Storing a Battery Over Winter Without Killing It

Both approaches to battery storage work. What differs is what they cost you, and the cost is what this sets out.

Two variables, not ten

The ageing that happens to a lithium pack sitting on a shelf is driven mainly by how full it is and how warm it is kept. A cell held near the top of its voltage range sits under constant internal stress, and the chemistry degrades faster while that stress is applied.

Warmth accelerates every chemical process involved, so the same pack ages considerably faster in a hot room than in a cool one. Combining both, a full pack in a warm cupboard is the worst realistic storage condition available in an ordinary home. A partially charged pack in a cool, dry place is the best, and the difference over a winter is not trivial.

The number to leave it at

Manufacturers commonly advise storing somewhere around the middle of the charge range rather than full or empty, and the reasoning is consistent across the industry. A mid charge keeps every cell comfortably away from both the high-voltage stress at the top and the protection cut-out at the bottom.

Exact recommended figures differ between manufacturers and chemistries, so the instructions that came with your pack beat any general rule. If your charger has no way to stop partway, charging fully and then riding a short distance gets you approximately there. Displays that show a percentage are estimating from voltage rather than measuring stored energy, so treat the figure as a guide rather than a reading.

Cold is not the enemy that heat is

Cold storage slows the chemistry down, which reduces the rate of ageing, so a cool garage is genuinely better than a warm hallway. What cold does not tolerate is being charged while the cells are near or below freezing, which can cause lasting internal damage.

With a loaded rack, the safe procedure is to bring a cold pack indoors, let it reach room temperature, and only then plug it in. Many management systems refuse to charge below a set temperature for exactly this reason, and that refusal is protection rather than a fault. Damp matters separately from cold, because condensation forms on a cold pack brought into a warm room and water finds connectors.

Checking on it during the layoff

Every pack loses a small amount of charge each month even when switched off, because the management circuit and the cells both draw a little. Over a long storage period that slow drain can take a pack from a comfortable middle charge down towards the protection threshold.

In the saddle, looking at it once every month or two and topping it back up to the middle is the whole of the routine. A pack that has fallen very low may refuse to accept a charge at all, which is a deliberate safety behaviour rather than a dead cell.

Recovering a deeply discharged pack is a job for someone with the right equipment, and improvised charging is exactly how these packs catch fire.

Where to put it

Somewhere cool, dry, out of direct sun, away from anything flammable and on a hard surface is the practical description of a good spot. Charging is the higher-risk activity, so if the pack will be charged where it is stored, that location matters more than usual. Fire services in several countries have raised concerns about lithium batteries charged in hallways and stairwells, because a fire there blocks the exit route.

Storing a pack somewhere it can be knocked over, crushed or driven into is worth avoiding for obvious reasons. Any pack that is swollen, damaged, has been in a crash or smells unusual should not be stored indoors and needs professional disposal advice.

Anything structural — forks, steerer, brake mounts — is worth having a mechanic sign off.

Waking it up in spring

Bring the pack up to room temperature before the first charge, and do that charge somewhere you can see it rather than overnight in a cupboard. Let the first charge run to completion so the management system can balance the cell groups after months of drifting apart. Expect the first ride to show slightly less range than you remember, and expect it to recover somewhat over the following few charges.

In the saddle, a pack that has genuinely lost significant capacity over one winter was probably stored full and warm, or was already near the end of its life. Range that has halved, or a pack that heats up noticeably during a normal charge, is a signal to stop using it and get it assessed.

Side by side

The takeaway

Half charged, cool and checked occasionally is almost the whole of battery storage, and it costs nothing.

Most of riding well is being boring and predictable to everyone else on the road.