Have the batteries EVER been completely drained and not recharged immediately upon return of driver?

Symptoms:

Low battery voltage.

Cause:

When the battery is drained, an electric current introduced in the opposite direction of the ion flow can return the system to a charged state to begin the process again. Because recharging does not restore the previous state perfectly, the battery eventually loses its ability to take a charge

Lithium-ion batteries typically have a lifespan of two to three years, starting from the date of manufacture (not the time of first use, as some believe). Depending how the battery is treated, 300 to 500 discharge/charge cycles is normal. For maximum lifespan, avoid discharging lithium-ion batteries completely.

The worst situation occurs when the battery is fully charged and kept at a high temperature. Batteries can be worn out by overcharging or storage at high or low temperatures.

Resolution:

Follow a proper charging and storage procedure for batteries.

More Information:

Follow battery manufacturer’s recommendations.