The acid in a working flooded cell is not one liquid — left to itself, it quietly sorts into layers, strong at the bottom and weak at the top. This is acid stratification, and it is one of the least-discussed reasons batteries disappoint their owners. It falsifies your hydrometer readings, wears the bottom of your plates faster than the top, and it does all of this invisibly, inside a cell that looks perfectly healthy from above. It is also the unspoken reason behind two things this library keeps recommending: the equalising charge, and immobilised electrolyte. This guide explains the gradient itself — how it forms, what it quietly costs, and which batteries are immune by construction.
How the gradient forms
The mechanism is simple physics doing its honest work in an inconvenient place. During charge, concentrated sulphuric acid is released at the plate surfaces. Concentrated acid is denser than the surrounding electrolyte, so it sinks, sliding down the plate faces and pooling at the bottom of the cell. During discharge the chemistry runs the other way — acid is consumed and water is produced at the plates — diluting the upper electrolyte further. Charge and discharge a cell repeatedly without ever stirring it, and each cycle deposits another instalment of dense acid at the bottom and leaves the top a little weaker. Nothing mixes the column back together on its own: the cell has no pump, and gentle diffusion works over weeks while cycling separates the layers daily. Batteries that live on partial charges — cycled hard, recharged in a hurry, never gassed — stratify fastest, because the one natural stirring mechanism a flooded cell has is the bubble stream of a proper full charge, and a hurried charge never gets there.
What the gradient costs you
A stratified cell is really two bad cells stacked on top of each other. The lower plate area works in acid far stronger than the design intended: local chemistry runs harsher there, corrosion and hard sulphate formation accelerate, and that part of the plate ages on fast-forward. The upper plate area works in acid weaker than intended: it is effectively undercharged even when the charger says full, and chronically undercharged lead sulphate is exactly how sulfation gets its start. The result is uneven wear the datasheet never imagined — capacity quietly falls, and the loss concentrates where you cannot see it. Worse, the cell starts lying to you. A hydrometer samples the electrolyte it can reach, near the top of the cell, and in a stratified cell the top is the weak layer: your state-of-charge reading comes back low even though the charge went in. Chasing that false reading with more charging punishes the already-overworked bottom of the plates. When a whole bank shows specific-gravity readings that refuse to come up after a full charge, stratification belongs on the suspect list before any verdict about the battery itself — a point our maintenance walk-round guide builds into its sampling routine.
Tall cells feel it most
Stratification scales with height. A short automotive cell has little column to sort; a tall industrial 2 V cell gives dense acid a long way to sink and a deep basement to pool in. That makes the tall flooded stationary cell — the OPzS class and its relatives — the classic candidate, and it is precisely why the maintenance culture around such cells is so insistent about periodic full charges and the equalising ritual. The same logic follows tall cells into traction duty, where deep daily cycling manufactures gradient faster than standby duty ever could. Some traction installations answer with electrolyte agitation — air-lift systems that bubble the column back into uniformity as part of every charge — an engineering solution worth asking about when specifying fleets that cycle hard and charge on tight schedules.
The cure is deliberate gassing — and prevention is a proper charge
The traditional remedy is elegant: use the cell’s own chemistry as the stirring spoon. Push a controlled overcharge and the plates begin to gas; the rising bubbles drag electrolyte upward, and the column mixes itself back to uniformity. That is the equalising charge doing one of its two jobs — it is not folklore, it is fluid mechanics — and the settings discipline belongs entirely to that guide. Prevention is cheaper than cure, though, and prevention is simply charging properly: regular full charges that reach the gentle gassing stage give a flooded cell its routine stir before the gradient can consolidate. A battery that only ever receives hurried opportunity charges is a stratification machine, whatever its nameplate promises.
Who is immune — and what that immunity costs
Immobilise the electrolyte and there is nothing left to sink. That is one of the quiet engineering victories of the VRLA construction: gel locks the acid in a silica matrix, AGM holds it in the pores of a glass mat, and in both cases the gradient largely loses its medium. Gel is the stronger answer of the two — a gelled tall cell is how you get the height of an industrial 2 V cell without the stratification tax, which is a real part of why the OPzV gel tubular cell exists at all. As ever in batteries, immunity is a trade, not a gift: sealed construction gives up the hydrometer, the visual check and the correcting equalise, which is a different maintenance philosophy with its own disciplines. But on the specific question of stratification, immobilised electrolyte is the honest answer by design. If your bank’s gravity readings have gone strange, or you are weighing flooded against gel for a tall-cell duty, talk to our engineers — reading a stratified bank correctly is a ten-minute conversation that saves expensive wrong conclusions.