The mower goes in the shed in October. The batteries should not go in with it, fully charged, to sit for five months.
Lithium-ion packs don't mind being idle. They mind sitting full and warm. The fix takes five minutes and saves real money, because a 40V pack costs more than most people expect to replace.
What the data says
Battery University publishes retention tables for lithium-ion cells. Two of them matter for a winter in the shed.
Charge level. After one year of storage at 25 C, a lithium-cobalt cell kept at 40 percent charge retained 96 percent of its capacity. The same cell stored at 100 percent retained 80 percent.
Temperature. At full charge, a year at 40 C left 65 percent of capacity. Three months at 60 C left 60 percent. Battery University names 15 C as the best storage temperature.
Our caveat: those figures come from lithium-cobalt cells. Many tool packs use other lithium chemistries, so treat the numbers as the direction and the rough size of the effect, not a spec for your pack. The direction is consistent across lithium-ion: high charge plus warmth is the expensive combination.
What 16 points costs
Say you paid $150 for a pack. Losing 16 points of capacity, 80 percent versus 96, is roughly a tenth of the pack's value gone in a single winter, about $24 of capacity at that price. That's our arithmetic on a round example, not a quote. Skip the step once and you might not notice. Skip it for three winters and the mower stops finishing the lawn.
The five-step routine
- Run it down a bit. If the pack is full from the last mow, use the blower or trimmer for a while. You're aiming near half.
- Check the gauge. Most 40V packs have a four-LED charge indicator. Two lit LEDs is a sensible target. Your manual may give its own recommendation, and if it does, follow it.
- Take it off the tool and the charger. A pack left in a tool can keep a small standby drain, and a charger left plugged in is a pointless draw and a fire-safety question.
- Pick the spot. Indoors, cool, dry, away from heaters and sunlight. A closet shelf in an unheated spare room works. A hot attic or a car trunk doesn't.
- Label it. Write the date on masking tape. You'll thank yourself in spring.
Should I check on it?
Yes, once or twice. Battery University notes there is virtually no self-discharge in lithium-ion below about 4.0 V per cell at 20 C, so a half-charged pack shouldn't fall to empty on a shelf. Even so, packs with built-in electronics draw a little, and it's cheap to look. If you see a single LED in January, top it back up to about half.
Spring: wake it up gently
If you stored the pack in the cold, let it come to room temperature before charging. Don't charge a pack that's frozen. Then charge it fully, use it, and you're back to normal.
Already left a pack full all summer?
Don't panic. The data shows storage at full charge costs capacity, but it's a loss measured in percentage points over a year, not a dead pack on day one. Use the pack down in normal work, charge it, and see how long a mow lasts compared with last year. If the runtime has dropped a lot, the pack's aging, not the charger. Then start the routine so it doesn't get worse.
A quick autumn calendar
Put it on the same weekend as the last mow. Run the pack down while you do the final pass, pull it off the tool before you hose the deck, and let it cool before it goes on the shelf. A pack that's still warm from work shouldn't go straight onto a charger or into a closet. Then set a phone reminder for late January. One alarm, one glance at the gauge, done. It sounds fussy until you've bought your first replacement.
What not to do
- Don't store it fully charged because "it'll be ready."
- Don't store it fully drained. A pack that hits zero can trip its protection and refuse to charge.
- Don't leave it in a parked car, a metal shed in July, or a heated garage next to the furnace.
- Don't stack loose packs where metal can bridge the terminals.
If you own several packs
Most households with one 40V tool soon own two or three packs. Run the same routine on all of them. With three packs at $150, a 16-point loss on each is about $72 of capacity gone in a single winter, on the same rough arithmetic as before. The routine takes ten minutes for the whole shelf.
A few chargers and packs offer a storage mode. If yours does, use it and skip the guesswork. If not, the half-charge rule works. The same logic applies if you won't touch the tools for a month in summer: don't park a full battery in a hot shed.
Why bother, in dollars
An hour of work from a cordless tool costs more in battery wear than in electricity. Our cordless leaf blower cost-per-hour article shows the split, and the payback calculator's battery tab lets you put your own pack price and cycle count into the same formula. Extending a pack's life by a year drops the cost of every hour you use it.
Is it dramatic? No. It's a five-minute habit that protects a hundred-dollar part. We'd do it every autumn.