Lawn mower batteries typically range from 20 to 40 amp-hours, depending on the mower type and engine size
The amp-hour rating you need depends on whether you have a cordless electric mower or a gas mower that uses a battery for starting. Cordless mowers — where the battery powers the entire cutting deck — usually need 40 to 80 amp-hours to run for a full yard. Gas mowers that use a battery only to start the engine need much less: usually 20 to 35 amp-hours is enough, because the battery only has to crank the starter motor for a few seconds.
The voltage matters as much as the amp-hours. Most cordless mowers run on 40 to 80 volts, while traditional gas mowers use 12 volts. A 12-volt battery rated at 30 amp-hours stores less total energy than a 40-volt battery at 40 amp-hours, even though the amp-hour numbers look similar. When you are shopping for a replacement, match both the voltage and the amp-hour rating to your mower's original battery — they are not interchangeable.
Key Takeaways
- Cordless electric mowers need 40 to 80 amp-hours at 40 to 80 volts to cut a typical residential yard on one charge.
- Gas mowers with electric starters need only 20 to 35 amp-hours at 12 volts, because the battery only powers the starter motor.
- Voltage and amp-hours work together to determine how much energy a battery holds, so you cannot swap a 12-volt battery for a 40-volt one just because the amp-hour number is similar.
- Your mower's manual or the original battery label will show the exact voltage and amp-hour rating you need for a replacement.
Cordless mower batteries and runtime
If you own a cordless electric mower, the battery amp-hour rating directly affects how long you can mow before the battery dies. A 40-amp-hour battery at 40 volts will run longer than a 20-amp-hour battery at the same voltage. Most residential yards take 30 to 60 minutes to cut, so a 40 to 60 amp-hour battery is usually enough for one session without needing a second battery.
Brands like DeWalt, Makita, and Ryobi make cordless mowers that use their own battery systems. A DeWalt 40-volt mower might use a 5.0 amp-hour battery, while a Ryobi 80-volt model could use a 6.0 amp-hour battery. The smaller number here refers to the battery's capacity in that specific system; the voltage is what matters most for power. Higher voltage batteries deliver more cutting force and run faster, which means they drain quicker on thick grass but finish the job sooner.
If you mow a large property or want to cut without stopping, buy a second battery so one charges while you use the other. This costs less than buying a larger single battery and gives you more flexibility.
Gas mower starter batteries and cold-weather performance
A gas mower's 12-volt battery only needs to turn the engine over for a second or two. A 20 to 30 amp-hour battery is standard for most residential gas mowers. The battery does not power the cutting blade or the engine itself — once the engine starts, the alternator takes over and keeps the battery charged while you mow.
Cold weather reduces a battery's ability to deliver current. In winter, a 12-volt battery that works fine in summer may struggle to crank a cold engine. If you store your mower outdoors or in an unheated shed, a slightly larger battery (30 to 35 amp-hours) can help it start reliably when temperatures drop. You can also remove the battery and store it indoors during off-season months to prevent damage from freezing.
If your gas mower has trouble starting even with a new battery, the problem is usually not the amp-hour rating but the battery's cold cranking amps (CCA) rating. CCA measures how much current the battery can deliver at 0°F. A battery with a higher CCA rating will crank a cold engine more reliably, even if the amp-hour capacity is the same.
How to find your mower's battery specifications
The easiest way to know what battery you need is to check your mower's manual or look at the label on the old battery itself. The label will show voltage (usually 12V, 40V, or 80V) and amp-hours (written as Ah). Write down both numbers before you shop for a replacement.
If you cannot find the manual, search the manufacturer's website using your mower's model number. The model number is usually stamped on the frame or printed on a sticker near the engine. Most manufacturers list battery specifications in the parts diagram or the owner's manual PDF.
Do not assume that a battery from a different brand will work, even if the voltage and amp-hour rating match. Cordless mower batteries have different connector types and charging systems. A Ryobi battery will not fit a DeWalt mower, and forcing it could damage both the battery and the mower. Stick with the original brand or a certified replacement made for your specific model.
Replacement battery cost and lifespan
A replacement 12-volt battery for a gas mower costs between $40 and $100, depending on the amp-hour rating and brand. Cordless mower batteries are more expensive: a 40 to 80 amp-hour battery for a cordless mower typically costs $150 to $400. Buying a battery from the mower's original manufacturer usually costs more than a third-party option, but the original is more likely to last as long as the mower itself.
Most lawn mower batteries last three to five years with regular use. Cordless batteries degrade faster if you leave them fully charged for months at a time or expose them to extreme heat. Store cordless batteries in a cool, dry place and charge them only when you plan to use the mower within a few days. Gas mower batteries last longer if you remove them before winter storage and keep them in an unheated garage or shed where they stay cool but do not freeze.
If a battery is more than four years old and no longer holds a charge, replacement is usually cheaper than repair. Battery cells cannot be replaced individually in most mower batteries — you buy the whole unit.
Amp-hours versus cold cranking amps: what the difference means
Amp-hours (Ah) measure how much total energy a battery stores. A 30 amp-hour battery can deliver 30 amps for one hour, or 15 amps for two hours, or 60 amps for 30 minutes. For cordless mowers, this rating tells you how long you can run the mower before the battery is empty.
Cold cranking amps (CCA) measure how much current a battery can deliver at freezing temperature (0°F). This matters only for gas mowers with electric starters. A battery with a high CCA rating will crank a cold engine reliably, while a battery with low CCA might not turn the engine over fast enough to start it. A 12-volt battery might be rated 400 CCA, for example. If you live in a cold climate, look for a battery with at least 300 CCA for reliable winter starting.
For cordless mowers, CCA does not explore because the battery is not used to crank an engine. The amp-hour rating is what matters.
Frequently Asked Questions
Can I use a higher amp-hour battery than my mower came with?
Yes, if the voltage matches. A cordless mower designed for a 40-volt 5.0 amp-hour battery will work with a 40-volt 6.0 or 8.0 amp-hour battery from the same brand. The mower will run longer per charge, but it may charge more slowly and the battery will be heavier. Check your charger's manual to make sure it can handle the larger battery.
What does it mean if my battery is rated 5.0 Ah but the mower uses 40 volts?
The 5.0 Ah is the capacity at 40 volts. Total energy is measured in watt-hours (voltage × amp-hours), so a 40V 5.0Ah battery holds 200 watt-hours. A 12V battery would need 16.7 amp-hours to hold the same energy. The voltage and amp-hours together determine runtime and power.
Why does my gas mower battery die if I do not use it for a few months?
All batteries self-discharge over time, even when not in use. A 12-volt mower battery can lose 10 to 20 percent of its charge per month in storage. If you store your mower for winter, remove the battery and keep it in a cool place, or use a battery tender (a low-power charger) to keep it topped up.
Is it safe to use a battery from a different brand if it has the same voltage and amp-hours?
For gas mowers, yes — a 12-volt battery is a 12-volt battery. For cordless mowers, no. Cordless batteries have proprietary connectors and charging systems. Using the wrong brand can damage the mower or the battery, or both.