40V Max Kobalt Lawn Mower Replacement Battery 4.0Ah

My Kobalt 40V mower starts fine but dies halfway through the lawn and won't restart until it sits for a while — what's happening?

That's thermal cutoff activating — the battery's protection circuit trips when cell temperature spikes under sustained high-draw load, which happens fast in thick or wet grass. The pack needs to cool before voltage recovers enough to restart. Cutting in those conditions puts more strain on a 40V 4.0Ah pack than most users expect, and a degraded or deeply cycled replacement battery will hit that threshold even sooner than a new one. Let the pack cool completely, then check resting voltage with a multimeter — a healthy 40V lithium pack should read between 40V and 42V at rest; anything under 36V means the cells aren't recovering properly.

The charger light goes solid green after only 20 minutes but the mower still cuts out after one pass — is the battery actually charged?

A fast green light on Kobalt 40V chargers usually means the charger detected a high resting voltage and assumed the pack was full — this happens when a battery has one or two cells sitting above threshold while others are significantly lower, a condition called cell imbalance. The mower then collapses under load because the weak cells can't sustain current draw even though overall voltage looked fine to the charger. We see this frequently on batteries that were stored discharged for a season or run repeatedly to full cutoff without a complete recharge cycle. Charge it, let it rest 10 minutes, then measure voltage at the pack terminals — if it reads above 41V but the mower still stalls on the first strip, cell imbalance is the likely cause.

My new replacement battery fits and powers on but the mower runs slower and has noticeably less blade power than the original — what did I do wrong?

Nothing you did — this is almost always a voltage sag issue where the replacement pack can't hold voltage steady under the high current the mower's brushless motor demands during cutting. Sag shows up as reduced blade speed and torque even though the battery indicator looks full. It's more common in colder weather or when the pack hasn't gone through two or three full charge/discharge cycles to condition the cells. Run the battery through three complete charge and full-use cycles on a lighter Kobalt 40V tool first, then retest on the mower — if blade performance is still noticeably weak compared to the original, check pack voltage under load; it should stay above 34V while the mower is actively cutting.