TL;DR

Rapid discharge is usually a dead cell or a charger that is not completing full charge cycles. Measure individual cell voltages fully charged to identify a dead or reversed cell.

What you might see

battery indicator drops rapidly during shift
lift height reduced before end of shift
battery cells hot after a partial cycle
charger shows incomplete charge cycles

Likely causes

Tap to mark as ruled out

One or more battery cells failed (dead or reversed), reducing total capacity and causing the full pack to collapse early
Charger fault or damaged charger connector, delivering incomplete charge cycles
Battery deeply discharged and sulfated from chronically running to near-empty before recharging
Parasitic draw from a wiring fault keeping a control circuit energized when the truck is parked

Required tools

Digital multimeter (DC voltage)
Clamp ammeter
Battery hydrometer (specific gravity check for flooded cells)
Deionized water for topping
LOTO kit

Safety first

Forklift batteries produce hydrogen gas during charging. No open flames or sparks near a charging battery.
Wear acid-resistant gloves and safety glasses when checking electrolyte or handling battery connectors.
Lock out the forklift before testing the battery connector with the truck parked.

Procedure

7 steps · ~30 min

1
Charge the battery fully. Allow it to rest for 1 hour after charge completes before measuring.
2
Measure individual cell voltages across each 2V cell with a digital multimeter. A healthy fully-charged cell reads 2.1-2.15V. Any cell below 1.95V or above 2.2V after equalization is suspect.[1]
3
Measure the total pack voltage. A 24V pack should read above 25.5V fully charged.
4
Check charger output. Connect a clamp ammeter to the charging cable. The charger should deliver rated amps in bulk phase for at least 30 minutes at the start of a charge cycle.
5
Inspect the charger connector and battery connector for heat damage, corroded pins, or a loose locking bail causing high resistance.[1]
6
Check the battery water level. Low electrolyte exposes the plates, accelerating sulfation. Top off with deionized water to the max fill line.
7
If multiple cells test below 1.95V or the pack capacity is below 80% of rated amp-hours, the battery pack needs replacement.

Sources

Toyota 8-Series Forklift Operator and Maintenance Manual, Toyota Material Handling
Toyota 8FGU Series Forklift Service Manual, battery maintenance and charging procedures (general)
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