What Is The 'Battery Barrel' Effect, And How Does It Affect Battery Life?

When it comes to batteries, lots of factors impact things like capacity, efficiency, and longevity, one of which is the battery barrel effect. In the simplest of terms, this means that an energy storage system, such as a multicell battery, is only as powerful as its weakest component. Think of it as a barrel made out of different sized staves; if you pour water into it, it's going to start spilling once it rises past the shortest stave.

Before getting into specifics, it's worth understanding how single-cell and multicell batteries work. Each battery cell contains two electrodes — a positively charged cathode and a negatively charged anode — which are separated by a chemical material called electrolyte. Multicell batteries are made by stacking alternating electrodes, including separators between each one.

In single-cell batteries, capacity loss occurs due to long-term chemical changes caused by the interaction between the electrolyte and the electrodes. There are also external factors like too much heat that can speed up battery degradation. With multicell batteries, the weakest component within the system determines overall capacity, meaning all cells stop charging when one reaches its maximum, even if other cells could hold much more. The barrel effect has a major impact on rechargeable battery lifespans, which is why engineers are working to improve battery management systems (BMSs) to improve energy efficiency and overall longevity.

A BMS monitors a battery's voltage, current, and temperature to gauge its health and prevent overcharging and overheating. It has different responsibilities when monitoring single-cell batteries like the ones in your phone versus multicell batteries like you'll find in your laptop or Tesla. In your phone, a BMS is protecting against too much current, overcharging, over-discharging, and heat.

With multicell batteries, one of the core functions of the BMS is cell balancing, which regulates charging and keeps cells at their optimal voltage to maximize overall capacity. There are two ways to do this. Passive balancing is when a BMS senses that a cell's voltage has reached a certain point and burns off excess energy, which is a computationally cheap but wasteful way to handle the situation. More efficient is active balancing, which involves moving extra power from cells with more charge to cells with less. There are different techniques used to accomplish this, but complexity and cost make active equalization more difficult to implement. 

Researchers are also pursuing other methods for mitigating or fully removing the barrel effect. One possible solution is developing more efficient electrodes that can level out ion distribution and transport. These kinds of improvements, as well as other emerging battery innovations, could make for batteries that last longer and with less energy loss.