Battery pack discharge voltage is uneven

What happens if a battery reaches a discharge cut-off voltage?

Once one individual cell in a series connection reaches the discharge cut-off voltage, the entire series connection will stop discharging. Thus, many cells are never fully charged or discharged, and the available capacity of the battery pack is subject to the minimum capacity of the individual cells.

How do you get batteries to discharge evenly?

Getting the batteries to discharge evenly is essentially impossible in a 'real world' application. In my flashlight test experiment the battery closest to the bulb always discharged soonest, the other batteries discharged inconsistently sooner/later. Using rechargeable batteries and changing their position didn't affect this result.

Why do flashlight batteries not discharge evenly?

This is common and apparently no viable study explains exactly why. Getting the batteries to discharge evenly is essentially impossible in a 'real world' application. In my flashlight test experiment the battery closest to the bulb always discharged soonest, the other batteries discharged inconsistently sooner/later.

What is a voltage vs discharge C-rate curve?

On each cell’s voltage vs. discharge C-rate curve, data points can be found where they have the same voltage and the sum of their current values equals the total current of the parallel connection.

How much energy does a battery have at the start of discharge?

For one cell to be at 1.5V while the others are fully exhausted then they would have had only 5% - 10% of their new energy content at the start of discharge. SO this is not a batch variation - two of the batteries were very close to dead at the start of discharge OR something else has happened not mentioned in your question.

Why do lithium ion batteries need to be connected in series?

To meet the power and energy requirements of the specific applications, lithium-ion battery cells often need to be connected in series to boost voltage and in parallel to add capacity . However, as cell performance varies from one to another [2, 3], imbalances occur in both series and parallel connections.