Why do car batteries go flat?

Thanks to Rhod Jervis for the answer!

Will - Returning to a car after a few weeks or so can lead to a frustrating struggle as the only thing preventing the car from operating is one flat battery. But why does this happen? We put in a call to University College, London's Rhod Jervis

Rhod - There's a process called self discharge, which all batteries undergo to some extent. And then there is sort of the battery becoming flat, fully degraded and not possible to be regenerated. And that can unfortunately happen with lead acid batteries, which are the type of sort of starter batteries in internal combustion cars. So the process of self discharge is really one of fighting against the nature of all batteries. When we charge batteries up, what we're doing is putting them into a semi unstable state, thermodynamically speaking, and this is the equivalent of pumping water up a hill, up to a high reservoir. We're giving the battery that potential energy, but it also means it's in a state where it really wants to get rid of some of that energy. And in some cases that can lead to favouring some what we call side reactions that end up using a small percentage of the charge stored in a battery. So that happens in lead acid batteries, lithium ion batteries, lots of different types of batteries. Some of them lose their charge relatively quickly and have a shelf life of maybe a few, a few months. Others will lose just a couple of percent in a year or something like that. Most often it's reactions with the electrolyte. This is the sort of liquid in the batteries that carries the charged ions back and forth between the electrodes and then often that will use a very small percentage of the capacity. These side reactions, these self discharge reactions, are more likely to happen when the battery is in a high state of charge. And so leaving the battery very, very charged will accelerate those reactions as will increase temperature. So what we can do to avoid that is to maybe store the battery partially charged, not leave it in high states of charge for a long time, not overcharge it, not leave it in warm conditions, et cetera.