Floor Scrubber Battery Chargers: Onboard vs Offboard and Choosing the Right Charge Profile

Key takeaways

• The charger profile must match the battery chemistry, and a mismatch can ruin a new battery set in months.
• Onboard chargers suit walk-behinds that charge where they park, offboard chargers suit riders and fleets that need more amps.
• Flooded batteries need a higher finishing voltage than AGM, and gel needs the lowest; lithium needs its own CC/CV profile with no float or equalize.
• Every time you change battery type, change the charger setting the same day, and write the setting on the machine.

A floor scrubber battery charger converts wall power into a controlled sequence of current and voltage stages, called a charge profile or algorithm, that is tuned to one battery chemistry. Flooded lead-acid, AGM, gel and lithium each need a different profile. The charger can be onboard (built into the machine) or offboard (a separate unit), but matching the profile to the battery matters far more than where the charger sits.

Most premature battery failures in scrubber fleets trace back to charging, not to the batteries themselves. A set that should give 1,000 or more cycles gives 250 because it was undercharged every night, overcharged on the wrong profile, or never allowed to finish. This guide covers how the two charger formats compare, what the stages actually do, the voltage differences between chemistries, and a field check to confirm your charger is set correctly.

Onboard vs offboard chargers

An onboard charger is mounted inside the machine and has a power cord you plug into a wall outlet. An offboard charger is a separate box, usually wall-mounted or on a cart, with a DC cable and connector that plugs into the machine's battery connector.

Decision rule: if the machine moves between buildings or floors and charges wherever it ends the shift, onboard is the practical choice. If machines live in one charging room, offboard chargers usually last longer, charge faster, and can be replaced without opening the machine. Many ride-on and industrial machines are sold with offboard chargers for that reason.

What a charge profile actually does

A modern "smart" charger runs a multi-stage algorithm. Names vary by maker, but the sequence for lead-acid looks like this:

1. Bulk (constant current). The charger pushes its maximum rated current while pack voltage rises. This restores roughly the first 70 to 80 percent of charge.
2. Absorption (constant voltage). Voltage is held at a set point while current tapers. This is where the chemistry-specific voltage matters most.
3. Finish or gassing stage (flooded only on many chargers). A low, controlled current at elevated voltage stirs the electrolyte with gas bubbles and fully converts the plates. Skip this and flooded batteries stratify and sulfate.
4. Float or maintenance. A lower voltage holds the pack full if it stays plugged in. Some chargers shut off and restart periodically instead.
5. Equalize (flooded only, optional). A deliberate controlled overcharge, covered in equalizing charges.

Lithium (LiFePO4) uses a simpler constant current, constant voltage (CC/CV) profile, then stops. There is no float stage, no gassing stage and no equalize. Cell balancing is handled by the battery management system (BMS), not by overcharging.

Typical voltages by chemistry

The numbers below are typical ranges for a 2 V lead-acid cell at about 77°F (25°C). A 24 V lead-acid pack has 12 cells, a 36 V pack has 18. Always use the battery manufacturer's published values, which take priority over this table.

Lead-acid set points should also be temperature compensated, typically by about 3 to 5 millivolts per cell per °C, lower when hot and higher when cold. Many chargers do this internally or with a sensor; cheap ones do not.

Why profile mismatch kills batteries

The differences in the table look small, a tenth of a volt per cell, but over a 12 or 18 cell pack and hundreds of cycles they decide battery life.

• Flooded battery on an AGM or gel profile: chronic undercharge. The pack never reaches the gassing voltage, electrolyte stratifies (acid settles at the bottom), sulfate hardens on the plates, and runtime fades steadily over months. Operators notice short runtime long before anyone suspects the charger.
• AGM battery on a flooded profile: chronic overcharge. AGM cannot have water added, so every bit of gassing is permanent electrolyte loss. Cells dry out, heat up, and can swell. In the worst case this leads to thermal runaway.
• Gel battery on a flooded or AGM profile: gel is the most voltage sensitive. Excess voltage creates gas pockets in the gel that never recover, killing capacity quickly.
• Lead-acid profile on lithium: the float and equalize stages hold voltage on cells that should be at rest, or push beyond the BMS limit, causing BMS cutoffs, error messages, or accelerated aging. A lithium pack on a lead-acid profile may also never reach full charge.
• Lithium profile on lead-acid: no finish stage, no float, no temperature compensation. The batteries are undercharged nightly.

The most common real-world scenario is simple: a machine is sold with AGM or gel batteries, a later replacement set is flooded because it was cheaper or in stock, and nobody changes the charger setting. On many chargers the profile is selected by a DIP switch, a menu, or a jumper that nobody looks at after delivery. See the battery replacement guide for the commissioning steps.

Sizing the charger to the battery

For lead-acid, a common rule of thumb is a charger output of about 10 to 20 percent of the battery's 20-hour amp-hour rating. A 24 V pack rated 235 Ah at the 20-hour rate pairs sensibly with a charger of roughly 25 to 45 A.

Worked example (Scrubber Guide model): a scrubber uses 120 Ah in a shift. Flooded batteries need roughly 110 to 120 percent of the removed amp-hours put back (a charge factor of 1.1 to 1.2). That is about 140 Ah to return. With a 25 A charger, bulk charging is fast but the absorption taper is slow, so expect roughly 8 to 10 hours in total, plus any finish stage. With a 15 A onboard charger, the same recharge can run past 12 hours, which is why some fleets with long shifts find packs are not full by morning.

A charger that is too large for the pack can overheat it; too small and the pack never finishes. If you are adding runtime with bigger batteries, check that the existing charger is still in range. Lithium packs typically accept much faster charging, often a full charge in 2 to 4 hours, but only at the rate the battery maker and BMS allow.

Field check: is this charger set correctly?

Use this five-minute check when you take over a fleet or after any battery change:

1. Read the battery label: flooded (removable vent caps), AGM, gel, or lithium.
2. Find the charger's profile setting in its manual: switch, menu, or label. Record what it is set to.
3. Confirm the pack voltage matches the charger output voltage (24 V on 24 V, 36 V on 36 V).
4. Near the end of a charge, measure pack voltage with a multimeter and compare with the absorption range for that chemistry.
5. For flooded batteries, after a full charge and rest, check specific gravity with a hydrometer. Readings consistently below about 1.250 on a "full" pack point to undercharging.
6. Write the chemistry and profile on a label on the charger and on the machine.

If the charger shows a fault light or code, see charger fault codes, and if it will not charge at all, work through battery not charging.

Charging habits that matter as much as the charger

• Charge lead-acid after every use longer than about 15 to 20 minutes, and let the cycle finish. Partial charges are the subject of opportunity charging.
• Do not unplug mid-cycle to "grab the machine for a minute" unless it is unavoidable.
• Charge in a ventilated area. Lead-acid batteries, especially flooded, release hydrogen during charging.
• Keep the recovery tank lid or battery compartment open during charging if the manual allows, for ventilation and cooling.
• Water flooded batteries after charging, as described in watering flooded batteries.

Frequently asked questions