Here is a two pack alternative battery if you cannot get Ridgid to warranty your non registered Ridgid battery.  Here are some specs on these non Ridgid batteries.

• Condition: Brand new Sub-Type: Replacement Battery Voltage (V): 18V Wattage: 90W Item Weight: 680g
• professional lithium battery for Ridgid 18V lithium battery R840087, R840083, R840085, R840086, AC840089, AC840085, AC840086, AC840087P Series.
• Works Fine: fit RIDGID 18V Lithium system of cordless drill tools, works with Ridgid 18-Volt lithium battery charger R86092.
• Fade-free Power: over 500 cycle times, high capacity 4.0Ah lithium battery 18V, long lasting/ more run-time and powerful in working
• Helper Design: with on-board fuel gauge, integrated LED battery charge level indicator allows user to monitor battery using. Built-in integrated microchip prevents overcharging and self-discharge.

18V Lithium Ion Replacement Battery Compatible with for RIDGID R840083 R840085 R840086 R840087 R840089 AC840085 AC840086 AC840087P AC840089 Series Drill Battery (2 Pack)

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Let us know how they work for you.