Conservation and the loss of charge of NiMH batteries - Page 1
Forum section: electronics / general

author: msuffidy
author_rank: Frequent Contributor
timestamp: January 09, 2021, 09:03:07 pm
content: Since energy is supposed to be converted but not lost, what happens to the energy that charged a NiMH battery had when it loses it when on the shelf for a long time? Obvious interpretations are loss through slow cirucit path through air, and internal chemical degradation.

author: ogden
author_rank: Super Contributor
timestamp: January 09, 2021, 09:22:22 pm
content: https://en.wikipedia.org/wiki/Self-dischargehttps://batteryuniversity.com/learn/article/elevating_self_discharge

author: msuffidy
author_rank: Frequent Contributor
timestamp: January 09, 2021, 09:37:10 pm
content: I guess maybe in the least damaging case, somehow the electrons escape and slowly leave the battery in the form of heat.

author: ogden
author_rank: Super Contributor
timestamp: January 10, 2021, 12:37:11 am
content: Quote from: msuffidy on January 09, 2021, 09:37:10 pmYour guess is wrong in case of NiMh cells because theyabsorbheat during (self)discharge.

author: magic
author_rank: Super Contributor
timestamp: January 10, 2021, 09:41:57 pm
content: So where the energy goes and if it stays inside, how can you charge the battery back to the initial state by shoving a nominal load of energy into it, without it getting unusually hot in the process?My vote is for heat, unless you have some convincing argument.

author: beanflying
author_rank: Super Contributor
timestamp: January 11, 2021, 01:49:52 am
content: Also understand that not all NiMh technologies self discharge at the anything like the same rate. The Sanyo Enerloop cells were some of the first very very low self discharge technologies which is now fairly widespread across a range of brands. Before those it was common to see self discharge rates of 20-30+% per week once again depending on technology.

author: BravoV
author_rank: Super Contributor
timestamp: January 11, 2021, 06:01:16 am
content: Quote from: magic on January 10, 2021, 09:41:57 pmNot a chemist, but my kid is, told me not in details though, some self generated heat was used in fueling non reversible chemical reaction (that can not be reversed back at charging process) , that in the long run this formed chemical residue after accumulated for a while, will contaminate the cell it self like capacity degradation, higher self leakage and etc, its called max. charge/discharge cycle.

author: ogden
author_rank: Super Contributor
timestamp: January 11, 2021, 08:21:57 pm
content: Quote from: magic on January 10, 2021, 09:41:57 pmYes, heat it is. NiMh battery itself will absorb heat during discharge, but it's load - will heat-up. Self- discharging NiMh battery is both chiller and heater if I can say so. Both most likely will reach thermal equilibrium. Honestly I don't know because don't actually care. Self-discharge currents of healthy NiMh cell are so low that you don't notice any resulting thermal effects anyway.

author: magic
author_rank: Super Contributor
timestamp: January 11, 2021, 09:12:09 pm
content: A random figure for NiMH self-discharge I found on the net is 1% per day. That's some 20mAh for a fully charged typical AA cell and therefore 1mW of power dissipation. That's not very easy to detect, but a few cm of foam can provide better than 100°C/W thermal isolation for an object this size, so it can be boiled down to detecting a fraction of °C difference between two points with and without the battery present inside. The outside of the isolation would need to be isolated from thermal gradients, air drafts and IR to maintain uniformity. Might be doable if somebody really needs to know

author: Rick Law
author_rank: Super Contributor
timestamp: January 12, 2021, 10:29:25 pm
content: This reply is not an attempt to explain the OP's question directly. I want to remind folks thatenergy content is not the same as useful energy. Useful as in the physics definition of the energy being able to do work. Most of you already knew it, but might not have think about that in this manner.Imagine if you have two batteries, A & B. Their negative terminals are connected together, and their positive terminals are connected together via a light bulb.(Case 1) If battery A is at 4.2V and battery B is at 3.2V, as you already know, the light bulb will be powered by 4.2V-3.2V=1.0V. So, "work" can be done here with 1V and the bulb lights up.(Case 2) You guess it, if both A&B are 4.2V, the bulb is powered by 4.2V-4.2V = 0.0V = NOT powered!So, while case 2 is a system with more energy content than case 1, you can't use it. To do "work", it doesn't matter how much the energy content is, there has to be a differential. Even if A & B are both 420V in case 2, a huge amount more than 4.2V, it still doesn't matter. No differential, no work.In so far as the self-discharged NiMH goes, the energy may still be in there (different molecules or different arrangement of same molecules such as forming crystal), but that energy is of no use to you or anyone else except if you want to reshape/rearrange the molecules again.So, similar to the "heat deaths" of the universe, energy may still be in there - just not useful kind.EDIT:Modified the cases as 1 & 2 instead of A & B, since I already called the first battery as A and called the second battery as B.