Why does a lithium forklift battery show a lower SOC after a weekend idle?

Direct answer: A weekend drop is usually truck key-off loads and BMS electronics, sometimes plus SOC estimation drift, not a sudden capacity failure. Controllers, telematics, DC-DC converters and displays can draw current while the truck looks ‘off’, and the pack’s own BMS also draws a small standby current. Measure the key-off current, decide a disconnect or sleep policy for long idle periods, and compare voltage-based checks with the displayed SOC before opening a warranty claim.

Separate real drain from display drift. If pack voltage is clearly lower on Monday, energy left the battery. If voltage is almost unchanged but SOC jumped down, the gauge may have recalculated; see reading SOC and alarms. LiFePO4 voltage is flat in the mid range, so SOC estimates can move after rest.

Truck key-off loads. Many forklifts keep modules alive for immobilisers, fleet trackers or cabin electronics. A few hundred milliamps over 60 hours is several amp-hours. On a smaller pack such as a 24V 230Ah that is more visible as a percentage than on a 36V 700Ah (690Ah) class pack.

BMS standby. Smart packs with displays, CAN, RS485 or 4G, including models such as the 48V 420Ah, keep monitoring circuits alive unless a sleep or main switch is used. Exact standby current is not usually published; treat it as ‘small but continuous’ until Redway provides a figure for your model.

What is not normal. Double-digit SOC loss with a large voltage drop, heat, or alarms points to a parasitic fault, a truck that was partly awake, or a pack problem. Investigate before blaming ‘lithium self-discharge’ alone; chemistry self-discharge is typically low.

Policy options. For weekends and holidays: leave packs at a mid-high SOC, switch battery power off if the truck allows, or plug into a maintainer/charger that is lithium-safe. Do not store for weeks at empty.

How to measure. Use a DC clamp on the battery negative with the key off and all aftermarket trackers accounted for. Record amps, then estimate Ah over the idle hours.

Related symptoms. Voltage sag under load is a different issue from weekend idle loss; see voltage drop under load for load-related diagnosis patterns.

Warranty hygiene. Send Friday and Monday voltage, SOC, alarm logs and key-off current with any claim so real defects are not mixed with normal standby drain.