Equalizing Charges for Floor Scrubber Batteries: When, Why and How

Key takeaways

• Equalizing is a controlled overcharge for flooded lead-acid batteries only, used to break up stratification and soft sulfation.
• Equalize when specific gravity varies by more than about 0.030 between cells, or on a schedule of roughly every 10 to 30 cycles or monthly, per the battery maker.
• Never equalize AGM, gel or lithium unless the manufacturer explicitly calls for it.
• Ventilate, check water before and after, and stop if a battery gets hot.

An equalizing charge is a deliberate, controlled overcharge applied to a fully charged flooded lead-acid battery, typically at around 2.55 to 2.65 V per cell for a few hours. It mixes stratified electrolyte, converts soft sulfate back into active material, and brings every cell to the same state of charge. Only flooded batteries should be equalized; AGM, gel and lithium should not.

In a scrubber pack of 12 or 18 cells wired in series, no two cells age identically. Some run slightly warmer, some lose a bit more water, and after enough partial charges they drift apart. The weakest cell sets the pack's usable capacity, because the machine's low voltage cutout trips when the pack voltage drops, and a weak cell drags it down early. Equalizing is the maintenance tool that pulls those cells back together.

What equalizing fixes, and what it cannot

If a battery does not recover after one or two equalizing charges, stop. Repeating the process just consumes water and corrodes the positive plates. That battery is likely due for replacement, see the battery replacement guide.

When to equalize

There are two approaches, and many fleets use both.

Condition based (preferred). After a full charge and a rest of at least a couple of hours, measure specific gravity in every cell with a hydrometer. Equalize if:

• The spread between the highest and lowest cell is more than about 0.030 (30 "points"), or
• The pack average is below roughly 1.250 after a full charge, suggesting chronic undercharging.

Schedule based. Battery makers commonly suggest equalizing flooded deep-cycle batteries every 10 to 30 cycles, or about monthly in regular use. Machines that get opportunity charged or run on undersized chargers usually need it more often.

Equalizing too often is also harmful. It shortens life through water loss and positive grid corrosion. "Weekly just in case" is not a good policy.

Typical equalizing parameters

These are typical ranges at about 77°F (25°C). Your battery maker's values take priority.

Many smart chargers have an equalize mode that runs automatically on a schedule or can be triggered by a button or menu. Some chargers add a short equalize to every charge cycle; check whether yours does before adding manual equalizing on top.

How to equalize, step by step

1. Confirm the chemistry. Removable vent caps mean flooded. If the label says AGM, gel, VRLA, sealed or lithium, stop here.
2. Check water first. Open each cell. If the plates are exposed, add just enough distilled water to cover them. Do not fill to the final level yet, because the electrolyte will rise and gas heavily.
3. Start with a normal full charge. Equalizing is applied to a charged battery, not a flat one.
4. Ventilate. Open the battery compartment and make sure the room is ventilated. Equalizing produces significantly more hydrogen than a normal charge. No sparks, flames or smoking nearby.
5. Start the equalize mode on the charger per its manual.
6. Monitor. Every hour, check battery temperature by touch on the case and measure specific gravity on a few pilot cells. Expect heavy bubbling; that is normal.
7. Stop when specific gravity readings stop rising for about three consecutive checks, when the charger ends the cycle, or if any battery becomes hot.
8. Let it cool and water. After the pack cools, fill every cell to the correct level with distilled water, see watering flooded batteries.
9. Clean the tops. Equalizing often leaves a film of acid mist. Wipe with a baking soda solution, rinse lightly, and dry.
10. Record before and after readings in the battery log.

Worked example: reading the hydrometer results

A 36 V scrubber with six 6 V flooded batteries feels weak by the end of the shift. After a full charge and two hours rest, the supervisor reads 18 cells (corrected to 80°F).

The spread is 1.275 minus 1.225, or 0.050, above the 0.030 threshold, and battery 3 is low in all three cells. That pattern (one whole battery low, others fine) suggests battery 3 has sulfated, often from a low water level or a loose connection. Action: check battery 3's water and cable connections, then equalize. If battery 3 still reads below about 1.240 afterwards while the others are near 1.265 to 1.275, replace the set or, if the pack is under a year old, that single battery with a matching one.

A different pattern, for example one single cell at 1.150 while the other two cells in that battery read 1.265, usually points to a shorted or failed cell. Equalizing will not fix it.

Common mistakes

• Equalizing a sealed AGM or gel battery because the charger "has the button." This dries the cells out and can cause swelling.
• Equalizing a lithium battery. LiFePO4 packs use the BMS to balance cells; an equalize voltage can trip protection or damage cells.
• Filling to the full level before equalizing, causing overflow and acid on the floor.
• Running equalize in a closed closet with no ventilation.
• Using equalizing to cover up a charger problem. If the pack needs it every week, check the charger profile and charging habits.
• Forgetting the floor. Acid drips from a boiling-over battery can etch sealed concrete and damage floor finishes in the charging area.

Frequently asked questions