Battery and Low Voltage Faults on Floor Scrubbers: What the Controller Sees and How to Fix It

Key takeaways

• Low voltage faults are triggered by voltage under load, so a battery that looks fine at rest can still cause them.
• Check charge state and battery type setting before suspecting the controller.
• A voltage drop of more than about 0.2 V across any one cable or connection under load points to a bad connection.
• One weak battery in a series pack drags the whole pack below cutout.
• Lithium machines add BMS faults that cut power regardless of the scrubber controller.

A battery or low voltage fault means the scrubber's controller measured pack voltage below a threshold, usually while the motors were under load, and shut down some or all functions to protect the batteries. The most common causes are an undercharged pack, one weak battery in the series string, a loose or corroded cable, or the wrong battery type selected in the controller.

Most scrubbers shed loads in stages. The brush motor usually stops first because it draws the most current, then the vacuum, and finally drive, so the operator can still get the machine back to the charger. That staging is why operators commonly report "the brush quit but it still drives". It is not a brush fault; it is battery protection working as designed.

What the controller is detecting

The controller watches the battery voltage at its own terminals. Typical thresholds are expressed per cell. For lead-acid, a cutout somewhere in the range of roughly 1.7 to 1.8 volts per cell under load is common, which works out to roughly 20.4 to 21.6 V on a 24 V pack and 30.6 to 32.4 V on a 36 V pack. The exact value depends on the maker and on the battery type chosen in the settings, so treat these as typical, not as your machine's number.

There are usually three distinct conditions in this family:

Some controllers use a delay of a few seconds before acting, so a brief dip on startup does not trip them. Others latch the cutout until the key is cycled, and a few will not let brushes restart until voltage recovers above a higher reset threshold.

Typical display wording

Expect messages that refer to low battery, battery discharged, battery voltage low, undervoltage, battery voltage high, or overvoltage. A battery gauge flashing or showing one bar is often part of the same family. LED machines frequently flash the battery indicator or a dedicated fault LED. Lithium machines may show wording about the BMS, battery communication, or battery temperature. Look up the exact code in your manual; do not assume a number from another machine.

Common triggers, ranked by likelihood

1. Pack not fully charged. The charger was unplugged early, was not plugged into a live outlet, or the machine was opportunity charged for a few minutes only.
2. One weak or failed battery. In a 24 V or 36 V series string of 6 V, 8 V or 12 V batteries, one bad unit sags under load and pulls the whole pack below cutout.
3. Loose, corroded or undersized connections. High resistance at a terminal creates voltage drop only under current, so it looks fine with the meter at rest.
4. Wrong battery type setting. A controller set for flooded lead-acid on an AGM, gel or lithium pack can cut out early or late. This often appears right after a battery replacement or lithium conversion.
5. Low electrolyte (flooded batteries). Exposed plates lose capacity fast. See watering flooded batteries.
6. Battery at end of life. Lead-acid packs commonly last roughly 500 to 1,500 cycles depending on care; capacity falls off steeply near the end.
7. Excess load. Heavy down pressure, a grit brush on a rough floor, or a dragging drive raises current, which deepens voltage sag.
8. Cold. Lead-acid capacity drops noticeably in cold storage areas and freezers.

Step-by-step diagnosis

Safety: wear eye protection and gloves around batteries, keep sparks away from flooded batteries, and remove rings and watches.

1. Check state of charge at rest. Charge fully, then let the pack rest at least 2 hours (ideally longer). A healthy flooded lead-acid 12 V battery reads about 12.6 to 12.7 V at rest when full; a 6 V reads about 6.3 V. A 24 V pack should read roughly 25.2 to 25.5 V, a 36 V pack roughly 37.8 to 38.2 V. Lower readings after a full charge point to a charging problem; see battery not charging.
2. Measure each battery individually. Batteries in the same pack should be within about 0.1 V of each other at rest (about 0.05 V for 6 V units). A battery noticeably lower than the rest is the first suspect.
3. Test voltage under load. Run brush and vacuum together (on a wet test floor or with the deck safely raised if your machine allows brush-on with deck up; otherwise scrub a test area). Read total pack voltage and each battery. A good pack sags but stays well above cutout; one battery that drops far more than the others is failing.
4. Hunt for voltage drop. Still under load, put one meter lead on a battery post and the other on the cable lug bolted to it. You should read close to zero, typically under 0.1 V. More than about 0.2 V across any connection means corrosion or looseness. Repeat across each cable, the main fuse or circuit breaker, the key switch circuit and the main contactor.
5. Compare pack voltage to controller reading. If the service menu shows battery voltage, compare it to your meter at the controller terminals. A large difference points to wiring between the pack and controller, or to a controller sensing fault.
6. Check specific gravity (flooded only). A hydrometer shows weak cells that a voltmeter averages away. A cell much lower than its neighbors after a full charge suggests a failing battery.
7. Verify the battery type setting in the service menu against the batteries actually installed.

Fixes

Reset behavior

Low battery warnings clear on their own once voltage recovers, which is why the machine can seem fine again after resting a few minutes. That recovery is not real capacity. Cutouts usually clear after a key cycle, but some controllers will not restart brush functions until the pack is recharged above a reset threshold. Overvoltage warnings after charging usually clear as surface charge bleeds off within minutes. BMS faults on lithium packs may require a specific wake procedure or connection to a charger.

Fault record worksheet

Write these down before calling for service:

• Exact code or message, and whether it is a warning or a shutdown
• Hour meter reading and time into the shift when it appeared
• Battery type, age, and battery type setting in the controller
• Resting voltage of the pack and of each battery after a full charge
• Voltage of each battery under brush and vacuum load
• Highest voltage drop found across any connection under load
• Hydrometer readings by cell (flooded batteries)
• Charging history for the past week: full charges, partial charges, missed charges
• Ambient temperature where the machine is stored and charged

If runtime is the real complaint rather than a code, see short runtime and estimate expected run time with the battery runtime calculator. Good daily habits are covered in battery maintenance.

Frequently asked questions