Where Do Electrons Go When a Battery Loses Charge?

[hippysmith]

I am no physicist or student and am asking question in order to out try and understand why my old tractor is not charging. This got me into understanding the concepts of how the generator and voltage regulator work to create a current to charge the battery.

My investigations pulled me down in to understanding the concept of free electrons from battery flowing through a conductor to make up the current. I can understand how a generator driven by the fan belt can create current I think, but this is the same number of electrons as in the system isn’t it. My understanding !!! Is the dynamo is creating a current or flow of electrons that end up in the positive side of the battery.

Apologies if my wording is not very scientific. I kind of get that and I read the circuit will be net neutral in terms of number of electrons in the system ( battery/circuit). But here is my question. If the charge is being used to create light or heat by passing through a resistor? Then are those electrons being converted to heat or light and then doesn’t that mean electrons are escaping the circuit?

If so, do they ever get replaced some how? Is this why you might need to replace electrolyte in battery???? I did physics A level 30 years ago and only just scrapped a pass then and can only remember the basics.

 

[hippysmith]

Wondering to myself, is this why you can only charge a battery so many times you effectively run out of electrons.

 

[TonyStewart]

There is a conversion of energy between mechanical and magnetic forces that is reciprocal, so that generator internals are like motors where Force is measured by Torque and Current with some constant that depends on design. Current is the rate of change of electron flow, so in a loop with nodes + and - (called ground if = 0V) the electrons flow in a loop through rectifier diodes (4) so that DC is created from AC. An lost energy in heat or light must be replaced from the "prime mover" or prime storage ( battery or extremely large capacitor).

So the 1st failure is a lack of measurements for voltage and current. A cheap DMM is a good investment for voltage and many results give clues. This would tell one if the fault is in one battery cell being too high resistance (aged plates) and excess cell voltage (boiling acid) or one or more diodes, or anything else in the loop, wires, loose or oxidized battery terminal etc.

Refurbished or recycled alternators or batteries are a common solution from auto junk yards.

A website called the "battery university" is a great resource to learn more.

 

[hippysmith]

By golly I think I understand. The mechanical energy turning the generator is converted to fast moving electrons ( a strong charge). When that circuit is connected with a light, essentially increased resistance, the rate of flow of the electrons is diminished and the energy is given off in light or heat, but the electron is still in the circuit it’s just now not flowing so fast.

Yeah I’ve bought new voltage regulator and got a 101 diagnostics planned to work out if the dynamo is generating properly and if not it’s the field / electo magnet component or the generator circuit. I ve got some new brushes etc. in fact I ve bought a whole new wiring harness. But yep than you very much. They should teach tractor repairs in physics lessons!

 

[Baluncore]

I am no physicist or student and am asking question in order to out try and understand why my old tractor is not charging.

Welcome to PF.

Is the belt slipping? Maybe the generator, maybe the regulator, maybe the battery. The troubleshooting procedure will depend on what you have. It does not require understanding electrons.

First fit a temporary good battery and see if it works again. Measure the battery voltage or look at a filament globe, while the engine is running, does the voltage / brightness rise with higher RPM from idle? When you turn on the headlights, what does the voltage fall to?

Tractors went from dynamo generators to AC alternators during the 1970s. Generator charge regulators almost never fail. Generator brushes fail, then the winding insulation fails. Generator bearings fail, then destroy the rotor windings. Cross your fingers. Sometimes changing to an alternator is the easiest way to fix a generator.

What is the make, model and year of your tractor?
Is the fuel diesel, gasoline, vaporising oil, or kerosene?
Is it magneto or distributor ignition?

 

[hippysmith]

He he.
It’s a Massey Fergusson 1974, Mf 165 with original dynamo. Engine diesel A4 212.

I advised a have purchased new wiring loom. This came with different colour coded wires from original spec. But does look to have all correct wires. I ve drawn the following wiring diagram if you are interested. And have got new bushes for dynamo and new regulator.

The tractor has been rewired before I got it and so I m hoping to resolve a few issues such as not charging and fuel guage not working.

Yep I appreciate I don’t need to understand flow of electrons buts nice to understand if I can.

I have answer I wanted but am happy to keep posting on here with progress if anyone interested. I m off to Ireland in Aug and will be back around 20th.

 

[hippysmith]

I m not sure if I uploaded image so having another go

 

[hippysmith]

Ooh and tractor

 

[snorkack]

Wondering to myself, is this why you can only charge a battery so many times you effectively run out of electrons.

No. Not this.
The thing is that a good rechargeable battery is a lucky and well-designed system.
If everything goes right then a lead battery, when unloaded, has Pb²⁺ (PbSO₄) on both electrodes. When you load the battery, you move two electrons per each Pb through your power source so you have Pb on one side and Pb⁴⁺ (PbO₂) on the other side.
Even though the actual charge is neutralized (you get PbO₂, not bare Pb⁴⁺), this is still unstable. The electrons want to return to the situation where it is PbSO₄ on both sides - and they have some energy to release in the process.
What you want to happen is for all the electrons to return through your resistor and not elsewhere, and spend as much as possible of their energy in your resistor and not elsewhere.

They have alternatives. Since your battery is well designed to work best, when it changes it changes for the worse. On the unloading, maybe some parts of the electrodes have found alternative ways for their electrons to reach the other electrode, so you get out fewer electrons than you put in - because some have leaked away. Or maybe some parts of electrodes from which the electrons do go to your resistor, but spend too much of their energy in getting out of the electrode.
And on loading, you may find that parts of the electrodes remain PbSO₄ and don´t load. Which is why the capacity of the battery falls.
There are several ways for a battery to go wrong, because electricity has many ways to leak out.

 

[Baluncore]

It’s a Massey Fergusson 1974, Mf 165 with original dynamo. Engine diesel A4.212

That is a nice tractor. It must be one of the last with a dynamo. I think the dynamo and parts are all available from Bepco or Sparex.

 

[Drakkith]

But here is my question. If the charge is being used to create light or heat by passing through a resistor? Then are those electrons being converted to heat or light and then doesn’t that mean electrons are escaping the circuit?

No, not at all. The number of electrons in the circuit (which includes the battery) remains the same. Otherwise a charge would build up and you'd quickly get an enormous discharge to or from the circuit that would destroy or severely damage it.

As electrons move through any part of the circuit, they lose energy. High resistance components like resistors and insulators rob a great deal of energy from the electrons, while conductors take only a little (a simplification, as energy loss also depends on thickness and length of the components). A light-emitting-diode, or LED, is a semiconductor device that has higher resistance than a conductor, but less than an insulator. Some energy will be lost as heat in the LED, but a great deal is also lost in the form of the emitted light. No electrons are lost in this process.

If so, do they ever get replaced some how? Is this why you might need to replace electrolyte in battery???? I did physics A level 30 years ago and only just scrapped a pass then and can only remember the basics.

The only electrolyte I know of that can be replaced is the one in a lead-acid battery, and that's because some of the water either evaporates or is separated into hydrogen and oxygen gas that then dissipates. In such a case you just add distilled water to the battery to bring it back to full. It has nothing to do with losing electrons.

Additionally, over time the electrodes in the battery will degrade over time due to various issues and the battery will eventually require replacement. See here.

 

[snorkack]

No, not at all. The number of electrons in the circuit (which includes the battery) remains the same. Otherwise a charge would build up and you'd quickly get an enormous discharge to or from the circuit that would destroy or severely damage it.

True. But the number of electrons which will go through the circuit can and will diminish. For example due to self-discharge - when some electrons, instead of going through the terminals into the circuit, find another way to the opposite electrode.
All batteries have some self-discharge. However, batteries can fail through "soft short" of elevated self-discharge, or hard short, which can proceed to a thermal runaway. These are some failure modes, but there are more modes.

 

[sophiecentaur]

The mechanical energy turning the generator is converted to fast moving electrons

This is a common misconception; KE is the wrong model to use. The electrons flowing along a wire will have an incredibly slow average drift velocity of around 1mm/s. How so? Well there are a lot of electrons involved but the main thing is that the Power that's transferred from battery to light bulb is due to the forces that 'push' the electrons round and cause the filament to get hot due to its resistance.

This may be very counter-intuitive but consider a bicycle chain (equivalent to the electrons going round the circuit) the total mass of the chain is only a few 100g and the speed could be 1m/s. Its kinetic energy would be only (0.1x 1 X1)/2=0.05J (ish). That amount per second would be around 0.05W . But that chain can be transferring 1kW when accelerating a fit rider. In that case, the Power is the Force times the speed of the chain - i.e. a very high force (riders weight times the mechanical advantage of the crank).

 

[sophiecentaur]

But the number of electrons which will go through the circuit can and will diminish.

For the purposes of this basic level question, I feel that discussing a discharging battery is of limited usefulness. There are the same number of electrons in the whole circuit all the time. When the cell goes 'flat' it's because the chemical reaction has completed and all the molecules are in their minimum energy state (very crudely described). It's available Energy that runs out and not Charge.

 

[snorkack]

For the purposes of this basic level question, I feel that discussing a discharging battery is of limited usefulness. There are the same number of electrons in the whole circuit all the time. When the cell goes 'flat' it's because the chemical reaction has completed and all the molecules are in their minimum energy state (very crudely described). It's available Energy that runs out and not Charge.

No, this is a non-rechargeable battery, which indeed exhausts its energy when the chemical reaction completes and the electrons go to their minimum energy state.
But the original post discussed rechargeable battery. Or rather, a battery that initially was rechargeable - you could put in energy, force electrons into higher energy states, and then recover the energy by discharging the battery - but then lost the capability to charge. What happens to a rechargeable battery to cause it to stop being rechargeable? What becomes of the electrons, in the process?

 

[sophiecentaur]

@snorkack on a primary cell the chemical reaction is not simply reversible but, in a rechargeable cell the chemical process is reversed by a ‘forced’ reverse current. But recharging is not perfect and the electrolyte and electrode interface degrades so fewer molecules / ions are available BUT there are the same number of electrons in the cell. Where would they go??? If you got rid of electrons you’d have to get rid of atoms too.
PS it is possible to partially recharge some primary cells but it is not always safe.

 

[snorkack]

@snorkack on a primary cell the chemical reaction is not simply reversible but, in a rechargeable cell the chemical process is reversed by a ‘forced’ reverse current. But recharging is not perfect and the electrolyte and electrode interface degrades so fewer molecules / ions are available BUT there are the same number of electrons in the cell. Where would they go???
If you got rid of electrons you’d have to get rid of atoms too.

You might.
Consider trying to charge a lead battery.
On the wanted reaction
PbSO₄+2e^-=Pb+SO₄^2-
the electrons go to Pb atoms, from where they can be recovered. The net charge of the battery remains unchanged (near zero), because the electrons are taken from other atoms which are also in the battery, but on the other electrode:
PbSO₄+2H₂O=PbO₂+SO₄^2-+4H⁺+2e^-
The total number of electrons in battery is unchanged but energy is stored to force electrons to other atoms. That stored energy can be recovered by letting the electrons flow back, which is why the battery is rechargeable.
But if you force electrons into battery, they can find other places to go. Like
2H⁺+2e^-=H₂
The electrons may actually physically leave the battery, taking the H atoms with them. But whether or not they do, you have another problem: you have stored energy in atoms (H atoms instead on the Pb atoms you wanted) but the thing is that the energy in H molecules cannot be turned back into electric energy like the energy in Pb atoms. Worse, the energy stored in H molecules can readily be turned to mechanical energy of blowing the battery up, which would not have happened with energy stored in Pb atoms.

 

[sophiecentaur]

On the wanted reaction
PbSO4+2e-=Pb+SO42-
the electrons go to Pb atoms,

Of course you're right but the equation is only a partial description of real life. You could not have a bucket full of either side of the equation because it would disintegrate / explode with the coulomb forces. It only works in the context of the whole battery.
also

electrons may actually physically leave the battery, taking the H atoms with them.

You end up with a different system - with a different mass - so what has that proved? Also the H atoms have different potential from the potential the H and the e had on their own. But they will associate with other H atoms to form H₂ molecules and the energy again re-arranges itself. Whatever experiment you perform, you get conservation laws.

Good fodder for students to make them think.

 

[Mister T]

You don't run out of electrons when a battery dies. You run out of the energy necessary to make those electrons do work.

 

[sophiecentaur]

You don't run out of electrons when a battery dies.

any more than you run out of chain links when the cyclist gets tired.

 

[snorkack]

any more than you run out of chain links when the cyclist gets tired.

Or the chain may seize. Or the chain may snap.
Both chain getting stuck somewhere and chain snapping have the same gross result: input effort will not propel the cycle. In both cases all the links still exist somewhere. Yet side effects differ. It is quite meaningful and relevant to ask specifically where the chain has gone.

 

[sophiecentaur]

It is quite meaningful and relevant to ask specifically where the chain has gone.

Hmm. You are trying to take the analogy a bit too far here. The original question morphed a bit and there is no proper correspondence between electrons which can only be actually removed by applying a (very high, aamof) electrical potential and chain links. Once a link is dropped off, the whole chain stops working. conservation law will still apply; a missing link has to be 'somewhere' but the situation takes you outside the very crude analogy I offered.

 

[hippysmith]

So many great answers. And I am much better informed than I started, on how batteries work and how electrons are used to carry charge.

FYI I did get my battery charging. The issue was in the dynamo loosing its residue magnetic field and even when connected to a new battery via a voltage regulator it (dynamo field terminal) needed to be ‘flashed’ to enable the voltage regulator to send a variable current to modify the magnetic field in order to regulate the charge it generates. I did that early doors but could not detect much of a charge which confused me for a while. This was also hampered by my ammeter needle getting stuck after years of non use and also having a new battery that was fully charged and thus did not need charging. I started the tractor with battery and could see no charge on ammeter but a variable voltage from dynamo terminal. Voltage at ammeter at about 12v. I disconnected battery whilst still engine running.
I turned lights on and they came on with out battery.
So I knew charge was being generated. I could now see a volt meter reading of 14.n volts at ammeter but still gauge needle not showing charge. I was confident there was charge now. And after some gentle tapping and shaking the needle sprang into life. So reattached battery and now when revving engine can see charge on ammeter. So am concluding all is now working.

You are a great community and very generous with your wisdom. I only wish I had been a lot more interested when I was at school.