Battery discharge process - Page 1
Forum section: electronics / beginners

author: mitrynicolae
author_rank: Contributor
timestamp: August 12, 2019, 06:55:03 pm
content: I have one silly yet complicated question: Why batteries (of any kind) discharge over time even if they are not connected to anything?I want an explanation at electron level. The fact that there are some chemical reactions happening that lead to the loss of charge I already know. You can choose one type of battery it it is easier but please describe in detail the process.Also does the internal battery resistance has any role in this process? I already know that battery type, temperature, battery age, charge current and other factors affect the speed of the battery discharge, but the internal resistance has any role in it?Thanks!

author: Zero999
author_rank: Super Contributor
timestamp: August 12, 2019, 08:28:21 pm
content: The chemical reaction doesn't stop, just because the battery is not connected to a circuit. The exact answer to your question depends on the battery chemistry, of which there are many: alkaline, NiMH, lead acid, lithium ion, zinc air.

author: ArthurDent
author_rank: Super Contributor
timestamp: August 12, 2019, 10:14:38 pm
content: "Why batteries (of any kind) discharge over time even if they are not connected to anything?"Have you tried checking for discharge at absolute zero?The answer to this question may help answer your question.

author: Rick Law
author_rank: Super Contributor
timestamp: August 12, 2019, 10:17:14 pm
content: Besides Chemistry, another way to think of it is in terms ofenergy and stabilityof the electrons.Say if you have a billiard ball and a plank of wood slightly tilled at some angle. You put the ball on that plank of wood and the ball would roll down. The ball was at a higher energy level (potential energy) which when rolled down, that potential energy is released in the process.Let say you put some barrier there, such as, some baby powder sprinkled onto the plank of wood just at where the ball was - just enough powder to hold that ball there. The ball is now stationary at that higher potential energy level as before - because of the small barrier (made of baby powder), it was held there. For the ball to roll down, it must first overcome that small barrier. Now, let say some vibration is introduced. A heavy truck passed by near where that plank of wood with the ball, the road way shake just a bit thus vibrating the plank of wood just a bit... or may be a small earth quake, whatever... That little vibration could be just the kick it needed to overcome the powder-barrier and roll down. As it rolls, the potential energy is released.You can also visualize in this thought experiment that if you have a stair case where each step by itself is perfectly flat and level, but each step is lower than the last. If you leave a ball on the top step, eventually it will end up at the bottom of the stair case. In fact, even after the stair case, if there is a way further down, it just may roll there and go all the way to the basement. That is how nature works, and that is how energy works.In fact, more interestingly, you may not even need a "kick start"vibration as in this billiard ball thought experiment. When you are dealing with something as small as an atom or as small as a sub-atomic particle, it can happen by itself. It doesn't need a kick start to climb that energy-hill (powder barrier) to get to the other side of the powder barrier. When in quantum level sizes, the "Tunneling Effect" becomes far more significant. The particle in question somehow just tunnel though the barrier. The smaller the barrier, the more likely it can tunnel; likewise, the smaller the particle, the more likely it can tunnel. When you learn quantum mechanics, you will also learn the how to evaluate the probability of tunneling. If on the other side of the barrier is a way to release more energy, well, like the billiard, it will release that energy by rolling down.You may assume that, well, quantum mechanics is so esoteric, what does it has to do with our day-to-day batteries.It is interesting to think about your light switch with copper contacts. Why the hell does it still works a day after it was put on the wall? A perfectly cleaned copper contact should be fully covered by a thin layer of copper oxide in the matter of mere minutes, and copper oxide like other oxides is non-conductive. So, how the hack can a light switch in work at all? It is generally assumedthat tunneling effect is what started the electron avalanche. So, without tunneling, none of the switches in your house would work. But for quantum effect, you are living in the dark.As long as there is an energy gradient, be it by chemical reactions, or simply by the laws of quantum mechanics,that battery is going flat.In nature, high energy state will in time self-release and dissipate that energy until you have no energy differential. There is no way to keep your battery at a higher energy state forever.References:Electric Contacts, Theory and Applicationby Ragnar Holm, Published by Spinger-Verlag Berlin Heideberg GmbH4th edition, Page 136 :"...It is generally assumed that the first stage of any high resistivity film breakdown is the injection of electrons into the film [ of oxides ] by kind of field emission or Zener effect. The strong field makes the boundary barrier steeper and consequently thinner so that electrons can tunnel though the hill... ...the material within the path is strongly heated by the current and thereby the cohesion of the film material is diminished... ...a channel through the film is produce"

author: David Hess
author_rank: Super Contributor
timestamp: August 12, 2019, 10:56:26 pm
content: Two that I know of include:1. The insulation has a shunt resistance just like a capacitor.2. There are side reactions which convert the compounds to other unusable forms. Some of these reactions depend on charge voltage so occur faster at higher charge.

author: floobydust
author_rank: Super Contributor
timestamp: August 12, 2019, 11:02:43 pm
content: I understand leakage current is mainly due to impurities in the battery's electrolyte causing those ions to migrate. The separator also plays a role.A flooded lead-acid battery, put some tap water in it and the self-discharge rate skyrockets, because you have added impurities.Eneloop NiMh LSD batteries were developed by making a high purity electrolyte and an improved separator.