36V Bosch EasyRotak 36-550 Replacement Battery 5.0Ah

Bosch EasyRotak 36-550 cuts out halfway through the lawn and won't restart until it cools down — is that the battery or the mower?

That's thermal cutoff triggering in the battery pack, and it's more common when a higher-capacity replacement is pushed hard on thick or damp grass after years of the mower running on the original 2.0Ah cells. The deck motor draws significantly more current in dense grass, and if the pack's internal temperature sensor trips, it locks out until the cells drop back to a safe temperature. We saw this on the bench when running 36V packs hard through sustained high-draw cycles without rest — the cutoff is working correctly, but the fix is operational: mow in overlapping passes on a dry day and let the pack rest between battery swaps. Check the pack casing for heat after a cutout — if it's hot to the touch at the cells, the mower's load pattern is the cause, not a faulty battery.

My EasyRotak 36-550 runs noticeably slower and the blade sounds laboured after fitting the new battery — the old one seemed stronger even though it was dying

A new Li-ion pack at full charge can actually deliver higher voltage than a degraded old pack running in a worn state, so if the mower sounds slower the issue is usually poor contact between the pack's terminals and the mower's battery slot — oxide buildup or slightly bent contacts reduce current transfer and cause exactly this symptom. Clean the gold contacts on both the battery and the mower slot with a dry cloth or pencil eraser, then reseat the pack firmly until it clicks. If the blade still sounds laboured, check whether the blade itself is blunt or unbalanced, since a new battery will expose any mechanical drag that the old pack was too weak to reveal. Measure resting voltage on the new pack with a multimeter — it should read 40–42V fully charged; anything below 38V on a fresh charge points to a charger or contact issue, not the pack itself.

Bosch 36V charger just blinks and won't charge the new replacement pack — works fine with the original battery

This usually means the replacement pack's cells have shipped in a deeply discharged state and the Bosch charger's wake-up circuit isn't recognising them as a valid 36V pack. The charger checks cell voltage before starting a charge cycle, and if any cell group reads below roughly 2.5V, it refuses to begin. Leave the pack connected to the charger for 15–20 minutes without unplugging — some Bosch chargers will attempt a slow recovery pulse before the fault light settles. If the blinking continues, check whether the charger LED pattern matches a communication fault (rapid continuous blink) versus a temperature fault (two-blink pause pattern), as the Bosch AL 36V chargers use different blink codes for each — the user manual table maps them directly.