About this replacement
Ryobi CTH1802 Series — 18V Ni-MH Replacement Battery (B-8288)
This is an 18V 2100mAh Ni-MH replacement battery for the Ryobi CTH1802 cordless hedge trimmer and related 18V models. It fits the CTH1802, CTH1802K, HD1800M, HP1802M, and over a dozen additional Ryobi 18V tools that share the same battery platform. Voltage and capacity match OEM specification exactly.
- CTH1802 platform compatibility: These models share a common 18V Ni-MH rail, identical connector footprint, and compatible BMS handshake protocol. The battery slots and locks the same way as the original, and the charger recognises it without needing any reset or pairing step.
- Bench tested on actual hardware: We ran this pack through charge cycles on a CTH1802-class platform and monitored BMS response through motor-start inrush on the dual-action blade assembly. Cell voltage held within spec across repeated trigger-pull events with no overcurrent trip.
- Ni-MH break-in on hedge trimmer blades: Run the trimmer at light load — trimming thin stems only — for the first two cycles before cutting dense hedges. Ni-MH cells benefit from a gradual capacity formation period, and the BMS uses early cycles to calibrate its overcurrent thresholds for the blade motor's inrush signature.
BMS cutoff on CTH1802 blade motor inrush surge
The dual-action blade motor draws a sharp current spike the instant the trigger pulls — this inrush can be several times higher than steady-state cutting current. A new pack from storage has not yet profiled this load, so the BMS may interpret the spike as an overcurrent fault and cut the output. Running two light-load cycles before heavy cutting allows the BMS protection thresholds to adjust to the motor's inrush signature. After those cycles, the pack sustains full trigger pulls without tripping.
Charger not recognising the new pack after storage
Ni-MH packs stored for extended periods can self-discharge below the voltage level a charger accepts as a valid battery. If the charger LED blinks an error or stays off entirely, the pack cell voltage has likely dropped under the charger's minimum acceptance threshold. Place the pack in the charger and wait — some Ryobi 18V chargers run a trickle pre-charge mode that recovers cells over 10–20 minutes before switching to full charge. If the charger does not respond at all after 30 minutes, check that resting cell voltage is at or above 1.0V per cell, which for an 18V Ni-MH pack means at least 15V total on a multimeter across the terminals.