If your lawn mower won’t start, a dead battery is often the culprit. Before you spend money on a replacement, learning how to restore a lawn mower battery is a practical skill that can save you time and cash. Many batteries can be brought back to life with some simple maintenance and the right tools.
This guide walks you through the process step-by-step. We’ll cover everything from safety to testing, so you can get your mower running smoothly again.
How to Restore a Lawn Mower Battery
Restoring a battery isn’t always possible, but it’s worth a try for batteries that have been neglected or slightly sulfated. The key is understanding what type of battery you have and why it failed.
Understanding Your Lawn Mower Battery
Most riding mowers and zero-turn mowers use a 12-volt lead-acid battery. There are two main types:
- Flooded (Wet Cell) Batteries: These have removable caps so you can check and refill the electrolyte fluid. They are the most common type for restoration.
- Sealed (AGM or Gel) Batteries: These are maintenance-free and sealed. While you can attempt to charge them, they are harder to restore if deeply discharged.
The most common reason for failure is sulfation. This happens when a battery sits discharged for too long. Lead sulfate crystals form on the plates, preventing the battery from holding a charge.
Safety First: Essential Precautions
Working with batteries involves acid and flammable gases. Always prioritize safety:
- Work in a well-ventilated area, away from sparks or flames.
- Wear safety glasses and acid-resistant gloves.
- Remove all jewelry to prevent short circuits.
- Have baking soda and water nearby to neutralize any spilled acid.
- Never lean directly over the battery while working.
Tools and Materials You’ll Need
Gather these items before you begin:
- A digital multimeter
- A smart battery charger or a specialized desulfating charger
- Distilled water (for flooded batteries only)
- A wrench set to remove battery terminals
- A wire brush or terminal cleaner
- Baking soda
- A small funnel
Step-by-Step Restoration Process
Step 1: Remove and Inspect the Battery
First, disconnect the mower’s negative (black) cable, then the positive (red) cable. Carefully lift the battery out. Look for any obvious damage like cracks or bulges. If you see damage, the battery is not safe to restore and should be recycled.
Clean the terminals and case with a baking soda and water solution. This neutralizes acid and provides a clean surface to work on. Scrub the terminals with a wire brush until they’re shiny.
Step 2: Test the Battery Voltage
Use your multimeter to check the voltage. Set it to DC volts (20V range). Touch the red probe to the positive terminal and the black probe to the negative.
- A reading below 10.5 volts indicates a deeply discharged battery.
- A reading between 10.5 and 12.4 volts shows a partial charge.
- Around 12.6-12.7 volts means it’s fully charged.
If the voltage is zero, the battery may have an internal break and cannot be fixed.
Step 3: Check and Refill Fluid (Flooded Batteries Only)
If you have a flooded battery, carefully remove the cell caps. Check the fluid level in each cell. The plates should be completely covered by about ½ inch of fluid.
If it’s low, top it up only with distilled water. Do not use tap water, as minerals can damage the battery. Fill each cell evenly. Overfilling can cause the battery to leak acid when charging.
Step 4: Attempt a Slow Charge
Connect your smart charger. Attach the red clamp to the positive terminal and the black clamp to the negative. Set the charger to the slowest charging rate (often called a “trickle” or “maintenance” charge) for a 12-volt battery.
A slow charge is gentler and more effective for reviving a dead battery. It may take 12 to 24 hours. If your charger has a “repair” or “desulfate” mode, use that. Never leave a charging battery unattended for extended periods.
Step 5: The Epsom Salt Method (A Last Resort)
For severely sulfated flooded batteries, some gardeners try the Epsom salt method. This is not always recommended by pros, but it can sometimes work as a last-ditch effort.
- Fully charge the battery first.
- Carefully drain the existing electrolyte acid into a proper acid-safe container. You will need to neutralize and dispose of this properly at an auto parts store.
- Create a new electrolyte solution by dissolving 7-8 ounces of pure Epsom salt (magnesium sulfate) in warm distilled water.
- Use a funnel to pour the new solution into each battery cell.
- Recharge the battery slowly for up to 24 hours.
This method can sometimes dissolve sulfate crystals, but it doesn’t work everytime and can shorten overall battery life if done incorrectly.
Step 6: Test Under Load
After charging, let the battery rest for a few hours. Test the voltage again with the multimeter. It should read at or above 12.6 volts.
The real test is a load test. If you have a load tester, use it. Otherwise, reinstall the battery in your mower. Connect the positive cable first, then the negative. Try to start the engine. If it cranks strongly, the restoration was likely succesful.
Preventing Future Battery Problems
Regular maintenance is the best way to avoid needing to restore your battery again.
- Regular Charging: Use a float charger or maintainer during the off-season. This keeps the battery at optimal voltage.
- Clean Connections: Check terminals for corrosion a few times a year and clean them.
- Fluid Levels: For flooded batteries, check fluid levels every few months and top up with distilled water as needed.
- Full Storage: Always store your mower with a fully charged battery, especially over winter.
When to Give Up and Replace
Not every battery can be saved. It’s time to buy a new one if:
- The battery case is cracked, leaking, or bulging.
- It won’t hold a charge after a proper restoration attempt.
- The battery is more than 4-5 years old, as capacity naturally degrades.
- It reads zero volts, indicating an internal short or open circuit.
Recycle your old battery responsibly at an auto parts store or recycling center.
FAQ
Can you really fix a dead lawn mower battery?
Yes, you can often fix a dead lawn mower battery if it’s due to sulfation or low fluid levels. However, physically damaged or very old batteries usually need replacement.
What household items can I use to clean battery terminals?
A paste of baking soda and water is excellent for neutralizing and cleaning corrosion. A stiff wire brush or an old toothbrush works well for scrubbing.
How long does a restored battery last?
A successfully restored battery might last another season or two, but its overall lifespan and capacity will be reduced. Consider it a temporary fix.
Is it safe to jump-start a lawn mower battery?
You can jump-start a mower from a car battery, but ensure the car is not running. The higher alternator output from a running car can damage the mower’s electrical system. Connect jumper cables in the correct order: positive to positive, then negative to a ground on the mower’s frame.
Why does my mower battery keep dying?
Common reasons include a faulty charging system on the mower, a parasitic drain (something drawing power when off), old age, or simply leaving it discharged for too long. If a new or restored battery dies quickly, have your mower’s stator and voltage regulator checked.
Restoring your lawn mower battery is a satisfying project that can extend the life of your equipment. By following these steps carefully and prioritizing safety, you can often avoid an unnecessary trip to the store. Remember, consistent maintenance is the key to preventing most battery issues in the first place. Now you’re ready to get that mower back to work.