Dealing with a battery that doesn’t seem to be working quite right can be sort of confusing, and nobody wants to have to buy a new battery for their lawn mower if it can be avoided. Knowing how to chase down all the symptoms of a bad lawn mower battery definitely comes in handy when you find yourself with a battery that has died and are wondering if it can be revived or if a replacement is needed.
I’ll go through some of the methods that I’ve used to get to the bottom of this issue. Hopefully, you can follow along and be certain that you have a dead lawn mower battery before you shell out cash for a new one.
What You’ll See if Your Lawn Mower Battery is Dead (The Short Answer)
The telltale sign of a lawn mower battery that’s dead is failure to charge fully or hold a charge. Usually, small engine battery problems reveal themselves when you try to turn the key and get the motor started. If the engine is having a hard time turning over or is clicking, your battery probably doesn’t have enough of a charge, and might be dead.
5 Symptoms of a Bad Lawn Mower Battery
Batteries require a decent amount of attention and care in order to function for as long as possible. If you leave batteries out in the cold, undercharged, or even on the battery charger for too long, the internal components of the battery can be affected.
Batteries that are struggling to stay alive will usually: have a hard time starting the engine, have low fluid levels, be bulging or overly corroded, show low voltage, have a hard time charging completely, or will be unable to hold a charge altogether. I’ll go into a bit more detail about these symptoms of a bad lawn mower battery below.
Inability to Crank or Start Engine
The number one sign of potential battery issues is definitely trouble at the point of ignition. A lawn mower battery that’s dead won’t be able to provide enough power to generate the spark that gets the engine going.
When this happens you will notice that your engine is either cranking super slowly or not at all. You might also notice a distinct clicking sound when you try to use the ignition.
Though this is a typical sign that your lawn mower has a dead battery, you can’t conclude that the battery is to blame quite yet. Problems with your alternator, spark plug, or starter can also result in an inability to crank the engine.
Improper Fluid Levels
If you suspect that your lawn mower battery is dead, a good first thing to do is check the fluid levels in each of the cells. Lead-acid batteries use an electrolyte mixture of sulfuric acid and distilled water.
Over time, the water level in this mixture can lower. This can be attributed to a bunch of different things whether it’s overcharging, leaking, or even just old age (read here about how long lawn mower batteries last typically). But, if the cells in your battery don’t have enough fluid, the battery won’t function.
This is where knowing how to restore a lawn mower battery comes in handy. In some cases, you can add more distilled water to your battery and get it working as it should again. If you’ve got low fluid levels and some free time, get a pair of gloves and some distilled water and give this a shot.
Bulging, Damage, or Excessive Corrosion
External damage to a lead-acid battery is usually pretty easy to notice. Most commonly you will see bulging, cracked components, or a buildup of blue-green corrosion on the battery’s terminals.
While corrosion is normal and pretty easy to clean up, swelling and structural damage to a battery is a more serious issue. You don’t want to end up with a battery that leaks all over or ends up exploding from excess pressure. If you’re worried about the integrity of your lawn mower’s battery, swap it out.
Low Voltage Readings
After going through all of the other symptoms of a bad lawn mower battery, doing a voltage test will tell you definitively if the battery is dead or alive. A healthy 12-volt battery with a full charge will read between 12.6 to 12.8 volts. If you have a 12-volt battery that is completely discharged (dead) it will read 12.0 volts or lower.
The same is true for 6-volt batteries. A healthy 6-volt battery will produce a reading of just over 6 volts. Anything 5 volts or lower means you have a dead battery in this case.
But, don’t throw in the towel quite yet if your battery shows a bad reading. Trying to recharge your battery is the first thing to do. Just make sure you use the correct charger and don’t accidentally overcharge the battery.
Can’t Charge Fully or Stay Charged
If you connect your charger and your battery won’t charge completely, you might be in trouble. This is also the case for a battery that will charge fully but begins to lose charge right away without any use.
If, after checking the fluid levels, looking for external damage, running a voltage test, and recharging the battery you still have problems, it is usually safe to say that your lawn mower battery is completely dead. I’ll describe how to run this quick test in detail below.
An Easy Test to Quickly Diagnose a Dead Lawn Mower Battery
In my opinion, the easiest way to diagnose the state of your battery is to check its voltage, try to charge it fully, and then monitor its voltage again. All you really need to get this done is a battery charger and a multimeter.
Though some folks might not have these tools already, they come in handy all the time and I’d recommend investing in both. Here is a step by step guide to follow if you want to run a quick battery health test and end up with a trustworthy diagnosis:
1. Assess battery and measure voltage.
If you’ve run through the symptoms of a bad lawn mower battery that I listed in the previous section, you’ve probably already done this step. But, if not, go ahead and make sure that the fluid in each cell is filled to the fill line. Then, check the exterior of the battery for damage or corrosion before using your multimeter to read the voltage.
To take a voltage reading for a 12-volt battery just set the multimeter to the DC or A setting at 13 volts, put the red lead on the positive battery terminal, and the black lead on the negative terminal. Your multimeter should then display the voltage of your battery. If you get a reading of less than 12.6 volts, go on to step two.
Follow the same process if you have a 6-volt battery but look for a reading of at least 6 volts.
2. Charge the battery fully.
If your battery isn’t fully charged (which is quite likely) connect your battery to a charger and allow it to recharge completely. Wondering, how long will it take to charge, and whether you can overcharge a lawn mower battery? Well, it really depends on what kind of battery charger you’re using, and yes it’s possible to overcharge and ruin batteries.
I recommend using a low voltage or trickle charger over a longer period of time to charge the battery fully.
3. Test voltage again.
Once you have charged your battery according to the time recommended with whatever type of charger you’re using, test the voltage again. You’ll be looking for a voltage that shows the battery is fully charged.
If you get a good reading, it’s a good sign. Now you know that your battery is able to recharge completely.
4. Rest the battery overnight and measure voltage once more.
The final step is to make sure that your battery can hold a charge. After leaving your battery to sit without use for over 8 hours, check the voltage again. If you have a healthy battery, the voltage should stay the same.
If you get a reading that shows your voltage has lowered since charging, something is off with the battery. At this point, if you’re unable to charge your battery completely, or your mower’s battery drains and won’t hold a charge, your best bet is to replace it.
If your battery is unfortunately done for, you might find the post I’ve linked to below useful to learn what you should do with your old battery (please don’t just throw it in the trash).