Efco Sirius 1200 Replacement Battery 25.2V 13800mAh

Efco Sirius 1200 — 25.2V Li-ion Replacement Battery

This 25.2V Li-ion battery replaces the original pack in the Efco Sirius 1200 cordless lawn mower. Capacity is 13800mAh (347.76Wh). It slots into the same battery bay and communicates with the mower's existing BMS handshake.

• Sirius 1200 fitment: The Sirius 1200 blade motor draws a sharp inrush current every time you engage the deck. This battery's cell configuration matches the original 25.2V rail, so the BMS doesn't see a voltage mismatch on startup and won't cut power before the blade reaches speed.
• Bench tested on actual hardware: We cycled this pack through repeated high-draw discharge events simulating blade engagement under load. The BMS held the output rail stable and the protection circuit tripped at the expected over-current threshold without latching — it reset cleanly on the next power cycle.
• Seasonal storage tip: Before the first cut of spring, bring the battery indoors overnight if temperatures are below 10°C. Cold Li-ion cells carry higher internal resistance, and the blade motor's inrush current on a cold pack is enough to trip the BMS before the mower even moves forward.

BMS cutoff on thick-grass or wet-grass blade load

Wet grass and dense overgrowth force the blade motor to draw significantly more current than normal mowing conditions. If that draw exceeds the BMS over-current threshold, the pack cuts out mid-row to protect the cells. This isn't a faulty battery — it's the protection circuit doing its job. Let the mower sit for 60 seconds, then resume at a slower walking pace to reduce blade load per pass.

Charger showing a fault light after the mower sat all winter

Li-ion cells self-discharge slowly in storage. A pack left uncharged from autumn through winter can drop below the charger's acceptance voltage — typically around 20V on a 25.2V pack — and the charger refuses to start a charge cycle. Some chargers include a recovery or "boost" mode that trickle-charges the pack up to acceptance voltage first. If yours doesn't, a bench charger set to Li-ion recovery at 0.1A can bring the pack back to 22V, after which the standard charger will accept it normally.