Equalisation Charging Explained: When, Why and How to Balance Your Battery Cells

Every cell in a flooded lead-acid battery starts life as a near-perfect match. Months of daily charging and discharging slowly change that. Some cells end up working a little harder than others, and before long the battery is only as good as its weakest cell.

Normal charging doesn't always put that right. Partial charges, busy shift patterns and opportunity charging can leave some cells slightly undercharged, with acid settling at the bottom of the cell and lead sulphate starting to harden on the plates. The result is shorter run times and a battery that needs replacing earlier than it should.

In this article, we'll look at what an equalisation charge actually does, how to tell when your batteries need one, how to do it safely, and the watering and record-keeping habits that go with it.

What is an equalisation charge?

An equalisation charge is a controlled, deliberate overcharge carried out after a normal full charge. The battery is held at the end of its charge for longer than usual, so that the cells that are lagging behind have time to catch up with the rest.

The extra charge does three useful jobs:

• It brings every cell up to the same, full state of charge.
• The gassing it produces stirs the electrolyte, reducing stratification, where heavier acid collects at the bottom of the cell.
• It helps convert soft lead sulphate back into active material before it hardens into permanent sulphation.

Equalisation is intended for flooded (vented) lead-acid batteries. Sealed or valve-regulated (VRLA) batteries are generally not equalised unless the battery manufacturer specifically recommends it, so always check first.

How often should you equalise?

There's no single answer for every fleet. Many traction battery manufacturers recommend a regular equalisation, often weekly, and many modern chargers carry one out automatically on a schedule. The right frequency depends on the battery, the charger and how hard the truck is worked, so your battery manufacturer's guidance should always come first.

Fleets that rely on opportunity or fast charging deserve particular attention. Short top-up charges during breaks keep trucks moving, but they rarely bring the battery to a true full charge. A planned full charge followed by an equalisation, for example at the weekend, is often part of keeping those batteries healthy.

Signs your batteries need equalising

The clearest signal comes from measuring the specific gravity (SG) of each cell. If readings taken after a full charge show a noticeable spread between cells, the battery is out of balance. A Hydrometer (FLT-E018) makes this a quick check, with a colour SG range from 1.10 to 1.30 and a 4mm nozzle for narrow openings.

Other warning signs include:

• Trucks running out of charge earlier in the shift than they used to.
• Individual cells running hotter than their neighbours.
• SG readings that stay low even after a full charge.

If one or two cells stay low even after an equalisation, that points to a deeper problem such as a failing cell, and the battery should be checked by your battery service provider.

How to equalise safely

Because equalisation pushes the battery into prolonged gassing, it needs a little more care than an everyday charge. Use this checklist before and during every equalisation:

• Check the battery manufacturer's instructions and the charger settings first.
• Equalise only flooded lead-acid batteries, never sealed or VRLA types unless the manufacturer approves it.
• Check electrolyte levels before you start. If any plates are exposed, add only enough deionised water to cover them.
• Open the battery lid or cover and make sure the charging area is well ventilated.
• Keep sparks, flames and smoking well away while the battery is gassing.
• Let a hot battery cool before equalising, and keep an eye on cell temperatures.
• Wear the correct PPE: splash goggles or a face visor, chemical-resistant gloves and an apron.

Watering after equalisation

Equalisation uses more water than a normal charge, because the extra gassing breaks water down into hydrogen and oxygen. That's why it is so important to check and top up afterwards, not before. Electrolyte expands during charging, so a cell filled to the top beforehand is likely to boil over, spreading acid across the battery top and into the tray.

Once the equalisation has finished, top up with deionised water after charging to bring each cell back to the correct level. The HydroPure Water Deioniser System lets you make your own pure battery water directly from tap water, so you're not adding unwanted minerals to your cells. Pair it with the GUN-X Battery Watering Gun, which fills each cell and automatically shuts off at the right level, and your technicians can water quickly without overfilling.

Avoid too much of a good thing

Equalisation is a maintenance tool, not something to run every night. Over-equalising wastes energy, drives up water consumption, raises battery temperature and can speed up corrosion of the positive plates, shortening the very battery life you're trying to protect.

The best approach is to equalise on the schedule your battery manufacturer recommends, and to keep a simple record for each battery: the date of each equalisation, SG readings before and after, and when it was watered. Over time, those records make it easy to spot batteries that are drifting out of balance and deal with them before they fail on shift.

👉 Build equalisation checks into your routine with the Hydrometer (FLT-E018) and our battery watering and maintenance products, or email us at hello@batteryaccessories.net for advice on keeping your fleet's batteries balanced.