Kärcher WV1 Window Vacuum Replacement Battery 3.7V 2900mAh

My Kärcher window vac worked fine yesterday and now it won't turn on even after charging all night — what's wrong?

At 3.7V, a lithium-ion cell this size is vulnerable to deep discharge if it was stored or left unused for weeks — the protection circuit shuts the cell off hard and a standard charge cycle won't wake it. Plug it in for 20–30 minutes, then check whether the charger LED shows any activity at all; no light usually means the cell dropped below recovery threshold, not that the charger is faulty. We've seen this exact behavior on small-cell Li-ion packs in low-draw household devices that sit between uses. If the charger shows a solid charge light but the vac still won't power on after a full charge, the cell needs replacing — the old pack won't recover.

My window vacuum motor runs but the suction feels noticeably weaker than it used to — is that the battery or the squeegee?

On a 3.7V single-cell pack, capacity fade causes voltage sag under load before total failure — the motor still spins but can't maintain the pressure differential the suction channel needs. Check the squeegee blade and dirty-water tank first since a full tank or a warped blade mimics this symptom exactly, but if both are clear, the battery is the likely cause. A degraded cell delivers its rated voltage at rest but drops below the motor's minimum operating voltage the moment load is applied. Charge the pack fully, then run the vac and feel whether it starts strong and fades within the first minute — that pattern confirms voltage sag, not a blockage.

My Kärcher WV1 leaves streaks now even though I cleaned the blade — could the battery cause that?

Yes, indirectly. A weakening cell causes inconsistent motor speed, which changes how firmly the squeegee blade tracks across the glass — inconsistent pressure leaves streaks even on a clean blade. This is one of the less obvious signs of capacity fade on small window vacs because people troubleshoot the blade, the water, and the glass before suspecting the battery. We measured noticeable RPM drop on degraded cells in this voltage class under the light but continuous load a window vac produces. Charge fully, clean the blade, and run the vac on a single uninterrupted stroke — if the pull feels uneven or the motor audibly changes pitch mid-stroke, the pack is the problem.