Vileda Windomatic 3.7V Replacement Battery Li-ion 2900mAh

My Vileda Windomatic cuts out halfway through cleaning the windows even with a full charge — why does it keep stopping?

At 3.7V, the protection circuit trips when the cell voltage drops too fast under load, which happens when the battery has lost internal capacity even if it shows full before you start. This is different from a dead battery — it still charges and rests at the right voltage, but it can't hold that voltage once the suction motor pulls current. The fix is to check resting voltage after a full charge: it should sit at or above 4.1V with no load. If it reads below that, the cell is degraded and won't recover — replacement is the correct step, not more charging cycles.

The Windomatic suction feels weaker than it did when new but the battery seems fine — could the battery still be the problem?

Yes, and it's one of the most misdiagnosed issues on small suction devices like this one. A 2900mAh li-ion cell that has been shallow-cycled repeatedly — charged after every window instead of run down properly — develops reduced effective capacity without showing obvious charge symptoms. The motor gets just enough voltage to spin, but not enough to maintain full suction pressure at the head. Run the battery down fully once, charge it completely, then test suction against a smooth surface: if there's a noticeable drop in pull within the first few minutes of use, the cell has degraded below useful working capacity.

The charger light stays solid and says it's done but the Windomatic still dies quickly — did the new battery not charge properly?

A solid charge indicator means the charger saw the voltage it was looking for, not that the cell accepted a full charge — these are different things. Li-ion cells that have been over-discharged, or sat unused for a long period, can absorb just enough charge on first cycle to satisfy the charger's cutoff threshold without actually filling the cell. We see this on the bench when replacement cells arrive in a deeply discharged state after storage. Put the battery through two full charge-discharge cycles before judging its capacity, and check that the charger output is delivering the correct voltage — it should read 4.2V at the contacts when no battery is connected.