my ryobi mower battery shows full on the charger but dies in like 5 minutes when i mow.

Diagnosis

The issue you are experiencing with your Ryobi 40V 20-inch Brushless Push Mower (RY401018) is known as "voltage sag" or "surface charging." The charger monitors the total voltage of the battery pack; once that total reaches the target threshold, the charger signals a "full" status with a green light. However, the battery pack consists of multiple lithium-ion cells connected in series. If one or more of these cells has degraded or developed high internal resistance, it can no longer hold a significant amount of energy, even though it can still reach the target voltage. When you engage the brushless motor, the mower demands a massive surge of current. Because the weakened cells cannot sustain this discharge rate, the voltage "plummets" instantly. The mower's Battery Management System (BMS) detects this sudden drop below the critical cutoff threshold (typically around 32V-34V for a 40V system) and shuts the machine down to protect the cells from permanent damage. To confirm this, check if the battery indicator on the mower's handle drops from full to empty almost instantly the moment you hit the start button; this confirms a cell imbalance rather than a motor failure.

How to Fix It

While Ryobi batteries do not have a manual "reset" button, you can attempt to force the charger's internal balancing circuit to equalize the cells through a prolonged saturation cycle.

1. Ensure the battery terminals and the charger contact pins are completely clean. Use a cotton swab dipped in 90% isopropyl alcohol to remove any grass debris or oxidation from the gold-plated contacts.
2. Insert the battery into the charger and allow it to reach the green "charged" light.
3. Do not remove the battery. Leave it seated in the charger for a full 24 to 48 hours. The charger continues to perform "trickle balancing" even after the main light turns green, attempting to bring the lowest-voltage cell up to match the others.
4. Remove the battery and immediately perform a "deep discharge" by running the mower until it shuts off completely. Do not stop the mower manually before it dies.
5. Place the battery back on the charger for another full cycle. This "deep cycle" can sometimes recalibrate the BMS and force the charger to recognize the true capacity of the degraded cells.

If That Didn't Work

• Test with a Secondary Pack: Swap in a known-good 40V battery (such as a 4.0Ah or 6.0Ah pack). If the mower runs for the expected duration, the original battery has a failed internal cell and must be replaced. Lithium-ion cells that have dropped below a certain voltage threshold cannot be recovered by standard chargers.
• Check for Blade Obstructions: Inspect the underside of the mower deck. If the blade is bent or if there is a massive buildup of dried grass clippings around the spindle, the motor will work harder, drawing more current and causing a "false" low-battery shutdown. Clear all debris and ensure the blade is balanced.
• Inspect the Battery Connector: Check the plastic housing of the battery for any cracks or signs of heat warping. If the battery is not seating perfectly flush against the mower's terminals, it can create high resistance, causing the BMS to trigger a shutdown due to perceived voltage drop.
• Verify Charger Output: If you have multiple batteries and all are exhibiting the same rapid drain, the charger itself may be failing to actually push the full current into the cells, providing a "ghost charge" that looks full but lacks actual amp-hours.

Heads Up

• Temperature Management: Never charge your 40V batteries immediately after mowing if they are hot to the touch. Let them cool for 30 minutes; charging hot cells accelerates degradation and leads to the exact imbalance you are seeing.
• Winter Storage: Store your batteries in a climate-controlled environment. Leaving them in a freezing garage during winter can cause the voltage to drop below the recovery threshold, effectively killing the pack.
• Avoid Total Depletion: Try to swap batteries when you have about 20% power left. Repeatedly running the mower until it shuts down completely puts excessive strain on the lowest cell in the pack.
• Charging Frequency: Avoid leaving batteries on the charger for weeks at a time. While a 48-hour balance cycle is helpful for troubleshooting, long-term "float charging" can degrade the chemistry of the 40V cells.