The John Deere S120 uses a 12V lawn tractor battery rated at 300 CCA. A U1 or U1R battery with 300+ CCA is the correct replacement target.
John Deere S120 Battery Size and Specs
Battery Voltage
12 volts
Battery Setup
Single battery
Factory Battery Rating
300 CCA
Common Replacement Group
BCI Group U1 / U1R
Recommended Upgrade
350 CCA U1 battery
Amp Hour Rating
30 Ah common Deere U1 replacement
Battery Type
Wet charged lead-acid or AGM lawn tractor battery
Terminal Type
U1 lug post terminals
Charging System
Regulated charging system
Charging Output
8 amps
Ground
Negative ground
What Battery Does a John Deere S120 Use?
The John Deere S120 uses a 12V battery rated at 300 CCA. It also uses a regulated 8 amp charging system.
For replacement, use a U1 or U1R lawn tractor battery with at least 300 CCA. A 350 CCA U1 battery is the better upgrade if it fits the tray and the terminal layout matches.
Use 12V, 300+ CCA, U1/U1R as the safe replacement target. Match the positive terminal location before buying.
Quick Battery Specs
12VVoltage
300 CCAFactory Rating
U1 / U1RCommon Size
8ACharging Output
Best Battery to Use
12V BCI Group U1 or U1R battery
300 CCA minimum
350 CCA preferred upgrade
30 Ah common premium replacement rating
Wet charged or AGM lawn tractor battery
Correct terminal orientation
Secure fit in the factory battery tray
U1 Battery Dimensions
Battery Group
BCI Group U1
Typical Length
About 7.7 inches / 195 mm
Typical Width
About 5.2 inches / 130 mm
Typical Height
About 7.3 inches / 186 mm
Fit Check
Confirm tray size, hold-down, post height, and cable reach before buying
U1 vs U1R Battery
U1
Standard terminal layout
U1R
Reverse terminal layout
Why It Matters
The S120 battery cables are short, so the wrong layout may not reach properly
Best Check
Match the positive terminal location on your old battery
Battery Upgrade Option
The best upgrade is a higher-quality U1 or U1R AGM battery with 350 CCA. It gives stronger starting without changing the mower’s electrical system.
Better starts after storage
Lower self-discharge
Less terminal corrosion
Better vibration resistance
Stronger cold-weather cranking
Battery CCA Comparison
Factory Spec
300 CCA
Best Target
350 CCA
Weak Battery
230 CCA
Charging System Checks
The S120 uses a regulated 8 amp charging system
A fully charged 12V battery should read about 12.6V at rest
Running voltage should usually be around 13.8V to 14.5V
Weak charging can ruin a new battery
Loose or corroded terminals can mimic a dead battery
Replacement Tips
Turn the mower off and remove the key
Disconnect the negative cable first
Disconnect the positive cable second
Install the new battery with the same terminal orientation
Reconnect the positive cable first
Reconnect the negative cable last
Secure the hold-down before mowing
Signs the Battery Is Bad
Clicking when turning the key
Slow cranking
Starts only after charging
Battery voltage drops quickly under load
Corrosion keeps returning
Battery is older than 3 to 5 seasons
Common Mistakes
Buying U1 when the mower needs U1R, or the other way around
Using a weak low-CCA battery
Leaving the battery flat over winter
Connecting the battery cables in the wrong order
Ignoring loose or corroded terminals
Installing a battery that is not clamped down properly
