Your Unsold Stock Is Ageing: Calendar Loss in Battery Inventory

Most battery ageing advice is written for the person using the battery. This is written for the person holding it before anyone uses it — the distributor with 400 packs in a godown, the OEM with a quarter’s buffer stock, the financier who has repossessed a fleet and is waiting for a buyer.

A lithium pack does not sit still. It ages while parked, on a clock that runs whether or not anyone switches it on, and the two things that set the speed of that clock are both under your control and both usually ignored.

• •Cycle ageing happens through charging and discharging. Everyone expects this, warranties are written around it, and it is what people mean when they say a battery is “used up”.
• •Calendar ageing happens whether the pack is used or not. It is driven by the slow growth of the passivating film on the anode, and it is worst at high state of charge and high temperature.

The second one is the one that shows up as an unexplained write-down.

A cell at high state of charge has a fully lithiated anode sitting at its most reducing potential — which is exactly the condition under which the electrolyte wants to decompose against it. The protective film that formed during manufacture keeps growing, slowly, and every nanometre of growth consumes lithium that never comes back.

Drop the state of charge and you drop the anode potential away from that maximum, and the reaction slows. Drop the temperature and it slows again, for the ordinary reason that all chemical reactions slow when cooled.

Neither of these requires equipment. They require a decision about how stock arrives and where it is put.

• •Store at 40 to 60 per cent charge. Not full. If cells arrive at 100 per cent because that is how the supplier shipped them, that is worth raising with the supplier — and worth discharging to storage state if the stock will sit for months.
• •Store in the coolest place available, which usually means not under a metal roof and not in the room that also holds the chargers. Ambient is the baseline that everything else adds to.
• •Rotate stock first-in-first-out and mean it. A pack that has sat eighteen months at the back of a rack is not the same product as the one at the front, whatever the label says.
• •Test a sample before it ships, not after a complaint. A capacity check on a few cells from a batch that has been in storage tells you what you are actually selling.
• •Record arrival date and storage state of charge on the stock record. If you cannot answer “how long has this been here and at what charge”, you cannot forecast the warranty exposure it carries.

Cells are a perishable asset. That sentence is uncontroversial in the industry and almost never reflected in how inventory is actually financed.

Two consequences worth carrying into a commercial conversation:

• •Inventory holding cost for batteries is higher than the interest rate on the working capital, because the asset is losing capacity as well as costing money to hold. A twelve-month stock position in a hot warehouse can lose more to calendar ageing than to financing.
• •A warranty term written for one market can be loss-making in another with an identical product. Ambient temperature belongs in the calculation alongside duty cycle — and if the stock spent six months at 45 °C before sale, part of the warranty period has already been consumed before the customer takes delivery.

None of this is an argument against holding stock. It is an argument for holding it cool, holding it part-charged, and knowing how old it is.

This piece deliberately covers only the parked pack. For what heat does to a battery in service — and which precautions are worth the trouble once a vehicle is running — see why Indian summers are hard on lithium batteries. For the charging side, the seven charging habits guide covers the in-use half of the same chemistry. And if you are testing stock before it ships, the capacity test procedure in Battery 101 is the bench method.