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Lawnmower Battery Keeps Draining? Most Common Causes & Fixes

Lawnmower Battery Keeps Draining? Most Common Causes & Fixes

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Paul Brown

I’ve battled draining lawnmower batteries off and on for years. Sometimes it’s a simple issue while other times the cause is more complex. If you are having this issue I want to help you understand what’s happening and how to prevent it.

A draining lawnmower battery can be caused by corroded battery posts and voltage regulator problems are common reasons, as well as a low charge, charging over its capacity, or having worn-out components. By catching these problems early, you can save time and money.

This article contains information on how different forms of battery drainage occur in lawnmowers. We’ll cover some testing procedures to help you diagnose the issue, as well as a chart showing the pros and cons of different battery choices if a replacement is needed. Proper maintenance can also extend your battery’s lifespan, so we’ll cover that as well.

Corroded Battery Post & Voltage Regulation

Corroded Battery Post: What To Look Out For

When a battery inside the lawnmower ages, it produces a bright teal or bluish powdery substance on the positive terminal which is a sign of corrosion. It is caused by the battery’s acid coming into contact with hydrogen gas.

When you purchase a new battery for the lawnmower, it’s a good idea to write down a reminder to purchase a new battery in 3 years. This way you can purchase a battery at the time the old battery expires.

How To Maintain

Both positive and negative terminals will need to be cleaned by using a wire brush. Battery terminal cleaner is the best cleaning solution to apply in order to keep the battery’s acid neutralized.

There is also the baking soda method where you can make your own terminal cleaning solution with water and a teaspoon of baking soda.

The following video demonstrates how to clean the terminals using this method:

How To Clean Your Battery Terminals with Only Baking Soda & Hot Water

You should clean your battery terminals monthly as a precaution. It’s easier to clean them regularly rather than let the build-up get worse.

When it comes time to replace the battery, clean the terminals the same way previously mentioned and reinstall the new battery. Check to see if the battery is functioning normally after installing to see if there are problems.

Below is a video on how to inspect and clean battery terminals:

Voltage Regulation Problems: What To Look Out For

Some Signs Of Irregular Voltage Include:

  • Dim lights
  • Engine stalling/sputtering
  • Instrument cluster isn’t providing power
  • No reading appearing on the voltage
  • Voltage level dial is jumping all over


How To Test:

You can test the voltage by measuring the battery voltage when the engine is on and off. The regulator should on average stay at 12V but a range of 13.8-14.5 voltage is acceptable too.

To begin the test, move the lawnmower onto your driveway and put it in park for a proper working space. Then turn on the engine and headlights. With work gloves, look at the voltage regulator (as referenced in the machine’s manual) where the recharge battery station is.

Turn the multimeter setting dial to voltage (∆V symbol represents voltage). See if the battery voltage reads 12 volts by taking the red lead and fastening it to the battery’s positive terminal. The black lead will then be placed on the negative.

If the voltage doesn’t show a range between 13.8-14.5 or the lights are dim, the voltage regulation isn’t running through. If it is less than 13.8, the battery needs to be recharged. If over 14.5 the battery has gone bad already or will be going down shortly.

Other Potential Problems: Your voltage can also provide no reading at all, which can either be caused by defective wires or a battery that isn’t getting the voltage. The voltage itself can also jump around to different levels on the multimeter instead of staying on 22 VAC.

How To Fix:

If you spot a problem during this test, your regulator will need to be replaced once it dies. If problems persist even when purchasing a new regulator, contact the store for further instruction.

Low Power Capacity & Electrical Drainage

Low Power Capacity: What To Look For

The amount of horsepower in small engines for lawn mowers should on average have a horsepower of 550 ft pounds per second. In wattage that is about 745.7. When purchasing the lawnmower you should note the torque ratings to calculate how much power your lawnmower will produce under normal circumstances.

How To Test

When checking to see if the power capacity is the same when purchased, use this formula: Torque Speed divided by 5252 equals the horsepower.

Example of a lawnmower engine type with the formula:

1450 Series engine (a lawn mower) has a 14.5 torque. When we apply the formula we get 14.5 x 3600 RPM =522000. Then divide by 5252 which brings the horsepower to 9.9390708.

How To Fix: Getting More Horsepower

First, remove the flywheel housing located at the top of the lawn mower’s engine. Then unscrew the bolts from the top of the engine from the governor flap. Unhook the springs from the flap that connects to the carburetor and replace the flywheel on top of the engine.

Look inside and check the muffler for any cracks or misplacement. If these signs are present the muffler will need to be replaced. Horsepower isn’t able to perform properly if the engine’s muffler isn’t tightened down. If you attempt to tighten the muffler and it still loosens after checking at a later time, it may need to be replaced.

You can also replace the gas with a gas containing higher levels of octane. Airflow also produces higher horsepower so replacing it monthly will help the engine perform better overall.

Electrical Drainage: What To Look Out For

A standard lawnmower engine should show that it has 6 or 12 electric volts. There is a specific kind of electrical drainage in lawn mower engines called parasitic electrical drainage. This is when the lawnmower engine is off and yet the electrical system drains the battery.

How To Test

Check how much the electric current draws after turning off the engine. Next, take off the positive cable from the battery. Then attach an ammeter between the positive terminal and battery cable. The ammeter can show if there is 1 milliamp or more. If this happens, the electrical system is experiencing parasitic electrical drainage.

How To Fix

By using an alternator, it gives voltage to a running engine while also recharging the battery. If this doesn’t work, try a multimeter to check the voltage output in the alternator. This is done by using a wiring diagram that should come with the package.

Your charger could also turn out to be useless and you can use the multimeter to check if it is. Though the defective charge could also be from a bad outlet so check before purchasing a replacement.

Check the voltage regulator to see if the lawnmower engine is getting the amount of electricity needed to run. This can be done while the engine is running. This causes small engine batteries to run out if the voltage regulator isn’t functioning properly. Source

Below is a video on how to check the voltage regulator on lawnmower engines:


Low RPM Inside Engine & The Fuse

What To Look Out For: Low RPM Inside The Engine

Does the engine have low RPM (revolutions per minute)? If it does, it is because the spinning flywheel is unable to create an electric current through the magnets. If this occurs then the battery doesn’t charge at all.

How To Test

Try putting the engine into idle then tightening the throttle cable. Moving the governor and throttle cables at different engine speeds might help the battery run better.

How To Fix

If the battery doesn’t run at the engine speed according to the owner’s manual, it will need to be replaced.

What To Look Out For: Fuse

Behind the dash near the battery, the fuse is under the rear fender. You won’t know if it is blown unless it is pulled out for inspection. A good fuse will have a clear casing with the continual line attached in an upside-down “U” shape.

A blown fuse will have the continual line snapped apart inside the now blurry casing. Electrical currents cannot travel through the fuse if the continual line isn’t attached therefore the battery doesn’t charge or run properly.

How To Test

The following video is a demonstration of how to test a small engine fuse.

Blown Fuse? How to Test Your Fuses.

How To Fix

A blown fuse will need to be replaced and the new fuse should be tested once installed in the lawnmower.

Try Different Lawn Mower Batteries

Is Your Battery A 6-Vot Or 12-Volt?

You might also have luck by changing out the current battery type for a new one. Lawnmower batteries can hold power in 6-volt and 12-volt models. Both battery types hold DC electrical energy but the 6-volt holds 6.3 DC and the 12-volt holds 12.6. You can also tell the difference in their caps as 6-volt has 3 and 12-volt has 6. Source

Lawnmowers built before 1980 will have a 6-volt battery and can easily be damaged when it is left to overcharge. If the battery has 10amps or more it will not function properly if overcharged. To avoid this, set a timer so you know when to unplug the charger from the lawnmower or buy a different charger that monitors the battery. Both options help to prevent overcharging which can cause the battery to stop working.

When shopping for new batteries, look for one that has 36-40 volts and high amp-hours. You’ll want both to help aid in long yard work hours. The battery will also last longer if the material is top-notch. Examples of strong materials are metal plating and aluminum. The sizing of the battery is important because if the size doesn’t fit well with your lawnmower, it can detach. Look for U1 and U1R and see if they match the terminal position from your older battery.

To help give you a broader outlook on different battery types and their features, I’ve provided a list of the top 6 batteries available. Although these are good products, there are also features that could be problematic so I’ve made sure to list those as well.

Top 6 Lawn Mower Batteries: Pros & Cons

  1. Weize 12V 35 AH Recharable Battery
Has SLA/AGM Technology
(can be applied with other equipment)
Battery loss occurs from quick overcharge
Rechargeable Deep Cycle feature
(can be used several cycles)
Needs to be checked for regular charging
Provides 30-day return policy and 1-year warranty
Provides handles for safer storage

2. Expert Power EXP12180 Battery-12-volt

Has non-conductive ABS plastic casingBattery not applicable with other equipment
Easy to install into lawn mower and is long-lastingRecharge battery regularly after each use
Uses AGM technology and is surge resistant
Prohibits spillage to provide safe operation

3. Mighty Max Battery ML35-12SLA Battery-12 volt

No need for wire harnessing or mounting The battery has to be charging 24/48 with no interruption
30-day refund
Deep discharge recovery
Has a deep cycle solar battery
(better than 33 Ah, 34 Ah, and 36 Ah battery types)

4. Expert Power EXP12200 Battery-12-volt

Has a rechargeable battery providing threaded terminalsIf fully used, it will need 24 hours to recharge
Doesn’t require maintenance
1-year warranty with a 90-day refund if issues are presented
User-friendly and lightweight

5. Chrome Battery Deep Cycle SLA Rechargeable Lawn Tractor Battery-12-volt

Has AGM technologyCharges slower than most batteries
Comes fully charge after purchased
Has higher amp hours with minimal maintenance
Can replace a Husqvarna YTH2448 battery

6. Universal Power Group 12V 35 Ah UB12350 Battery

Provides SLA and AGM technologyNeeds to be at full charge for each use
Has handles for placing with no maintenanceCan’t charge older models of John Deere products


Maintenance Tips For Lawn Mower Care

Do ThisDo Not Do This
Keep battery charged on a set scheduleDo not keep your battery charging all the time. This leads to sulfation.
Keep at full charge before storing in dry shedsDo not undercharge your battery because this can lead to sulfation as well.
Clean the battery often, get a fuel filter, damaged blades, or new tires as needed (or annually) Do not fail to replace damaged power leads because these can cause burn outs.
Replenish water levels under lawn mower capsDo not place a mower on its side when oil is refilling. It’s best to use an oil drain plug
Spray For Service SF fogging oil inside carburetor and spark plugs. Other good brands are SG, SJ, and SH.Never place your mower next to hazardous appliances or where moisture can get inside.
Clean out wet clippings by mowing a few dry patches of grassDo not mow your lawn if it is wet from recent watering.
Source 1, Source 2

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