On the flip side, they have a low cycle life, which we will discuss later. In addition, they have a low energy density, are difficult to manufacture in very small sizes, emit hydrogen gas which is a very explosive gas, are prone to thermal runaway if the batteries all the chargers are not properly designed, and they can’t be stored in discharge condition for long due to sulfation or irreversible polarization of electrodes.

Different Types of Lead Acid Batteries.

Lead acid batteries are commonly available in two forms – the conventional and the maintenance-free design. The latest maintenance-free versions’ latest designs are also called Valve-regulated lead acid batteries (VRLA). You don’t need to add/replace any electrolyte to these types of batteries.

The conventional design, sometimes referred to as the flooded type or ventilated lead-acid (VLA) design, contains large quantities of electrolytes. They have a small ventilated access by means of renewable plugs that allows you to measure the specific gravity and the state of charge of the cells. They emit large amounts of gasses, hence must be placed in a vented area with adequate care for circulation. In addition, the internal reactions reduce water levels in the battery. Hence distilled water has to be periodically added.

But, VRLA changed that by carrying only a limited amount of electrolyte in the form of a gel or absorbed in a separator (absorbent glass mat). The internal reactions maintain an equilibrium, such that the electrolyte is consumed and regenerated without actually consuming it unless you disturb the equilibrium by charging the batteries at higher voltages.

In case of excess pressure build-up in the cells of the set value, the valve opens and releases the internal gasses.

Ignition Batteries

Ignition batteries are a category within the types discussed above, with the difference that they are specifically designed for automotive or similar applications.

These types of batteries deliver a large value of current in a short amount of time (as required during the starting of an engine), as against those suited for long-duration low current applications.

Temperature and Lead Acid Batteries

The temperature has a huge impact on the performance of the lead acid batteries. High temperatures enhance the chemical reaction rate, increasing the battery’s instantaneous capacity but reducing its life. The lead acid batteries are designed to provide their full capacity at 25°C. Every 10°C increase in the operating temperature reduces the life span by half.

Reduction in outside temperature during winter reduces its capacity. A lead acid battery can provide only 65% of its nominal capacity at -20°C. The batteries should be charged at higher voltage and the charging current at lower temperatures. Similarly, you need to reduce the charging current (C-10) to 75% at higher temperatures.

How long does a generator battery last?

Many manufacturers specify the life of their batteries in a number of years. These are very misleading statements as what is usually specified is the design life, which is under the lab conditions where all the required parameters are maintained at optimal values, and the battery is just subjected to float charge without any load.

In actual practice, the life of the battery mainly depends on the depth of discharge (DOD) and the number of cycles the batteries are subjected to with this DOD value.

Consider a battery charged at 100%. It is then connected to a load and discharges to 20% of its nominal charge. The depth of discharge is then said to be 80%. The more DOD a battery encounters in service, the lesser the number of such cycles it can withstand and the lower its life span will be. One cycle constitutes a battery discharging to a particular DOD value and then getting charged back to its 100% capacity.

A typical battery can withstand 5000 cycles at 10% DOD, 2500 cycles at 20% DOD, 1200 cycles at 50% DOD, and 700 cycles at 80% DOD. If the battery is subjected to two 80% DOD cycles a day, its lifespan will be only one year. The presence of excessive temperatures can reduce it further.

Will the generator start without a battery?

The small engines can easily start with the rewind rope without any need for a battery. In fact, many small writing portable generators are sold as such. The main reasons and benefits for including the electric start technology is

  1. As a matter of convenience in small-size generators, if you can afford it.
  2. Some of the models are sold with the electric start feature only. You cannot start them without a battery.
  3. As the engine size increases, it becomes difficult for an average person to turn over the engine sufficiently fast to start it.
  4. Diesel generators have a much higher compression ratio than other generators, as they work without the spark plugs. This makes it even harder to rotate them and start.
  5. Batteries may be required to perform other complex control functions, so it is better to use them for starting.
  6. Larger portable generators and standby generators, equipped with an automatic transfer switch, are started automatically, necessitating the presence of the battery.

How do I choose a generator battery?

To begin with, thoroughly review your owner’s manual and note down the specifications, model number, and the Ampere Hour rating of the already installed old battery that came with your generator set.

Factors To Consider When Choosing A Generator Battery For Replacement

If, for some reason, you are missing these details or the prescribed battery is no longer available, you can follow the below recommendations:

  1. Ensure that the battery is fit for the application, giving specific attention to ambient air temperatures, starting current demands, and steady power requirements to power the control circuitry. The owner’s manuals and specifications for the majority of the generator sets are available online. Ensure that you follow those specifications.
  2. You should select a rechargeable type of battery.
  3. Most generator applications are easily fulfilled by flooded-type batteries unless you are installing them in an enclosed space where hydrogen emission can be dangerous.
  4. What warranty terms are offered by the sellers?
  5. Generac Generator’s website states that any standard car battery at auto part stores is suitable for their products.
  6. If you own a Briggs or Generac unit, you may select a 26R flooded type battery that provides 51 AH capacity with around 525 cold starting amperes.
  7. If your battery has a separate charger, instead of being charged by the alternator output, you must choose the AH rating, considering the expected outage times, multiple starts due to refueling, and expected running hours.

Does a generator charge its own battery?

Depending on the manufacturer and the model of the generator set, the generator may or may not charge its own battery. Most small-engine portable generators do not charge the batteries while running. However, many large portable generators (like GP 15000 and GP 17000) do charge their batteries during operation. You need to check your owner’s manual for your specific model.

How do I keep my generator battery charged?

Many manufacturers like Generac supply a separate external charger for the purpose and recommend you to charge your batteries at least once a month for about 48 hours. You may have to go for a third-party charger if you did not receive one with your machine.

In emergency situations like a power outage, if your battery is completely drained, you may start the generator with a recoil and plug your charger into the receptacles. However, this must be done in extreme emergencies and not as a regular practice.

Are Battery Failures the Most Common Reason For Generator Failure?

While this is a general perception that a lot of users have, my experience, having dealt with more than 50,000 batteries in my 30-year career, is different. If we take care of the batteries as per the laid down guidelines, they provide the intended performance. However, they are sensitive pieces of equipment that may start failing in the absence of adequate care.

You must check and test the batteries at regular intervals, including the load test as specified.

As far as the generators are concerned, other than the battery, there are four other sources in the starter circuit that can malfunction.

  1. The starter switch,
  2. The wiring between the starter switch and the solenoid.
  3. The solenoid, and
  4. The motor

To check the possible reason for your starting problem, carry out the following steps

  1. Trace the wire running from the starter switch to the solenoid. Ask someone to hold the starter switch in turned “ON” position. Check the voltage to the ground at the solenoid connection, starter switch, and battery.
  2. To test the solenoid, put a jumper cable on the battery-positive terminal. Touch the other end of the cable to the large solenoid terminal located near the starter motor. If the motor responds, the solenoid has failed; otherwise, the motor has failed. Usually, the shaft bushings and the starter brushes are more prone to failure.

How do you know if a generator battery is bad?

The flooded-type lead acid batteries contain sulphuric acid and emit hydrogen gas which is highly inflammable. Before working on these batteries, you must take the following safety precautions:

  1. Wear splash-proof polycarbonate goggles.
  2. Remove wristwatch, rings, or any other pieces of jewelry.
  3. Wear plastic or rubber gloves.
  4. Disconnect the negative cable before disconnecting the positive one. Reconnect in reverse order.

Checking the Battery

Start by cleaning the battery terminals. With a digital voltmeter, measure the battery terminal voltage. Batteries may develop a high surface voltage that can mislead you to believe that your battery is healthy, while it may not have any cranking power. Hence, it is always advisable to measure the terminal voltage after engaging the starter couple of times and at least 15 minutes after disconnecting the charger. The table below shows the relation between the measured voltage and the state of charge.

100% 12.7 – 12.9V

75% 12.4 – 12.5V

50% 12.1 – 12.2V

Discharged 12.0 or less

Why does my generator battery keep dying?

If your battery is directly charged by the generator, there could be problems with the voltage regulator or the alternator, whereby voltage levels are not adequate to charge the battery. If you are using a separate charger, you need to check that it is functioning properly. If no issues are detected in the charging system, check the condition of the battery by carrying out the load test.

How often should you start your generator?

Most manufacturers recommend that you should start your engine once a month and operate it for about half an hour before shutting it off and storing it. Maintaining and charging the battery is an independent exercise if your generator comes with a separate charger.


A generator battery is a lead acid battery that is used to start and run a generator. The most common reason for generator failure is a bad battery. Generator batteries should be trickle charged when not in use to prevent them from dying. You should also start your generator at least once every two weeks to keep the battery charged.

Leave a Comment